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Fire regimes in Alaskan Pacific maritime ecosystems

Abstract: For thousands of years the cool, wet climate along the southern Alaskan coast supported ecosystems dominated by old-growth conifer stands with large, old trees and multiple canopy layers in a mosaic with wetlands and small disturbance gaps. Historically, lightning was rare and fuels were typically too moist to burn, year-round. Dominance of late-successional, fire-sensitive species such as western hemlock and Sitka spruce, multiaged stand structures, and evidence from paleological charcoal studies indicate that stand-replacing fires have been extremely rare in coastal rainforests over the past several millennia. While some accounts suggest that rare, stand-replacing fires occurred historically on susceptible sites in southeastern Alaska, contemporary fire behavior and effects suggest that fires in these ecosystems are more likely to be small ground fires with mixed severity. Fire activity has increased in Alaskan coastal forests during the postsettlement period. Most recent fires were human-caused, ground fires that burned less than an acre due to variability in fuel moisture, slow fire spread, and effective fire suppression; however, impacts to burned areas, including tree mortality, erosion, and delayed regeneration, were severe.

While fuel loads and stand structures in areas that have not been altered by logging or other human activities are probably still within the historical range of variability, changes to fuel characteristics and fire activity are anticipated with climate warming. Over the past century, temperatures warmed, precipitation patterns changed, depth and duration of snowpack decreased, and storm intensities increased in southern coastal Alaska. Without substantial increases in precipitation during the fire season, warmer temperatures are likely to result in drier fuels for longer periods, and thereby increase the probability and impacts of wildfire. Global climate changes may also increase the frequency of lightning. Models project that the southeastern coast of Alaska and northern coast of British Columbia will transition from low to high probability of fire by 2040.

Table of Contents:

Figure 1—Western hemlock-Sitka spruce coastal forest on the Tongass National Forest, Alaska.
Citation for this synthesis:
Zouhar, Kristin. 2017. Fire regimes in Alaskan Pacific maritime ecosystems. In: Fire Effects Information System, (Online) U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: https://www.fs.usda.gov/database/feis/fire_regimes/AK_Pacific_maritime/all.html [].

INTRODUCTION

This Fire Regime Synthesis brings information on fire regimes together from two sources: the scientific literature as of 2017 and models developed by LANDFIRE for Biophysical Settings (BpSs) , which are based on literature, local data, and/or expert estimates. This synthesis:

This synthesis uses primary literature on fire when available and relies on several literature reviews covering disturbance regimes and related topics in Alaska (e.g., [35,55,101,110,118,127,139]) and British Columbia (e.g., [31,32,38,57]) for information on topics related to fire, including climate change. Much of the regional climate change information comes from reviews by Tillmann and Glick [124] and Haufler et al. [56].

The primary geographic focus of this synthesis is southeastern and south-central Alaska. Information from similar coastal rainforest communities in British Columbia and the Pacific Northwest is sometimes referenced to provide context when information from Alaska is lacking. A publication on Fire regimes of Pacific Northwest coastal forests is also available in FEIS.

Common names are used throughout this synthesis. For a complete list of common and scientific names of plant species mentioned in this synthesis and links to FEIS Species Reviews, see table A2.

DISTRIBUTION AND PLANT COMMUNITY COMPOSITION

Figure 2—Land cover distribution of Alaskan Pacific maritime ecosystems based on the LANDFIRE Biophysical Settings (BpS) data layer [72]. Numbers indicate LANDFIRE map zones. Click on the map for a larger image and zoom in to see details.

Geography and climate: The Alaskan Pacific maritime ecosystems covered in this synthesis occur along the Pacific Coast from eastern Kodiak Island south and east through southeastern Alaska and into northern British Columbia. This corresponds to the distribution of Sitka spruce in Alaska and northern British Columbia [30,129], the coastal rainforests ecological province described by Barrett et al. [16], and the subpolar and northern part of the perhumid temperate rainforest zones described by Veblen and Alaback [127]. Subpolar temperate rainforests occur in south-central Alaska—west and north of Yakutat Bay, mostly in the Prince William Sound region. Perhumid temperate rainforests occur from southeastern Alaska and northern British Columbia south to northern Vancouver Island [127].

Southeastern Alaska consists of a large number of islands (the Alexander Archipelago) and a highly dissected mainland strip. High, steep mountain ranges, rugged topography, glaciers, and icefields separate southeastern Alaska from Canada to the east and north, effectively isolating it from other vegetation zones [5]. The northern coastal limit of coast Douglas-fir (hereafter, Douglas-fir)—near Kemano, British Columbia [115] at about 55° N latitude [61]—marks a change from coastal forests with a history of rare to infrequent fire to the south to those with almost no history of fire to the north. Forests dominated by Douglas-fir are thought to be a product of infrequent (fire-return intervals as long as 500-700 years), stand-replacing fires during the past 3,000 years (reviews by [4,13]). The role of fire is less significant and less certain in rainforests to the north and along the narrow coastal strip to the south [74].

The climate in this area is maritime, with moderate mean annual air temperature, high mean annual precipitation, cool summers, and moderate winters [16]. The region is characterized by prevailing westerly winds from the Pacific Ocean, stable temperatures, and prolonged cloudiness. Cyclonic storm systems forming over the North Pacific Ocean and the Gulf of Alaska bring large amounts of precipitation, especially during winter. Windward slopes receive most of this precipitation [5,38,111,127]. Dry periods with above-normal temperatures that last 2 weeks or more occur every few years [55], but the summer dry season is not usually pronounced because of frequent summer fogs that often develop into light rain or drizzle [5,6,38,111,127]. Mean air temperature is generally less than 61 °F (16 °C) during the warmest month. Annual precipitation ranges from about 50 to over 190 inches (130-480 cm) and occurs mostly as rain, evenly distributed throughout the year. Heavy snowfall, glaciers, and icefields are common at high elevations [5,111,127]. Mean precipitation exceeding 118 inches per year (300 cm) is common on the northern coast of British Columbia, and extremes may reach more than 236 inches per year (600 cm) on upper mountain slopes. Much of the winter precipitation falls as snow at higher elevations and inland areas, but low-elevation coastal areas typically have little snow accumulation [38].

Vegetation and site characteristics: Alaskan Pacific maritime ecosystems covered in this synthesis were historically dominated by large, continuous tracts of all-aged, structurally diverse, old-growth temperate rainforest but included topographic, edaphic, and disturbance-dependent variants and inclusions such as scrub communities dominated by tall shrubs, low shrubs, or dwarf shrubs; wetlands dominated by herbs, shrubs, or small trees; alpine communities; and riparian areas (see table A1). The vegetation below timberline in southeastern Alaska is a mosaic of dense coniferous forest (approximately 60% of the land cover) interspersed with smaller patches of these other communities, such as shrub-covered avalanche and landslide tracks [6,38,98]. Wetlands occur in discontinuous patches on poorly drained terrain throughout much of southeastern Alaska, and muskegs—complex systems of bogs, open pools, forested wetlands, and poor fens—are the most common wetland type [87,98].

In perhumid rainforests, western hemlock and Sitka spruce dominate uplands and mix with shore pine, mountain hemlock, western redcedar, and yellow-cedar on wetlands. Mountain hemlock abundance generally increases with elevation [101,127]. The subalpine zone of perhumid rainforests is similar in composition to the coastal zone of subpolar rainforests [127]. Subpolar rainforests in south-central Alaska are dominated by mountain hemlock, western hemlock, and Sitka spruce. Along the eastern coast of the Kenai Peninsula, the coastal rainforest transitions into subboreal forest. Lutz spruce, a Sitka spruce × white spruce hybrid, occurs on the Kenai Peninsula, along the west side of Cook Inlet, and in the Skeena River Valley, British Columbia [19,30,53]. In the Kodiak-Afognak archipelago of Alaska, Sitka spruce is the only coniferous tree species, and it has been spreading into the grass-tundra communities in the area [53,95]. Shore pine and paper birch occur in areas with a history of fire [127].

The plant communities and BpSs covered in this synthesis are shown below. Brief descriptions of site affinities and successional relationships are provided for forest types, and NatureServe provides detailed descriptions for all ecosystems covered here [95,96]. The NatureServe report cross-references several other vegetation classifications including, but not limited to, the following: [22,23,35,36,39,86,119,128].

Upland forest and woodland: Closed-canopy Sitka spruce and western hemlock forests occur throughout southeastern Alaska at all elevations below the subalpine zone and in a narrow coastal strip in south-central Alaska. They mostly occur at low elevations on alluvial fans, floodplains, footslopes, and uplifted beaches, but may also occur at midelevations on steep slopes. Sitka spruce is more abundant and more often dominant at lower elevations and areas of periodic disturbance such as water movement, windthrow, soil mass movement, and wind and salt spray on beach fringes. Western redcedar may codominate in closed western hemlock-Sitka spruce forest communities south of 57° N latitude. These are late-seral or climax communities; floods, avalanches, and salt spray are the primary disturbances and typically affect small areas. Deciduous trees are uncommon; red alder (on the coast) and black cottonwood (more common inland) occur mainly on floodplains and landslide scars [13,128].

The following descriptions of site and successional relationships in Alaskan Pacific maritime upland forests and woodlands are modified from NatureServe [95] unless otherwise cited. Corresponding BpS series are given in parentheses after NatureServe's ecological system name; follow links to see detailed descriptions of vegetation and site characteristics provided by NatureServe [95].

Alaskan Pacific maritime Sitka spruce forest (16440) occurs on well-drained sideslopes and footslopes in the perhumid and subpolar rainforest zones. Western hemlock or mountain hemlock may be minor canopy associates. In southeastern Alaska, red alder may be an associated subcanopy tree species, especially in upland alluvial fans. Shrub associates are numerous. On Kodiak Island, in the Kenai Fjords, and Prince William Sound, Sitka spruce is frequently the dominant canopy tree from sea level to treeline on productive sites. It is the only conifer that occurs on Afognak and Kodiak islands, where its range is actively expanding. In the southern portion of the Alaskan rainforest, Sitka spruce is linked more closely with limestone substrates and frequently disturbed sites (e.g., unstable slopes, very steep sites, outer coast headlands).

Alaskan Pacific maritime western hemlock forest (16460) is the dominant forest system along the southeastern portions of the Alaskan coast. It occurs from the northern limit of Douglas-fir in coastal British Columbia, north through southeastern Alaska to Prince William Sound (the northwestern limit of western hemlock). Sites are typically well drained. Sitka spruce sometimes codominates. In the north (Yakutat through Prince William Sound), mountain hemlock may also be present, and in southeastern Alaska (Glacier Bay to British Columbia) yellow-cedar may be present. Ovalleaf huckleberry often dominates the shrub layer. Salmonberry or devil's-club may dominate the shrub layer on disturbed sites.

North Pacific mesic western hemlock-yellow-cedar forest (16460) occurs throughout southeastern Alaska at all elevations below the mountain hemlock zone and is most abundant on somewhat poorly to moderately drained slopes. Yellow-cedar and western hemlock codominate, while western redcedar, Sitka spruce, and Pacific silver fir are absent or rare. This system intergrades with mountain hemlock forest, and mountain hemlock may occur in transitional stands. The shrub layer is relatively well developed (>50% cover) in late-seral stands.

North Pacific mesic western hemlock-silver fir forest occurs entirely west of the Cascade Crest from Washington to coastal British Columbia, and extends north to about 56° N in southeastern Alaska. Pacific silver fir has a limited distribution in Alaska, where it is apparently confined to the extreme southern mainland and a few islands and occurs in nearly pure stands or mixed with Sitka spruce and western hemlock. The associated BpS series (10420) is not mapped in Alaska, and more information on this system is available in the FEIS publication Fire regimes of wet-mesic western hemlock forests.

Alaskan Pacific maritime Sitka spruce beach ridge (16540) occurs along the Gulf of Alaska coast in the following areas: Copper River Delta, Cape Yakataga, Yakutat Forelands, and the outer coast of Glacier Bay National Park. Western hemlock may codominate on older sites. Devil's-club is usually the most abundant understory shrub. As beach ridges form, they initially support brackish meadows and communities dominated by American dunegrass. The inland portion of these meadows transitions to Sitka spruce forest, which establishes about 130 years after beach ridge formation and may succeed to western hemlock forest.

Alaskan Pacific maritime mountain hemlock forest (16482) occurs from Kenai Fjords through southeastern Alaska, primarily in the maritime region (covered in this synthesis), but also in the subboreal transition on the inland side of the Kenai and Chugach mountains (covered in the FEIS synthesis Fire regimes of Alaskan mountain hemlock ecosystems. In southeastern Alaska, this system occurs from about 980 to 3,300 feet (300-1,000 m)—between the western hemlock and subalpine mountain hemlock dwarf-tree systems—on relatively stable sideslopes and benches with well-drained soils, typically on north-facing and rarely on south-facing slopes. Ovalleaf huckleberry typically dominates the shrub layer.

North Pacific maritime mesic subalpine parkland (16482) is very localized in Alaska, occurring in the eastern portion of the panhandle at high elevations at the transition from forest to alpine, forming a subalpine forest-meadow ecotone. As they approach treeline, mountain hemlock stands occur in clumps or patches of mature-height trees interspersed with mesic and wet meadows rich in dwarf-shrubs and forbs. Krummholz often occurs near the upper elevational limit. Major tree species are mountain hemlock, Pacific silver fir, yellow-cedar, and subalpine fir. Very deep, long-lasting snowpacks limit tree regeneration such that trees establish only in favorable microsites (usually adjacent to existing trees) or during years with low snowpack.

Riparian and wetland forest and woodland: Floodplains, fans, and estuaries subject to flooding are highly productive biodiversity hotspots [38]. Coastal wetlands are the richest plant communities of the coastal temperate rainforest zone in Alaska [98,123]. Depending on disturbance frequency and severity, wetlands support plant communities of various composition and ages, including deciduous forests, stands of massive spruce and hemlock, and shrub and herb communities [38]. Investigations in southeastern Alaska suggest that the development of muskeg vegetation may take 800 to several thousand years [99]. Muskegs may be a physiographic climax over much of southeastern Alaska, and they may be slowly increasing at the expense of forest cover in some areas [65,99,140].

The following descriptions of site and successional relationships in Alaskan Pacific maritime riparian and wetland forests and woodlands are modified from NatureServe [95] unless otherwise cited. Corresponding BpS series are given in parentheses after NatureServe's ecological system name; follow links to see detailed descriptions of vegetation and site characteristics provided by NatureServe [95].

Alaskan Pacific maritime floodplain forest and shrubland (16550) occurs along the Gulf of Alaska coast and south through southeastern Alaska. It does not occur on Kodiak Island. Frequent flooding, shifting channels, and sediment deposition characterize the system. Vegetation composition and disturbance cycle vary depending on type of input (glacial vs. nonglacial) and proximity to the glacier. On nonglacial floodplains, red alder or Sitka alder may be common in early-seral stands, and floodplain wetlands are common, but small. On glacial floodplains, wetlands are uncommon near the glacier, a high proportion of the floodplain is in barren and in early seral stages, and mature forest development is minimal. Vegetation on distal outwash plains varies with frequency of flooding and seral stage.

Alaskan Pacific maritime poorly drained conifer woodland (16810) occurs at low to midelevations on rolling terrain, benches, and gentle slopes from Kenai Fjords through southeastern Alaska. Soils are shallow to deep and usually have a thick organic layer or some peat development. In some places, stands are a fine mosaic of peatlands and better-drained inclusions. These are low-productivity sites that are intermediate between shore pine or mountain hemlock peatland sites and more productive forest systems. The forest canopy is open (less than 45% cover), and trees often show signs of stress. Standing dead trees are common. In the north, paludification on these sites may lead to conversion from mountain hemlock forest to mountain hemlock peatland over long time scales. Overstory trees may include western hemlock, mountain hemlock (often alone or with Sitka spruce in the subpolar rainforest zone), western redcedar (southern portion of the Alaska distribution only), and yellow-cedar. This system represents a topoedaphic climax that is relatively stable over time. Tree growth is generally very slow.

Alaskan Pacific maritime mountain hemlock peatland (16590) occurs from Kenai Fjords through southeastern Alaska and into British Columbia. It usually occurs on sloping terrain above 1,600 feet (500 m) in southeastern Alaska and British Columbia, and it may develop on fairly steep sideslopes in areas with very high rainfall and low permeability (such as Prince William Sound and Kenai Fjords). This ecological system is a mosaic of dwarf-tree (mountain hemlock, yellow-cedar, or Sitka spruce), dwarf-shrub, and herbaceous peatland communities.

Alaskan Pacific maritime shore pine peatland (16570) occurs from Yakutat south through southeastern Alaska. Shore pine does not occur north or west of Yakutat. This ecological system is a mosaic of shore pine, dwarf-shrub, and herbaceous peatland communities. It includes a range of canopy structures and compositions from mixed conifer peatlands on sideslopes and benches with yellow-cedar, mountain hemlock, western hemlock, and shore pine, to well-developed peatlands on flat, rolling, or sloping terrain with scrub shore pine. Soils are poorly drained with deep organic layers. Trees are typically stunted and the canopy has <30% cover.

Upland shrubland and grassland: Alaskan Pacific maritime upland shrubland communities include alpine dwarf shrubland communities that appear to be successionally stable; alder-dominated communities on floodplains and rivers; communities intermediate between marsh and Sitka spruce-western hemlock forest; and avalanche track, steep alpine, and tundra upland communities. Red alder is successional on disturbed forest sites [128]. In landslide and snow avalanche zones where disturbance is frequent, very densely vegetated patches of red alder and/or salmonberry occupy large areas. Depending on elevation, soils, and disturbance frequency these patches may succeed to western hemlock or other forest types [5]. Alaskan Pacific maritime grasslands include a variety of herbaceous vegetation types on sideslopes, rolling hills, and alluvial deposits. Graminoids, forbs, or ferns may dominate these herbaceous meadows. The following Alaskan Pacific maritime upland shrublands and grasslands occur in the area covered by this synthesis. Corresponding BpS series are given in parentheses after NatureServe's ecological system name. Follow links to see detailed descriptions of vegetation and site characteristics provided by NatureServe [95].

Alaskan Pacific maritime avalanche slope shrubland (16800)
Alaskan Pacific maritime subalpine alder-salmonberry shrubland (16520)
Alaskan Pacific maritime subalpine copperbush shrubland (16430)
Alaskan Pacific maritime alpine sparse shrub and fell-field (16430)
Alaskan Pacific maritime alpine dwarf shrubland (16430)
Alaskan Pacific maritime mesic herbaceous meadow (16530)

Riparian and wetland nonforest: Floodplains, fans, and estuaries subject to flooding are highly productive, diverse communities and may be dominated by a variety of shrubs and/or herbs. Wetland succession and species composition vary depending on site conditions such as water depth, substrate characteristics, and nutrient input. The following Alaskan Pacific maritime shrub and herbaceous wetlands occur in the area covered by this synthesis. Corresponding BpS series are given in parentheses after NatureServe's ecological system name; follow links to see detailed descriptions of vegetation and site characteristics provided by NatureServe [95].

Alaskan Pacific maritime shrub and herbaceous floodplain wetland (16560)
Alaskan Pacific maritime wet low shrubland (16600)
Alaskan Pacific maritime dwarf-shrub-sphagnum peatland (16600)
Alaskan Pacific maritime fen and wet meadow (16610)
Alaskan Pacific maritime alpine wet meadow (16730)
Alaskan Pacific maritime alpine floodplain (16760)

HISTORICAL DISTURBANCE REGIMES

Reference period: The coastal ecosystem in Alaska is, in many respects, a system of long time scales. Tree species are long-lived, intervals between large-scale disturbances are on the scale of centuries to millennia, and decay and decomposition progress equally slowly [38]. In order to understand how current vegetation and fire regimes developed, fire histories must be long enough to characterize the frequency and range of variability of fire occurrence in the system of interest [48]. In ecosystems where fire is rare or infrequent and fire-free periods last for centuries to millennia, it is useful to understand past changes in vegetation and fire occurrence over similar time scales. Paleoecological studies of charcoal records from lake sediments and soil profiles represent a key approach to understanding the natural variability of these fire regimes because they broaden the reference period and are well suited for reconstructing the incidence of past fire and its relationship to changing climate and vegetation, especially in ecosystems with fire-return intervals that exceed the age of the oldest trees [48]. Knowledge of the time since the last fire is not adequate for understanding historical fire frequency or assigning historical fire-return intervals in these ecosystems. Without long-term data, inferences about historical fire regimes are questionable and not likely to provide insights needed to manage for projected climate changes [137].

In this synthesis, the "historical" or "presettlement" period refers to the time beginning when the current vegetation types established, about 4,000 to 2,000 years ago (the late Holocene), and ending with European settlement in the late 18th and early 19th centuries. The time period beginning with European settlement and leading to the present day is referred to as "contemporary". Many of the dominant species and genera that make up the vegetation in contemporary Alaskan coastal ecosystems have been present for most of the Holocene, and components of the contemporary flora have evolved together for the past several millennia. In coastal areas of both Alaska and British Columbia, temperatures, precipitation, and vegetation composition began to resemble the contemporary record and landscapes about 4,000 years ago (table 1) (reviews by [48,60]).

Table 1—Generalized climate and dominant vegetation in southeastern Alaska during the late Pleistocene and Holocene (adapted from Viens 2001, as cited by [60]).
Years before present Climate Dominant vegetation
16,000-12,500 Cool, dry Tundra, shrubs
12,500-9,000 Warm, dry Pine, alder, willow
9,000-6,800 Warm, wet Spruce, hemlock
6,800-4,500 Trending wet, cool Hemlock, spruce, cypress
4,500-present Cool, wet Contemporary flora

Historical landscape variability: For thousands of years the natural vegetation along the southern Alaskan coast was dominated by old-growth conifer stands [13] with large, old trees and multiple canopy layers [95] in a mosaic with muskegs and other late-successional wetlands and small disturbance gaps [66]. Between ~7,000 and 4,000 years ago, decreasing summer insolation led to a transition from a relatively warm and wet climate to a relatively cool and moist climate. Rainforest ecosystems established during this time [74], and Sitka spruce, mountain hemlock, and western hemlock appeared in south-central Alaska about 4,000 to 3,000 years ago as a result of a migration northwestward along the Gulf of Alaska that took place as storm tracts strengthened during the late Holocene (review by [108]). These old-growth forests comprise a large carbon pool and support a disproportionately high diversity of plant and animal species, given their small areal coverage [139].

Landscape composition: Old growth is the dominant age class in upland coastal forest, with mostly small canopy gaps formed by topoedaphic variation (e.g., woody and herbaceous wetlands, including muskegs) and small-scale disturbances (e.g., avalanches, flooding, or windthrow) [66]. Susceptible sites may experience large-scale windthrow or geomorphic disturbances. The continual small-scale disturbance and renewal pattern in Alaskan coastal forests generates a shifting steady-state mosaic of mature forest with small patches in seral stages [35]. The different types of disturbances do not occur homogeneously across the forest landscape, and most are confined to, or occur predominantly in, specific site types or landscape positions [38].

Over most of the coastal landscape, stand-replacing disturbances are rare and stands are very old, with multiple canopy layers of large trees and occasional, small gaps [38,76,95]. In low-elevation Alaskan coastal forests most canopy gaps are small, disruption of the forest floor is minimal, and the percentage of area in gaps ranges from around 10% to 20%, though values as low as 4% and as high as 34% have been observed [23,38,76,101,103,127]. Most canopy gaps typically result from the death of 10 or fewer trees (gapmakers) [38]. Gap and gapmaker characteristics are generally similar among sites [101,103]. Disruptions to the forest floor may be largely restricted to storms with high winds and rain that saturates the soil, making uprooting more probable [23,101]. Ott and Juday [103] provide details on sizes and characteristics of gaps and gapmakers in a western hemlock association on the Tongass National Forest.

Disturbance regimes: About 97% or more of the coastal temperate rainforest was historically old growth, and younger stands were uncommon before logging began in the late 1800s [13,35,76,101,106]. Fire was absent to rare historically (see Historical fire regimes), and while large-scale windthrow and geomorphic disturbances occur, they are infrequent and generally restricted to susceptible landscapes [38,101]. Low-severity disturbances resulting in small-scale canopy gaps are common; and frequent, small-scale windthrow is the most important natural disturbance shaping old-growth stand structure [23,35]. Flooding, landslides, and avalanches are the primary geomorphic disturbances [127,139]. Snow, ice, frost, drought, insects, fungal pathogens, and mammals may also kill or injure trees [38].

The role of small-scale disturbance in controlling and maintaining forest structure in coastal temperate rain forests of North America has been well studied (e.g., [66,76,101]). Estimates of the average interval between successive gap creation events range from 230 to 920 years in southeastern Alaska [103] and from 280 to 1,000 years on the southern coast of British Columbia [38,75]. A substantial amount of gap infilling takes place between 20 and 80 years following tree death, but gaps, or portions of gaps, can persist for >80 years. On study sites in southeastern Alaska, forest turnover time was estimated to range from 230 to 920 years, and averaged 575 years [103]. Lertzman et al. [76] estimated a turnover time between 350 and 950 years in mature (100-250 years old) and old-growth (>250 years old) forest stands under a regime of small-scale, low-severity disturbance in the western hemlock zone on the west coast of Vancouver Island [76].

The following is a brief discussion of the dominant disturbance processes and related successional relationships influencing forest structure in these settings over the past several millennia.

Wind: Wind is the primary disturbance agent in the coastal forests of southeastern Alaska [4,10,66,106,127] and far more prevalent than fire in creating canopy openings in coastal forests in general [10]. Coastal rainforests are susceptible to wind damage because dominant trees (e.g., western hemlock and Sitka spruce) have tall, top-heavy crowns and shallow root systems, and high winds can occur during rainstorms that saturate the soil [23,54,101]. Gale-force winds may occur during any month in southeastern Alaska, but the strongest winds usually occur in autumn and winter [38,54].

Windstorms create a variety of opening sizes with a variety of internal remnant structure depending on site characteristics (e.g., exposure, position on the landscape, topography), wind intensity, and forest conditions [38,54,66,101]. In general, large-scale, infrequent blowdowns occur on exposed landscapes during severe storms, and small-scale canopy gaps (resulting from the death of one to several trees) are common on more protected landscapes and older forests [6,34,38,66,101,103,127]. Small-scale windthrow is much more common and is considered the most important natural disturbance in southeastern Alaska (e.g., [23,35,101]). Canopy gaps are typically <50 acres (20 ha) (e.g., [38,54,101,127]), and result in a very fine-grained landscape mosaic, such that the forest appears a homogeneous blanket of multiaged trees [38].

Estimated return intervals for large-scale blowdown on exposed sites range from 100 years [139] to 300 years [38,66,101]. Large-scale blowdown typically results in even-aged stands with a higher Sitka spruce component than the previous stand [125]. Where large openings are part of the wind regime, the resulting landscape mosaic can be patchy at multiple scales and made up of single-cohort stands, stands with multiple, even-aged cohorts, and all-aged stands maintained primarily through gap dynamics [54,66,101].

Because the understory is mostly left intact, regeneration after wind disturbance may follow different pathways than regeneration after mass wasting or fire. Gaps created by windfall are primarily filled through lateral growth of adjacent canopy trees or through the release of trees present in the understory. Large, decaying nurse logs favor establishment of advance regeneration [38]. Western hemlock, western redcedar, and Pacific silver fir tend to maintain dominance in a gap replacement regime, although Sitka spruce may be favored when openings are large or when mineral soil is exposed by uprooting [38,101].

Geomorphic disturbances: Geomorphic disturbances are among the most important naturally occurring, high-severity, stand-replacing events in southeastern Alaska. Due to the steep topography, they are important in shaping both vegetation and physical landscapes in coastal forests. The steep slopes and gullies along the sides of the trough-shaped valleys are susceptible to avalanches and episodic mass wasting. On the gentler slopes near the valley bottoms, more continuous fluvial processes dominate [38,127].

Episodic disturbances such as avalanches, landslides, and flooding result in seral communities typically dominated by hardwoods and/or shrubs, which dissect evergreen forest on susceptible landscapes. Landslides affect about 2% to 3% of the land area in temperate rainforests. Slopes >70% in areas of high rainfall are most susceptible. Successional patterns on landslide or mudflow surfaces are similar to those that occur on glacial deposits, with spruce and alder dominating early succession. Forests on avalanche slopes are usually distinguished from nearby old-growth forest by the presence of pure, even-aged, early-successional species such as Sitka spruce with alder and salmonberry mixtures. These forests succeed to western or mountain hemlock in about 100 to 200 years if not subject to additional avalanches [127].

Floods, triggered by rainstorms, rain on snow, or rapid snowmelt, can cause varying degrees of tree mortality [38]. During large floods and mudslides in riparian habitats, a lens of material is deposited and forms a new terrace, on which red alder and Sitka spruce establish over several hundred years. After the initial 100 to 200 years of stand development, gap processes become important in creating fine-scale structure. Riparian stands along large streams and rivers are most likely to have stand-replacement floods [127].

Insects and disease: Western hemlock and Sitka spruce have few insect pests in southeastern Alaska, likely due to the cool climate, although western black-headed budworm and hemlock sawfly outbreaks occasionally cause widespread defoliation, particularly to western hemlock (figure 3). Western spruce budworm and western hemlock looper have done little damage in southeastern Alaska. The spruce beetle occurs at endemic levels in old-growth stands but rarely reaches epidemic proportions in southeastern Alaska. Dwarf mistletoes and heart rot fungi are common in old-growth, Alaskan Pacific maritime forests and perpetuate western hemlock dominance [55].

Figure 3—Damage to western hemlock in southeastern Alaska from hemlock sawfly. Photo by USDA Forest Service, Region 10, Alaska, Bugwood.org.

Historically (from the mid-1700s to the late-1900s), spruce beetle outbreaks occurred every 30 to 50 years and have affected 3.7 million acres (1.5 million ha) since 1989 [20,120,124]. Spruce beetle outbreaks frequently follow windstorms or fires because trees that are blown over, broken by wind, or scorched by fire are ideal breeding sites for beetles [110]. In south-central Alaska, a spruce beetle outbreak of unprecedented magnitude and size caused a massive die-off of mature white, Lutz, and Sitka spruce forests on the Kenai Peninsula. The outbreak began in the 1980s and lasted about 20 years, peaking in the mid-1990s and affecting more than a million acres with up to 90% tree mortality, mostly white and Lutz spruce [19,43,110,117]. See Postsettlement fuels for more information.

Succession: Little information is available regarding postfire succession in Alaskan Pacific maritime ecosystems; however, fire is likely to favor species such as Sitka spruce, western redcedar, and Pacific silver fir by reducing thick organic layers and exposing patches of mineral soil, increasing nutrient availability, and removing advanced regeneration of western hemlock [38,122,127]. Sitka spruce usually regenerates on mineral or mixed-soil microsites, and western hemlock usually regenerates on organic substrates, regardless of the frequency and intensity of disturbance [34].

Successional patterns after stand-replacing disturbances in western hemlock-Sitka spruce forests vary due to differences in soils, microclimate, and disturbance type [6]. For example, even-aged stands grew faster when they originated after wildfire or logging than after blowdown; the authors speculate that this was due to higher soil temperatures leading to increased decomposition and greater nutrient availability [122]. Alaback [6] did not find differences in understory successional patterns between stands originating after fire and those originating after logging.

A general model of succession after (in order of most to least common large-scale disturbances) clearcutting, blowdown, or fire begins with a shrub stage in which residual shrubs such as blueberry, huckleberry, salmonberry, currant, menziesia, devil's-club, red elderberry grow quickly and dominate the site. Tree seedlings establish at about the same time and follow a pattern similar to that of the woody shrub species [6,55,101]. Production of shrubs and herbs increases linearly with time, for up to about 20 years, and understory biomass peaks 15 to 25 years after canopy removal [6]. Within about 8 to 10 years tree saplings begin to overtop the shrub layer [55,101], and shrub and herb abundance declines as forest canopies close after about 25 to 35 years. During this successional stage, clumps of rhizomatous ferns such as western oakfern and spreading woodfern occur on both decaying wood and the forest floor, and extensive carpets of mosses begin to form. Bryophytes and ferns begin to dominate the understory around 50 to 60 years after canopy removal. Moss biomass peaks around 140 to 160 years after canopy removal, and then declines as shrub and herb components increase during the last stages of successional development. Biomass of tree seedlings, mostly concentrated on well-decayed logs and stumps, also increases during the final successional stages [6].

Historically, western hemlock and Sitka spruce forests on low-elevation, upland sites were typically uneven-aged due to the predominance of small-scale disturbances, but developed even-aged stands after rare, stand-replacing disturbance [34]. After a stand-replacing disturbance and subsequent regeneration, forests may remain even-aged for up to 300 years before gradually becoming uneven-aged [55,101]. Old-growth forests are generally more structurally heterogeneous than any other age class [6], although in southeastern Alaska, tree species composition and stand structure were similar among even- and uneven-aged stands [34]. The age at which forests become old growth differs with site and forest type. It seems reasonable, however, that at least 350 years is required [101]. In many cases successional trajectories are very long, with directional change continuing to occur 400 to 500 years after disturbance [127].

Historical fire regimes: The ideal combination of lightning, fuels, and climate suitable for wildfires was historically very rare in wet coastal temperate rain forests [31]. The mild maritime climate allows for abundant biomass (fuel) production; however, temperate rainforests only rarely experience fire-conducive conditions [104]. Unlike interior Alaska [46,100], fire has historically been very rare in southern coastal Alaska [2,6,9,10,16,66,81,98,125,133] and northern coastal British Columbia [13,38,105] due to higher amounts of rainfall, wet conditions year-round, lower summer temperatures, and low incidence of lightning. Exceptions may occur where rainforest transitions into subboreal forest on the Kenai Peninsula [9,108] and in areas of severe rain shadow such as Lynn Canal north of Juneau [100,127]. As perhumid rainforests intergrade with drier forest types farther south (central and southern British Columbia, Washington, Oregon), increasingly more evidence of periodic fire is evident on the landscape [4], such as a greater abundance of Douglas-fir.

The model of episodic, stand-replacing fires developed for coastal Douglas-fir forests has sometimes been applied to the wetter, coastal rainforests, despite higher annual precipitation, less seasonality of precipitation, different stand structure, and different dominant species [74]. However, multiaged stand structures, dominance of late-successional, fire-sensitive species, and evidence from paleological charcoal studies in coastal forests farther south indicate that stand-replacing fires have been extremely rare in temperate rainforests over the past several thousand years. This rarity of fire has supported the development of ecosystems characterized by very large, old trees and the ubiquity of late-seral species and structures at stand and landscape scales [74]. While it is generally agreed that fire has not been a major disturbance factor in these forests for several millennia, the historical role of fire is not fully resolved [10], perhaps because stand-replacing fires that have occurred since European settlement have influenced our perception of fire in these forests (see Postsettlement fires). However, these fires are outside the historical range of variability for coastal rainforests, most of them were associated with logging [31], and statistics derived from recent fire history do not likely reflect presettlement dynamics [38].

The lack of fire applies not only to the forests, but also to the other community types in the coastal landscape, including riparian, wetland, shrubland, and alpine systems [81]. The nonforest BpSs that occur in the area covered by this synthesis occur as patches within the larger matrix of coniferous forest and, due to their small patch size, they would likely have burned with a similar frequency as the surrounding forest. In other words, they were historically unlikely to burn. Very little information was found in the literature that discussed fire in these communities. The southeastern variant of the mountain hemlock BpS is included in this synthesis because, similar to lower elevation Sitka spruce and western hemlock forests in southeastern Alaska, it has a nonfire history. The FEIS synthesis Fire regimes of Alaskan mountain hemlock ecosystems covers the northern variant of Alaskan mountain hemlock ecosystems in south-central Alaska, which are more likely to have burned historically.

Fire is not included in any of the LANDFIRE BpS models covered in this synthesis (table 2, table A1). Modelers of Alaskan Pacific maritime western hemlock forests in southeastern Alaska note that fire plays some role in inland areas near Haines, Skagway, and generally north of Lynn Canal, where the climate is drier and more continental [69]; however, contemporary fires are most likely anthropogenic in origin and not representative of the longer record. The conditions necessary for ignition and fire spread are more likely to occur in coastal forests of southern coastal British Columbia and the Pacific Northwest. Some information from these regions is referenced in this synthesis to provide additional context; however, for more detailed information on fire regimes in those ecosystems, see the FEIS publication Fire regimes of Pacific Northwest coastal forests.

Table 2—Data generated by LANDFIRE succession modeling for Alaskan Pacific maritime Biophysical Settings (BpSs) covered in this synthesis [72].
Fire interval¹
(years)
Fire severity² (% of fires)
Number of BpSs in each
fire regime group
Replacement Mixed Low I II III IV V NA³
NA       0 0 0 0 0 20
¹Average historical fire-return interval derived from LANDFIRE succession modeling (labeled "MFRI" in LANDFIRE).
²Percentage of fires in three fire severity classes, derived from LANDFIRE succession modeling. Replacement-severity fires cause >75% kill or top-kill of the upper canopy layer; mixed-severity fires cause 26%-75%; low-severity fires cause <26% [14,71].
³NA (not applicable) refers to BpS models that did not include fire in simulations.

Fuel characteristics: The mild, moist climate in coastal Alaska leads to substantial biomass production and slow decomposition, and therefore ample fuel availability over the long term (decades to centuries), but also poor combustibility in the short term (the fire season). Weather anomalies, topography, plant community composition, and disturbances (e.g., windthrow, clearcuts) affect fuel structure, moisture, and combustibility in Alaskan Pacific maritime ecosystems.

Quantities and types of fuels: Quantitative information on fuels in Alaskan coastal forests is limited. Although coarse woody debris generally accounts for most of the persistent surface fuel loads in coastal temperate rainforests [31], contemporary wildfires (e.g., on Portland Island in 2011, and on Bird Island and Sunset Bluff in 2015) were fueled by deep organic layers (duff) on the forest floor [82,90,91], which can be 3 to 4 feet (0.9-1.2 m) deep in some areas [82]. Most of these forest types also have large quantities of biomass and downed wood; however, mature Alaskan Pacific maritime Sitka spruce beach ridge forests usually have very little downed wood or snags [95]. Larson [73] collected data on downed woody fuel in 11 forest types and 19 plant associations in southeastern Alaska. For the 78 samples in predominantly Sitka spruce and western hemlock stands, downed wood weights averaged 33.2 tons per acre (74,332 kg/ha), with slightly lower weights in the 72 old-growth stands and considerably higher weights in the six cutover stands sampled. Downed wood weights ranged from 0.6 ton/acre (1,232 kg/ha) in muskeg (shore pine) stands to 52.4 tons/acre (117,483 kg/ha) in western redcedar stands. The author stated that "downed woody material is not much of a problem in southeast Alaska, as the nature of the nearly fireproof rain forest prevents fire hazard from becoming severe." For details on fuel weights by size class and plant community see Larson [73].

Stand structure and species composition of coastal forests may both reflect a history of fire and influence their susceptibility to fire. In the Clayoquot Valley on Vancouver Island, fire-return intervals were shorter in western redcedar/salal forests than in other forest types [49]. A review by Dorner and Wong [38] regarding disturbance regimes in coastal British Columbia suggests a potential link between high western redcedar cover and fire incidence in low productivity bog woodlands and western redcedar-western hemlock stands in the northern part of the province. They suggest that western redcedar is an indicator of past and potential fires because fire seems to favor western redcedar regeneration and persistence over associated tree species, and it is more flammable than those species. Low-productivity bog woodlands of Haida Gwaii are usually fire resistant, but they may become conducive to burning in unusually dry weather because they have a large component of western redcedar, a large number of snags, and peaty soils [38].

Fuel moisture: In general, fuel flammability in wet coastal temperate rainforests is low due to high moisture content [31,135]. With some local exceptions, the climate of southeastern Alaska has historically been too consistently wet to support major fires. The summer drought that affects coniferous forests farther south is not as apparent southeastern Alaska, and extended periods without precipitation are rare [6]. Even during years of extreme drought, high moisture content in fuels has historically prevented ignition and fire spread [31]. In southeastern Alaska, wildfires that began during dry summers in the 20th century usually died out upon reaching old-growth stands, where understories tend to be humid [35]. Observations of wildfires in the 21st century, however, indicate that the upper layer of the forest floor can dry quickly during a warm summer or dry period, and that 2 days of sunny weather can dry duff layers enough to allow fires to ignite and creep through the forest floor, sometimes deep (~3 feet (0.9 m)) under ground [82,90]. These types of ground fires can burn for several days [90] and char or consume tree roots (see Fire type and severity). Coarse woody surface fuels have high bulk density and typically remain moist beneath moss and herbs in the shade of multiple canopy layers [31,135].

Historically, truly exceptional droughts or successive years of drought were needed for fires to carry in coastal communities in the northwestern North America [4]. For example, Agee and Flewelling [3] estimated that it would take more than 4 weeks with no significant precipitation in a year with below-average rainfall (which recur at decadal intervals) to create flammable conditions in the Olympic Mountains of Washington [74]. Years of high fire activity in the Pacific Northwest were most likely to occur when persistent high pressure ridges formed along the Pacific Coast, reducing precipitation and allowing fuels to dry for extended periods. Under these conditions, dry winds could spread fire where fuels were available [31]. A study of the relationships between area burned and climatic variables throughout British Columbia between 1920 and 2000 found that area burned in the humid coastal western hemlock zone was significantly correlated with summer drought but not with drought in other seasons (P < 0.05), and that area burned was limited by fuel moisture rather than by fuel loads. Climate oscillations (21 climate oscillation indices used in this study) showed no significant relationships to area burned in this zone [89].

Topography influences fuel moisture and fire spread at both regional and local scales in northern Pacific coastal areas. At a regional scale, the drier climate in the rainshadow of mountain ranges is more conducive to fires, a pattern apparent around Lynn Canal north of Juneau [100,127] and on the mainland coast of British Columbia, Vancouver Island, and Haida Gwaii [38]. At local scales, topography determines fuel exposure to irradiance and wind and indirectly influences fuel moisture, making certain parts of the landscape more susceptible to fire. For example, southerly aspects may be more flammable than northerly aspects and valley bottom areas. In coastal forests of the Clayoquot Valley, British Columbia, fires were more than 25 times more likely to occur on south-facing slopes than on less susceptible landscape positions [38,47]. Within stands, subtle differences in microtopography can limit the spread of fire and restrict fire size in the wet coastal temperate rainforest [31].

Fire season and ignition sources: Conditions in Alaskan maritime ecosystems were historically too moist to burn year-round. However, contemporary fires occurred in south-central Alaska between April and October of some years, with most occurring in May, June, or July [110]; and fire risk is greatest in June and July in southeastern Alaska [82]. In coastal areas, fires have occurred from April through August (e.g., [62,82]), but the greatest potential for large fires is from the middle of May to the middle of July [100]. On the Kenai Peninsula, a brief dry period in June can cause low fuel moisture, making it the most favorable time for fires [36]. Lightning fires on the Kenai National Wildlife Refuge primarily occur in June and July. Fire danger indices tend to decline in late August and early September [113].

Lightning is generally infrequent in cold, rainy coastal climates, where it is associated with anomalously warm, dry, seasonal weather [31]; however, global climate changes may impact the frequency and distribution of lightning in coastal climates (see Climate change and fire regimes). For example, in May 2015—a period of high temperatures and dry conditions—lighting storms in southeastern Alaska generated hundreds of lightning strikes, several of which ignited wildfires on the Tongass National Forest [62].

The probability of lightning is greater in south-central than in southeastern Alaska [46], and it increases from north to south along the coast from southeastern Alaska through British Columbia [10,55]. Lightning-caused fires occur less frequently in south-central Alaska than in all other parts of the state except the Southeast and Arctic regions. From 1957 to 1979, 61 lightning-caused fires occurred in south-central Alaska, ranging in size from 1 to 5,600 acres (0.4-2,270 ha), which is fewer than three lightning fires/million acres (1.2 fires/100,000 ha), compared to 15.5 lightning fires/million acres (6.2 fires/100,000 ha) in interior Yukon, and 5 to 7.5 lightning fires/million acres (2-3 fires/100,000 ha) in western coastal Alaska [46]. Most contemporary fires in south-central Alaska are human-caused and occur around population centers, along transportation routes, and in high-use recreational areas [110].

Lightning was historically very rare in southeastern Alaska, striking near major towns only once every 1 to 2 decades [4], and lightning-caused fires were rare even during very dry summers [10,55,126]. Two lightning-caused fires were recorded in Southeast Alaska during the 20th century prior to 1974 [55], and 24 lightning-caused fires were recorded in southeastern Alaska between 1989 and 2010 [82]. However, most contemporary fires are human-caused [55,100,126], often due to campfires of recreationists [82,90,91] (see Postsettlement fires). Fire records from 1940 to 1982 in British Columbia indicate that fires were least common on the northern coast. Only four lightning-caused fires were reported for all of Haida Gwaii, and 63 lightning-caused fires were recorded on the northern coastal mainland [105].

Before Europeans came, relatively small and widely dispersed populations of Native Alaskans made little change in the forest, except in the immediate vicinity of villages and camps. Native Alaskans used fire for a variety of purposes such as cooking, warming, making canoes, and drying or smoking fish and meat, and fires may have occasionally escaped and burned the surrounding forests during infrequent periods of dry weather [55]. In the perhumid rainforest region, people who used fire were typically those whose settlements were in rainshadow climates. Fire was used to fell large trees and to clear burial grounds, but the effects were localized [127]. Intentional low-severity burning by First Nations peoples to enhance berry production has been documented in coastal British Columbia. Several western redcedar stands in the central and northern coastal areas are thought to have originated after fires that ignited when western redcedar logs were burned out to make canoes [38]. Fire use by tribes in the coastal wet forests farther south was also likely limited [25].

Fire frequency: Only one fire history study including any Alaskan Pacific maritime ecosystems [108] was available as of 2017. Few adaptations clearly related to fire are seen in coastal Alaskan vegetation, and evidence of past fires (e.g., even-aged stands or sediment charcoal) is sparse and inconclusive, suggesting that fire has not played an important role in the ecology and evolution of these ecosystems [6,126]. The cool, wet climate and low incidence of lightning support long fire-free intervals. Historically, exceptional droughts or successive years of drought were needed to create conditions conducive to large, severe fires in ecosystems along the northwestern coast of North America [4]. This is consistent with LANDFIRE's classification of Alaskan Pacific maritime ecosystems as nonfire ecosystems (table 2) [72]. Several, mostly small and mostly human-caused fires have been recorded in southern coastal Alaska since European settlement and extensive logging began (see Postsettlement fires).

South-central Alaska: While lightning is more common and fires are generally more frequent in south-central Alaska than in southeastern Alaska, fires mostly occur in forests dominated by Lutz and white spruce that are transitional between coastal rainforests and subboreal spruce forests. Some evidence also suggests a history of infrequent fires in the northern variant of Alaskan Pacific maritime mountain hemlock forests, which occur in south-central Alaska. See the FEIS Fire Regime Syntheses on Alaskan white spruce and Alaskan mountain hemlock (northern) for more information on those ecosystems. A single fire history study that examined some coastal rainforest stands on the Kenai Peninsula found that evidence of past fires (e.g., fire-scarred stumps and/or soil charcoal) was limited to mountain hemlock-Lutz spruce stands and even-aged stands codominated by Sitka spruce and Kenai birch. Most Sitka spruce stands showed no evidence of past fires [108]. Anecdotal evidence from the forest zone of the Kenai Mountains (i.e., charcoal observed in "most" soil pits) has been cited as evidence of "widespread, infrequent fires" in the area (USDA Forest Service 2002, cited in [70]). It is not clear in which forest types these soil pits were located. Estimated fire frequencies for forests on the Kenai Peninsula that are adjacent to Alaskan Pacific maritime ecosystems indicate that fire is rare in adjacent forests as well [19,108]. In white spruce-Lutz spruce forests across the central and southwestern Kenai Peninsula (BpS 7616440), radiocarbon dating of 121 soil charcoal samples taken from 22 stands suggests that time-since-fire ranged from 90 to 1,500 years. On average, these forests had not burned for over 600 years [19]. See Contemporary changes in fuels and fire regimes for more information.

Bluejoint reedgrass communities on the Kenai Peninsula (possibly BpS 7816530) may have been maintained by fire. In 1951, Hanson [52] anecdotally described fire frequency and effects in upland grasslands at about 900 feet (270 m) elevation northwest of Homer: "Grassland fires have been frequent in this region, retarding the growth of trees, which occur as thickets of Sitka spruce and alder mostly." Evidence of fires in similar grasslands in the region included charred fragments in soil profiles and scattered burned logs and stumps on the soil surface [52].

Southeastern Alaska: No fire history studies of ecosystems in southeastern Alaska have been published, although some authors cite evidence of past fires such as charcoal fragments in peat layers [98] and large areas of even-aged stands (e.g., [55,105]). Nonetheless, paleological studies in similar coastal temperate rainforests on Vancouver Island and southern coastal British Columba (e.g., [26,47,51,74]) suggest that fire was a rare and isolated phenomenon in these ecosystems throughout the Holocene.

Neiland [98] suggests that many forests in southeastern Alaska show scattered signs of past fires, such as charcoal fragments in the upper peat and soil layers. However, she admits that these were difficult to discern with certainty because the thick, almost continuously wet, organic layers made it difficult to distinguish whether discolorations on dead wood were caused by fire or moisture. She stated that low-severity fire may have been more common than has been realized because the wet, mild climate rapidly obliterates indications of past fires [98]. Harris and Farr [55] also noted that the rapid buildup of organic matter can hide charcoal under the forest floor and make it difficult to determine whether even-aged stands originated after fire.

Stand structure over most of southeastern Alaska was historically multiaged, although even-aged stands occur in several areas. The origin of some of these stands has been attributed to fires during the past few centuries [55,105]. For example, even-aged stands ranging from 150 to over 300 years old on Mitkof Island, Chichagof Island, Revillagigedo Island, and on the west coast of Prince of Wales and nearby islands were attributed to extensive wildfires in 1664, 1734, and between 1804 and 1824 [55,105]. The authors suggest that the wildfires were human-caused [55]. In his 1977 study of understory succession in Sitka spruce-western hemlock forests in southeastern Alaska, Alaback [6] stated that 13 of the 60 stands studied had originated after fire. Based on ages of codominant trees, these stands ranged from 71 to 187 years old, suggesting that fires occurred around the following years: 1790, 1823, 1835, 1839, 1877 (2 sites), 1887 (2 sites), 1905 (5 sites), and 1906 (2 sites) [6]. Others have reported fires in coastal rainforests on Vancouver Island and coastal mainland British Columbia during similar time frames (e.g., [47,114,116]), and several authors report frequent fires in the 1800s and early 1900s in the Pacific Northwest. See the section on Postsettlement fires in this synthesis, and the FEIS fire regime publications on Pacific Northwest coastal forests and Pacific Northwest mountain hemlock for additional details.

Coastal British Columbia: Little or no fire ecology research has been undertaken in the northern coast of British Columbia (Prince Rupert Forest Region, immediately adjacent to Southeast Alaska), probably due to the rarity of fire in that region. Fire history studies are rare for much of coastal British Columbia, with the exception of Vancouver Island and the southern mainland coast. Those areas have similar rainforests, but are adjacent to drier communities in rainshadows that have infrequent, stand-replacement fires [105].

Charcoal is common in the soils of rainforests in coastal British Columbia, indicating that they burned at some undetermined point in the past; however, temporal interpretation of this charcoal has been limited (review by [74]). Examination of soil profiles in Sitka spruce-western hemlock and western hemlock-western redcedar forests on Graham Island (in Haida Gwaii) revealed charcoal deposited at varying depths. At one site abundant pieces of charcoal in the lower depths of the organic horizon indicated that a fire occurred sometime before the present stand established (about 100 years prior), and numerous pieces of charcoal between depths of 27 and 47 inches (69 and 119 cm) in the mineral soil suggest a fire "in the distant past". At another site, buried charcoal layers occurred at 37 and 52 inches (94 and 132 cm). Traces of charcoal occurred on the soil surface in several locations in conjunction with reddish-brown mineral soil, suggesting more recent fires [33].

Other evidence of fire in coastal British Columbia comes mainly from the distant past on the southern coast, with one more recent example from the central coast. Examination of aerial photos taken in 2006 and 2007 along the central coast of British Columbia revealed only one patch that was created by fire during the past 140 years. The patch was 18,780 acres (7,603 ha) and was dominated by Douglas-fir [106], a different forest type than those covered in this synthesis. Paleological studies from western hemlock and mountain hemlock forests in southern British Columbia show that fires are rare events in coastal temperate rainforests on millennial time scales (e.g., [26,47,51,74]). Some sites on Vancouver Island showed no evidence of ever having burned during the past 6,000 years (longer than the time that temperate rainforest has existed in the region), and many sites show evidence of only one to three fires during that period [74]. Charcoal evidence from the west side of southern Vancouver Island suggests fire-return intervals of approximately 3,000 years [26]. In coastal western hemlock forests in Clayoquot Valley the median time since last fire ranged from 750 to 4,500 years. Drier southerly aspects were more likely to burn than northerly aspects and valley bottoms, and fire intervals were shorter in western redcedar/salal forests than in other forest types [38,47]. For more information on these and other fire history studies in and around seasonal temperate rainforests, see the FEIS publications on Fire Regimes of Pacific Northwest coastal forests and Pacific Northwest mountain hemlock communities.

Fire type and severity: Some accounts suggest that rare, stand-replacing fires occurred in Alaskan Pacific maritime ecosystems during the reference period (e.g., [55,105,108]); however, contemporary fire behavior and effects (e.g., [82,90,91]) suggest that fires in these ecosystems are more likely to be small ground fires with mixed severity. Evidence of small, mixed-severity fires is difficult to detect retrospectively, so if these types of fires occurred historically, they may not have been documented. Conversely, several even-aged stands in southeastern Alaska are thought to have originated after stand-replacing fires during the presettlement and settlement eras [55,105]. On sites where Sitka spruce codominated with Kenai birch in south-central Alaska, even-aged stand structure and charred stumps suggested some history of stand-replacing fire, but no additional data were collected [108]. Some generalized accounts of fire regimes in coastal western hemlock-Sitka spruce forests describe infrequent, high-severity, stand-replacing fires (e.g., [2,10]). However, this seems to apply more to contemporary forest fires in the seasonal temperate rainforest zone and to drier forest types, where Douglas-fir occurs.

Contemporary fire observations in southeastern Alaska indicate that low-intensity ground fires can occur in Sitka-spruce-western hemlock forests, and that the effects of these fires can be severe, although size of high-severity patches is typically small (e.g., [82,90,91]). Ground fires occur when the forest floor dries sufficiently to ignite and carry fire (see Fuel characteristics). These are typically low-intensity fires with no visible flames [90] but can be of high severity in areas where they consume the nutrient-rich duff layer and char or girdle the anchoring root systems of Sitka spruce and western hemlock, causing them to topple over [82,90,91]. Soils can become unstable and susceptible to erosion when the surface organic layers are consumed, and regeneration may be delayed by 5 to 10 years [82]. The Bird Island fire, for example, left an area of "burnt roots, bare rocks, and ash" [91], and 2 years after a 2009 wildfire near Juneau, the site was characterized as "a tangled mass of root wads, downed trees, exposed bedrock and a few green shoots of fireweed" [82].

A growing body of evidence suggests mixed-severity fire regimes were historically more common than previously documented in North America (e.g., [83]). A literature review by Dorner and Wong [38] suggests that historical fire regimes in coastal western hemlock forests in British Columbia were characterized by "a combination infrequent, high-severity crown and low-severity surface fires causing partial or complete stand replacement". A review and analysis by Daniels and Gray [31] suggests a regime of rare, low- and mixed-severity fires in coastal rainforests of British Columbia, resulting in complex stands with multiple age cohorts. Cumulatively, mixed-severity fire regimes result in complex landscapes comprised of sites that last burned at a range of fire severities, as well as sites that have not burned for long periods [83]. Documenting spatially and temporally mixed-severity fire regimes is data intensive, and much of the necessary evidence has been lost where these forests have been altered by logging and urban development [31].

Fire pattern and size: Generalized accounts of fire in coastal western hemlock-Sitka spruce forests describe large, stand-replacing fires (e.g., [2,10]); however, little to no evidence of stand-replacing fires in Alaskan coastal forests was documented in the scientific literature as of 2017. Harris and Farr [55] note "extensive areas of even-aged stands that apparently owe their origin to fire", but they do not report the size of these stands, and most of these fires occurred during the settlement and postsettlement periods. Postsettlement fires have mostly been small (<100 acres (40 ha)). Ground fires reported in southeastern Alaska during the 21st century are mostly very small (<1 acre (0.4 ha)) (e.g., [82,90]) due to variability in fuel moisture [82]. A lightning-ignited wildfire burned about 7.5 acres (3 ha) near Hoonah in 2010 before being suppressed; this was considered a large fire for southeastern Alaska [82].

CONTEMPORARY CHANGES IN FUELS AND FIRE REGIMES

Until the area was settled by European immigrants and logging began, the landscape of southern coastal Alaska was dominated by old-growth conifer stands in a matrix with small areas of wetlands and shrublands. In the late 1970s, 75% of the forests in southeastern Alaska were old growth (>150 years old), and most of the remainder originated after logging or windthrow [6]. Timber harvest, predominantly clearcutting, has created large areas of early-successional forest throughout southeastern Alaska (figure 4) [69]. Young stands were uncommon before clearcutting began on a large scale [13], and much of the information about postdisturbance dynamics in these forests comes from observing succession after clearcuts [127]. Clearcutting is the most common and widespread contemporary disturbance [44].

Figure 4—Recently clearcut section on the Tongass National Forest adjacent to older clearcuts in early seral stages in August 2010. Creative Commons photo by Alan Wu.

In general, forest types that historically burned infrequently (fire-return intervals longer than about 100 years) may still be within the historical range of variability—in terms of fuel loads, stand structures, and species composition [45]—in areas that have not been altered by logging or other anthropogenic disturbances. Fire regimes in coastal forests—where fire has been rare for millennia—have not been altered by fire exclusion; however, landscapes dominated by early successional forest resulting from massive timber harvests are far outside the historical range of variability. In addition, range shifts and widespread mortality of some tree species and changes in water relationships, likely due to direct and indirect effects of climate warming, may also result in conditions outside the historical range of variability in Alaskan Pacific maritime ecosystems.

Postsettlement fuels: Contemporary fuel characteristics in Alaskan Pacific maritime ecosystems differ from historical characteristics due to changes in plant community structure and composition resulting from widespread logging, increased forest cover and tree biomass in some ecosystems, widespread mortality of some tree species, and changes in water relationships. Some of these changes are apparently direct and indirect effects of climate warming. Additional changes to fuel characteristics are anticipated with further warming, and the overall expectation for the region is forest expansion and accretion of biomass, increasing its role as a significant carbon sink [27,139].

The most obvious and drastic change in contemporary fuel characteristics in Alaskan coastal forests is due to logging. Large areas of logged forest in southeastern Alaska can present a potentially hazardous fuel situation [100]. Logging slash is highly flammable when fresh, and can remain a fire hazard until the young stand reaches crown closure, at which point fuels dry out less easily [38]. During the 1900s, most fires in southeastern Alaska and northern coastal British Columbia [55,100,105] and many of the biggest fires in Haida Gwaii were associated with logging, likely due to the large amounts of slash fuels [38]. See Postsettlement fires for more information.

Tree species composition is changing and overall tree biomass is increasing in Alaskan coastal ecosystems, but biomass changes vary among individual species. Trends in live aboveground tree biomass on Forest Inventory and Analysis (FIA) plots in the temperate rainforest region of Alaska revealed an overall increase in conifer biomass (P = 0.05) between the periods of 1995-2003 and 2004-2010 that was driven in large part by increases in western redcedar and Sitka spruce. Shore pine was the only tree species that showed a net loss of biomass [123]. Overall aboveground carbon mass (assumed to be equal to 0.5 of dry biomass) in live trees did not change on the Tongass National Forest in southeastern Alaska, but it increased by 4.5% on the Chugach National Forest in south-central Alaska. Species composition has changed on both forests. Sitka spruce and white spruce significantly increased in carbon mass on the Chugach, and Sitka spruce, red alder, and western redcedar significantly increased on the Tongass. Yellow-cedar showed a significant decrease in carbon mass on the Tongass (P < 0.05) [15]; however, FIA data do not show an overall decline in yellow-cedar biomass in Alaska. Biomass increases for yellow-cedar were highest on steep, north-facing slopes (which are likely to be better drained and hold snow longer), while yellow-cedar biomass was unchanged on shallow, south-facing slopes [27].

The distribution, extent, and density of Alaskan temperate rainforests are shifting and increasing at the regional scale as trees establish in new sites and biomass accumulation exceeds mortality in unlogged forests. Observed gains in forest biomass are consistent with the hypothesis that climate change is allowing for colonization of previously non-forested locations. Overall, gains exceed losses but occur in different spatial and topographic contexts. Regionally, forest gains are concentrated at high latitudes, low elevations, and on northerly aspects, and losses are skewed toward low latitudes and southerly aspects. Gains greatly outpace losses in the northern part of the region (south-central Alaska) and are concentrated primarily in shrubland (such as above treeline, areas exposed by receding glaciers, and floodplains). Forest extent appears to be expanding in the north and declining slightly in the south; yellow-cedar mortality may be driving much of this spatial pattern (see Mortality, below). Wetland encroachment could explain some of the biomass and spatial gains on lower slopes; increases in precipitation in the future may decrease this growth if soils become waterlogged [27].

Alaskan wetlands are drying and succeeding to upland habitat in some coastal areas as a result of climate changes [21,64], such as increasing temperatures and decreasing water balance, in the Kenai Lowlands [64]. Wetlands that might have served as firebreaks in the past may become "fuel bridges" as they convert to shrublands and forests, potentially increasing fire spread and extent [21]. For details from these studies see the FEIS synthesis Fire regimes of Alaskan wet and mesic herbaceous systems.

Mortality: Mortality events lead to changes in stand structure and species composition of affected stands, thereby altering fuel characteristics and possibly increasing fire hazard. Widespread mortality of spruce (mostly white spruce and Lutz spruce, but also some Sitka spruce) in south-central Alaska, and of yellow-cedar at lower elevations throughout its range, has been well known for decades. More recently, reduced growth and biomass of shore pine has been detected in southeastern Alaska. Increased temperatures and reduced snowpack are implicated in these changes.

Widespread mortality of spruce following spruce beetle outbreaks in the late 20th century changed fuel properties over large areas of south-central Alaska [24,117,120]; however, Sitka spruce-mountain hemlock forests along the Gulf of Alaska did not show consistent directional changes in vegetation composition or fuel properties [24]. In white and Lutz spruce communities on the Kenai Peninsula, abundant dead trees and rapid growth of grasses, especially bluejoint reedgrass, led to concerns regarding increased fuel loads and altered fuel characteristics that increase the risk of severe fire [43,110,113,117]. Bluejoint reedgrass cover did not increase in Sitka spruce-mountain hemlock forests along the Gulf of Alaska, and these forests had the lowest regional mortality of spruce (22% reduction of Sitka spruce basal area) [24], suggesting that spruce beetle outbreaks are less likely to affect fire activity in Alaskan Pacific maritime forests.

Figure 5—Spruce beetle damage on Sitka spruce in the early 1980s. Photo by Andris Eglitis, USDA Forest Service, Bugwood.org.

Extensive yellow-cedar mortality and decline in southeastern Alaska and adjacent coastal British Columbia is a persistent feature on affected landscapes, not only because tree death occurs gradually, but also because yellow-cedar trees remain standing for 80 to 100 years after death (figure 6). Standing dead or dying yellow-cedars occur on >500,000 acres (>200,000 ha) in Alaska, from the southern part of southeastern Alaska north to the western coast of Chichagof Island (57.6° N), and on about 235,000 acres (95,000 ha) in British Columbia. At the northern extent of the decline on Chichagof Island, yellow-cedar death occurs in a narrow, low-elevation band from sea level up to about 500 feet (150 m). Every year, forest health reports document new areas of active yellow-cedar decline, which is strongly affected by warmer winters, reduced snow-pack, and earlier snowmelt that culminate in freezing damage to fine roots and eventual tree death [59,60]. See the FEIS Species Review on yellow-cedar and the comprehensive review of yellow-cedar by Hennon et al. [60] for more information and references to the primary literature on yellow-cedar decline.

Figure 6—Yellow-cedar mortality in southeastern Alaska. Photo by Paul E. Hennon, USDA Forest Service, Bugwood.org.

Shore pine also seems to be declining in Alaskan coastal forests, which could dramatically alter stand structure and species composition in shore pine peatlands, because associated tree species are not adapted to fill its niche in saturated, acidic soils [86,94,98,123]. Tree-ring analysis revealed a steep decline in shore pine growth in southeastern Alaska between the early 1960s and 2013 that was strongly correlated with the rise in growing-season diurnal minimum air temperature (P < 0.01) [123], although the exact cause of the decline is not clear. See the following sources for more information on shore pine mortality: [16,94,123].

Postsettlement fires: Fire occurrence increased in Alaskan coastal forests during the past 2 centuries compared to the reference period. However, these increases were not as dramatic as those observed in interior Alaska (e.g., [7]). Most fires in Alaskan coastal ecosystems were small, ignited by humans, and related to human activities such as logging, mining, and railroad construction. These fires may lead to changes that are outside the historical range of variability in Alaskan coastal ecosystems, including changes in species dominance and distributions. In addition, roadside fires may contribute to the establishment and spread of nonnative species into natural areas near roads [9,68].

South-central Alaska: Fire frequency increased in the late 1800s and early 1900s with the arrival of miners and the construction of railroads in south-central Alaska. From 1914 to 1953 on the Kenai Peninsula portion of the Chugach National Forest, an average of 22.5 fires occurred per year, about 73% of which were related to the railroad. Since the 1950s, the major causes of fires in the same area have been campfires and debris burning. By the end of the 20th century, over 99% of fires in that area were human-caused [108,110]. Several fires larger than 100 acres (40 ha) occurred on the Kenai Peninsula in the early 2000s [113]. It is unclear whether any of these fires burned in any Alaskan Pacific maritime ecosystems. It is more likely that they burned in white and Lutz spruce forests, and adjacent mountain hemlock and mountain hemlock-white spruce forests (e.g., [56,92,93,113,138]). Nonetheless, changing conditions in south-central Alaska have contributed to higher than normal fire hazard during recent decades that could affect coastal ecosystems. For example, temperatures over the last several decades have been warming, leading to longer and drier fire seasons [110] (see Climate change and fire regimes), and widespread mortality of some dominant tree species has altered fuel conditions and successional relationships in some areas (see Postsettlement fuels). As human populations (i.e., ignition sources) are growing, opportunities for fire ignition and spread are likely also increasing [19]. See the FEIS syntheses Fire regimes of Alaskan mountain hemlock ecosystems and Fire regimes of Alaskan white spruce communities for more information on contemporary fires in south-central Alaska.

Southeastern Alaska: Prior to the late 1800s, fire was very rare in southeastern Alaska. Since that time, several fires have been recorded. They were usually small (<100 acres (40 ha)) and of low-severity, typically associated with logging activity and extended dry periods, and predominantly human-caused [55,100,105]. Published references to fire in the region are few, although one account mentions a 100-acre (41 ha) fire in 1928 in an area logged in 1926. Two large fires burned on the east side of Prince of Wales Island in 1904 and "...smoldered all year in scrub and muskeg" [98]. A study of succession in Sitka spruce-western hemlock forests in Southeast Alaska included 12 stands that established after fires in the 19th and 20th centuries [6].

The three largest fires recorded in southeastern Alaska as of 1974 were the 15,000-acre (6,100 ha) Karta Bay fire, the 5,000-acre Skowl Arm fire—both of which occurred in the early 1900s—and the 2,000-acre Cleveland Peninsula fire of 1938. Only three other fires exceeded 100 acres: two in 1958; the other in 1968. During the summers of 1958, 1968, and 1971, parts of southeastern Alaska experienced extended dry periods, but total area burned in each of the three years was only 1,465, 137, and 71 acres (593, 55, and 29 ha), respectively. All but two of the 70 fires recorded during those years were human-caused [55]. Fire reports from 1956 through 1967 on National Forest land in Alaska included 243 fires that burned 1,705 acres of Sitka spruce and western hemlock cover types. Most of these fires (186, or 76%) were less than 0.25 acre (0.1 ha); 7% were 10 acres (4 ha) or larger, and only three fires reached 300 acres (121 ha). The three large fires burned when fire danger was high. Fires tended to be clustered around populated and high-use areas, and most were human-caused [100].

Several small, high-severity ground fires occurred in southeastern Alaska during the early 21st century, after warm, dry weather dried the forest floor enough to ignite and carry fire (e.g., [82,90,91]). Although they are typically small (<1 acre (0.4 ha)) (e.g., [82,90]) due to variability in fuel moisture [82], these fires can cause substantial damage because they kill trees by charring and girdling roots, and they consume the nutrient rich duff layer [82,90,91], leading to slope instability, erosion, and delayed regeneration [82]. These fires are often ignited from campfires of recreationists, although several lightning-caused fires have also occurred [82,90,91]. For example, in May 2015—a period of high temperatures and dry conditions—lighting storms in southeastern Alaska generated hundreds of lightning strikes, several of which ignited wildfires on the Tongass National Forest [62].

Because humans cause most contemporary fires, more fires may be expected in the future as human populations expand [55,110], especially if global climate changes lead to higher temperatures and periods of drought sufficient to dry the large biomass of fuels present in these systems (see Climate change and fire regimes).

British Columbia and the Pacific Northwest: The Provincial Fire Atlas of the Ministry of Forests in Victoria indicates that within British Columbia, fires are least common in the North Coast Forest District [38,105] adjacent to Southeast Alaska. Between 1950 and 1998 most ignitions (83%) in the province were by humans. Fires in the North Coast were typically small (median = 0.25 acre (0.1 ha) and maximum = 960 acres (390 ha)). Relative to many forest districts in interior British Columbia, the total area burned in the North Coast was extremely small (1,878 acres (760 ha)), and estimated fire-return intervals for the area during the study period ranged from 2,000 to well over 10,000 years [38].

Because they mostly result from human-caused ignitions, contemporary fires tend to burn in different locations and possibly in different seasons than presettlement fires [38]. In southern British Columbia and the Pacific Northwest, most large, stand-replacing fires that occurred in coastal forests since European settlement were associated with logging activity (e.g., the Seward and Chilliwack Valley fires in British Columba in the early 1900s, the 1902 Yacolt Fire in Washington, the 1920s fires in the Capilano River watershed near Vancouver, British Columbia, and the 1933 Tillamook Fire in Oregon). Logging created large fuel loads and microclimatic conditions that allowed the fuels to dry, making these sites conducive to burning. Humans ignited most of these fires [31]. A total of 6,329 wildfires occurred on Vancouver Island from 1950 to 1992. Fires were most likely in areas near municipalities, campgrounds, dirt roads, railroads, and paved roads when summer temperatures were high and precipitation was low (P < 0.001). The fire cycle calculated for lightning-caused fires in the wet coastal western hemlock very wet hypermaritime subzone on Vancouver Island was 8.4 million years for this time period [107].

Climate change and fire regimes: Global climate warming during the past century is unequivocal, and scientific evidence shows major and widespread ecosystem changes throughout the globe that are associated with increasing air and ocean temperatures [80]. Projections generally indicate that a warmer world will have more fire, although the potential impacts vary geographically and among ecosystems [40].

Climate change is caused, in part, by alterations in atmospheric concentrations of greenhouse gases and aerosols, and human activities are modifying both the average state and variability of climate by adding greenhouse gasses—particularly carbon dioxide and methane—to the atmosphere [121]. Current atmospheric concentrations of these gasses far exceed the natural range over the last 650,000 years and have increased markedly (35% and 148%, respectively) since the beginning of the industrial era in 1750. Both past and future anthropogenic emissions will continue to contribute to warming temperatures for more than a millennium, due to the time scales required for the removal of these gasses from the atmosphere [80].

Fire regimes in coastal Alaska are strongly climate-driven, and are therefore potentially susceptible to climate changes. Fire is rare in these ecosystems because lightning is rare and, although they are abundant, fuels are typically too wet to burn. Increasing temperatures, changing moisture relationships, and changing storm patterns have the potential to increase the possibility of fire occurrence by drying fuels; changing the composition, structure, and distribution of fuels; lengthening the fire season; and altering lightning patterns. However, the effects of climate changes are complex, and complexity increases at finer spatial scales due to local variability in factors that affect fuel characteristics such as local weather patterns, topography, dominant vegetation, disturbance history, and management history [57]. The following sections describe recorded and projected climate changes in southern coastal Alaska, how these changes are expected to alter fuels and possibly fire regimes, and climate change considerations for land managers:

Observed and projected climate changes in southern coastal Alaska: Climate datasets indicate substantial changes in southern coastal Alaska during the past century [123,139] that have the potential to affect the probability and characteristics of wildfire; these include increased temperatures, changing precipitation patterns, reduced depth and duration of snowpack, and increasing storm intensities [56,124]. It is very likely that further warming will cause changes that exceed those observed during the 20th century [80]. Projections of future effects of climate change are based on global circulation models that assume continued increases in greenhouse gases at varied rates, and therefore typically report a range of values [56].

During the late 20th and early 21st century, Alaska has warmed at more than twice the rate of the rest of the United States [7]. Increases in mean annual air temperature (table 3) and especially large increases in winter temperatures have occurred in southern coastal Alaska [56,63,124]. Near Juneau, winter temperatures increased 6.2 °F (3.4 °C) and summer temperatures increased 2.2 °F (1.2 °C) between 1949 and 2009 [124]. Temperature increases in south-central Alaska were driven, in part, by a warm-phase PDO that began in 1976 (Hartmann and Wendler 2005 cited by [120]).

Table 3—Increases in mean annual air temperature (MAAT) in southern coastal Alaska between 1971 and 2008 by location [56].
Place MAAT
Valdez 3.76 °F (2.09 °C)
Juneau 3.54 °F (1.97 °C)
Yakutat 2.75 °F (1.53 °C)
Kodiak 0.87 °F (0.48 °C)

Warmer winter temperatures have led to longer growing seasons and an average increase of 10 snow-free days throughout Alaska during the latter part of the 20th century [56], creating the potential for longer and more active fire seasons [7]. The length of the growing season increased almost 7 days per decade in south-central and southeastern Alaska between 1949 and 1997. The first snow-free week in Alaska occurred 3 to 5 days earlier per decade from 1972 to 2000, and the duration of the snow-free period extended 3 to 6 days longer per decade [124].

Regardless of the models or emission scenarios used, it is universally expected that warming will continue throughout the globe, and the magnitude of change will increase with increasing latitude. It is very likely that hot extremes, heat waves, heavy precipitation events, and more intense storms will become more frequent [56,80,124]. Predictions also include continued retreat of glaciers and icefields, increased flooding—especially in spring—and inundation of coastal areas [38,63], which will likely alter composition and structure of many Alaskan Pacific maritime ecosystems.

Air temperatures in Alaska [118,139] and British Columbia are expected to continue to increase [57], with winter temperatures increasing at a higher rate than summer temperatures [56,124]. Precipitation is projected to increase in coastal Alaska, particularly in fall and winter [63,118,139], but more will fall as rain instead of snow.

Five global climate models using two emission pathways projected an overall regional increase in mean annual temperature and mean annual precipitation, and a decrease in precipitation as snow in the northern coastal temperate rainforest of southeastern Alaska and northern British Columbia (table 4). Mean annual precipitation was projected to increase by 3% to 18%, and total precipitation as snow was projected to decrease by 22% to 58% by the 2080s. Projected changes in temperature and precipitation were greatest in the north and on the mainland and least in the south and along the coast [118].

Projections indicate that southern coastal Alaska will have the largest increase of frost-free days in North America in the 21st century, because current winter mean temperatures hover close to the 32 °F (0 °C) freezing threshold. For example, the projected temperatures for the Kenai Peninsula show that mean temperatures in March and November are expected to shift from below freezing to above freezing. Warmer winters would lead to longer growing seasons, altered distribution of snow cover, considerably reduced snow accumulation in some areas, and earlier snowmelt and peak runoff [29,56,60,124]. With a 1.8 °F (1.0 °C) increase in temperature, the growing season is projected to increase 20 to 40 days in Alaska by 2100 (vs. 1961-1990), particularly in coastal areas [139].

Table 4—Projections of temperature and precipitation changes in the northern coastal temperate rainforest of southeastern Alaska and northern British Columbia from five global climate models using two emission pathways [118].
Scenario Mean annual temperature

Mean annual precipitation

 Total precipitation as snow
Current (1961–1990) 3.2 °C 3,130 mm 1,200 mm
Low-medium emissions (2080s) 4.9–6.9 °C 3,210–3,400 mm 720-940 mm
High emissions (2080s) 6.4–8.7 °C 3,320– 3,690 mm 500-720 mm

Mean annual precipitation has generally decreased in south-central Alaska [21] and increased in southeastern Alaska [56,124] since the mid-20th century. Precipitation levels are projected to increase with climate warming; however, projections for precipitation changes are more variable and less certain than those for temperature [57]. Summer precipitation near Juneau is projected to increase by 5.7% between 2000 and 2099 [124]; however, projections for most of the northern Pacific coastal region are for increased cool-season precipitation and decreased summer precipitation [124].

Even with increased precipitation, many locations are expected to have decreased water availability, increased drought stress, and overall drier conditions during summer due to higher temperatures, increased evapotranspiration, longer growing seasons [40,56,124]. In south-central and most of southeastern Alaska, June water availability is projected to decrease 10% to 75% (June 2090-2099 vs. June 1961-1990) [124]. The drying trends observed in wetlands on the Kenai (see Postsettlement fuels) are likely to continue and increase [56]. Soil water stress is projected to increase in May and June in most of British Columbia (2070-2099 vs. 1961-1990) [124].

Projected changes in atmospheric circulation patterns suggest a higher frequency of weather extremes. For example, projections frequently show greater extremes of El Niño-Southern Oscillation (ENSO) and PDO indices suggesting long periods without precipitation and more severe droughts in some locations, and more precipitation, flooding, and stormy weather at other times and locations [40,56,57].

Projected changes in fuels and fire regimes: Both anthropogenic climate changes (especially increased temperatures, changing precipitation patterns, and earlier snowmelt) and natural climate variability (overlapping patterns of ENSO, PDO, etc.) are implicated in increased wildfire activity observed in the late 20th and early 21st century in the western United States [1,121,134] and interior Alaska [7]. Krawchuk et al. [67] used statistical models to project future changes in global fire patterns under simulated future climate conditions. Their models highlight the potential for widespread impacts of climate change on wildfire and suggest severely altered fire regimes in some areas, with substantial invasion and retreat of fire across large portions of the globe. They identified regional "hotspots" of change in fire probabilities. The southeastern coast of Alaska and northern coast of British Columbia were projected to transition from low to high probability of fire in the near future (2010-2039) [67]. High-elevation sites could become disproportionately more susceptible to fire under a warmer climate [50,51,74].

As of 2017 climate-induced changes in fire activity were not obvious in Alaskan Pacific maritime ecosystems, and contemporary changes in fire activity were mostly anthropogenic (see Postsettlement fires). Nonetheless, widespread changes in vegetation and fuel characteristics have been observed and are largely attributed to climate warming (see Postsettlement fuels). These types of changes are expected to continue throughout the northern Pacific coastal region and possibly become more widespread and severe with further warming [41,56,118,124]. These changes suggest a possibility for more frequent and larger fires than have occurred in Alaskan coastal forests than at any time during the Holocene. Projected changes and their expected impacts on fire regime characteristics are described in the following sections:

Climate change and fuel structure and distribution: Direct and indirect effects of climate warming are altering vegetation production, composition, and distribution in Alaskan coastal ecosystems with corresponding changes in the amount, composition, structure, and distribution of fuels on the landscape. Alaskan coastal forests may be particularly sensitive to these effects of climate change [118]. Many of the changes in vegetation already observed may be attributed to higher temperatures, reduced depth and duration of snowpack, elevated snowline, longer growing seasons, decreasing water balance, and altered disturbance regimes (see Postsettlement fuels), all of which are expected to continue along with associated changes in productivity, increased mortality, range shifts, severe insect outbreaks, and changing storm patterns and associated disturbances. These changes will continue to alter fuel characteristics, although there is uncertainty regarding overall impacts of these changes.

Productivity, range shifts, and mortality: Coastal forests are likely to see increases in productivity with warming temperatures, increased growing season length, and longer frost-free seasons, particularly in northern latitudes at low- and mid-elevation sites [56,63,112,124]. However, these gains may be offset by increased mortality of some tree species, increases in populations of damaging agents (insects, diseases, herbivores), altered disturbance regimes, changes in species compositions and competitive relationships, and moisture and nutrient limitations associated with longer and drier growing seasons [56,124]. Novel communities may develop as succession proceeds under novel climatic conditions [124,139]. Projected increases in temperature in south-central Alaska are also likely to increase the probability of nonnative, invasive plant establishment and spread [56,139].

Plant species' responses to climate change and other disturbances can include changes in vigor, phenology, growth rates, mortality, and occupancy of particular sites [29], leading to changes in the distribution of formations (forest, shrubland, grassland) across the landscape, and changes in species composition and structure within those formations [124]. Large vegetation shifts, such as those from forest to woodland or alpine tundra to forest, are expected to alter historical fire regimes [77]. Forests are projected to remain the dominant formation in northern Pacific coastal ecosystems, but their distribution and composition may change substantially due to range shifts, expansions, and contractions of tree species' distributions. Coniferous forests are expected to expand in south-central and southeastern Alaska and may serve as biome refugia [124].

The distribution of several dominant conifers in Alaskan coastal forests may shift as temperatures warm, because temperature is a driving factor in determining altitudinal distributions (e.g., treeline) and is a contributing factor determining latitudinal distribution at the edges of species ranges. Species range shifts due to climate are most likely to be detected at these extremes [29]. The six most common conifers in Alaskan coastal forests have ranges that extend from northern California to southeastern and south-central Alaska [28]. Many of these tree species (e.g., Sitka spruce, western redcedar, and Pacific silver fir) are physiologically adapted to grow much farther north or west than the limit of their contemporary distribution. The range of Sitka spruce, for example, is actively expanding westward into wet coastal tundra and shrublands at the western extreme of its distribution on Afognak and Kodiak islands [95,127]. With continued warming, western hemlock and western redcedar may expand their overall range while maintaining most or all of their current range [124].

As climate warms, lowland and subalpine forests may expand and alpine areas may shrink in south-central Alaska [37], southeastern Alaska, and northern coastal British Columbia [118,124,131]. With less snowpack limiting tree establishment, meadows may succeed to woodland or forest, resulting in a more continuous distribution of woody fuels at high elevations [51,74]. In the western Kenai Mountains of south-central Alaska, treeline rose very little between 1951 and 1996, but 29% of the forest-alpine tundra ecotone area increased in woodiness, closed-canopy forest expanded 14% per decade, shrubs expanded 4% per decade, unvegetated areas decreased 17.4% per decade, and tundra decreased 5% per decade. Area of open woodland remained constant but changed location [37]. Mean annual temperatures in alpine areas are projected to exceed those of current subalpine areas by the 2080s in all but the most northerly part of southeastern Alaska. Rising winter temperatures, reduced depth and duration of snowpack, and increased elevation of snowpack are anticipated to raise the elevation of treeline, resulting in a regional loss of high-elevation tundra ecosystems and raising the lower boundary of subalpine ecosystems [37,56,118,124,131]. A review by Haufler et al. [56] suggests a rise in treeline of 2,000 to 3,000 feet (600-900 m) in southern coastal Alaska. Modeling by Wang et al. [131] predicts that by the 2080s, British Columbia's coastal alpine habitat will shift upward in elevation more than 600 feet (208 m), coastal western hemlock will shift upward more than 1,000 feet (323 m) and northward about 43 miles (69 km), and subalpine mountain hemlock will shift upward almost 1,500 feet (455 m) and northward almost 50 miles (75 km).

Tree death rates and forest declines will likely increase due to the direct and indirect effects of warming temperatures, and many old forests may undergo abrupt changes when critical climatic thresholds are exceeded [32]. Projected habitat losses and transitions will tend to be exacerbated where insect disturbance (especially bark beetles) and disease are prevalent or occur in conjunction with drought stress [124]. Successional trends following large-scale mortality events are relatively unknown [27], although Oakes et al. [102] studied succession at the northern extremes of yellow-cedar decline.

Insects and disease: Tree mortality and injury from insect and disease outbreaks are expected to increase due to climate warming in southern coastal Alaska [56,118,124,139]. The relationships between climate and biotic disturbance agents are complex and interdependent. For additional details, the reader is encouraged to see the primary literature cited in these reviews: [38,56,57,124,139].

Changes in fuel characteristics are expected in conjunction with outbreaks (e.g., [24,117,120]), from both direct mortality and because trees weakened by infestation and infection are less tolerant of drought and heat stress; more susceptible to other insects, pathogens, and disturbances; and more likely to break or fall during wind storms [38,56,124,139]. For example, spruce beetle populations are likely to increase with rising temps (e.g., [18,20,56,97,120]), and host trees are likely to become more susceptible to attack and mortality from spruce beetles due to increased stress from drought and other climate-related impacts [57,120,124]. Other insects expected to have increasing impacts in Alaskan coastal forests include western balsam bark beetles, Sitka spruce aphids, and mountain pine beetles [56,63]. Climate models predict large potential increases in hemlock dwarf mistletoe abundance [17], and an increase in disease effects on western hemlock is expected [17,58]. In areas where host trees are stressed by drought, opportunistic pathogens that rely on poor host vigor, such as stem-decay fungi, may display greater virulence and increase frequency of wind-breakage [139]. Outbreaks of some root rots, blights, and rusts (e.g., Dothistroma needle blight expansion on shore pine [94]) are expected to increase where climate becomes warmer and wetter, but outbreaks may decline or remain unchanged in severity where conditions become warmer and drier [57].

Altered disturbance regimes: Projected climate changes are expected to impact the frequency and severity of disturbances other than fire, which will impact vegetation structure and species composition at landscape scales, changing fuel characteristics and having potential feedbacks on fire regimes. Altered weather patterns including a northern shift in storm tracks and greater storm intensities are one consequence of increasing ocean temperatures, and increased storm intensities are expected in southern coastal Alaska [56]. Wind damage, floods, and landslides can be expected to increase on terrain where they are already a risk factor [57]. Disturbance rates are likely to be substantially higher than what the landscape has historically experienced, especially near logged areas because timber harvesting can increase the likelihood and severity of natural disturbances [38]. The prediction of future forest disturbance regimes is in its infancy, but managers may wish to adjust plans accordingly where there is consensus among projections [57]. See the primary literature in the following reviews for more information [38,56,57,124,139].

Climate change and fuel moisture: Under the contemporary climate, high moisture content in fuels usually retards ignition and spread of fire in Alaskan coastal ecosystems, even during years of extreme drought [31] (see Fuel moisture). Warmer and drier conditions in systems with ample fuel loads and ignition sources will increase the incidence and likely the size and severity of wildfires. However, the effect of warmer and wetter conditions is less straightforward [56], and projections of future precipitation patterns have a higher degree of uncertainty than those for temperature [38].

Although projected increases in precipitation may temporarily increase the moisture content of slow-drying forest floors [57], warmer temperatures could result in drier fuels, despite increased precipitation [40]. Shanley et al. [118] suggest that lowland coastal forests in southeastern Alaska will remain wet, and fire will continue to be rare if seasonality of precipitation does not change substantially. However, soil water stress is projected to increase in the spring and summer in much of the northern Pacific coastal region [124]. Moisture content of slow-drying forest floors was generally projected to increase in northern British Columbia, particularly in June; however, moisture content of rapidly drying surface fuels, which are important to ignition and fire spread, may decrease even with increased precipitation [57]. Flannigan et al. [40] used three General Circulation Models and three emission scenarios to calculate how much precipitation would have to increase for every degree of warming to maintain fuel moisture levels in fine surface fuels, upper forest floor (duff) layers, and deep organic soils. They found that to maintain moisture levels precipitation has to increase by more than 15% for fine surface fuels, about 10% for duff layers, and about 5% for deep organic soils for every degree of warming [40]. The necessary precipitation increases (5%-15%) exceed most projected increases in Alaskan coastal ecosystems (see above), suggesting that fuel moisture is likely to decrease, even with projected increases precipitation.

In coastal areas where periodic droughts and fires occasionally occur, such as south-central Alaska and northern British Columbia, both are more likely to increase in frequency and severity with further warming [118]. Decreased water availability is projected to have a strong influence on fire regimes on the Kenai Peninsula [56]. Increased water limitation and drought have already been observed in south-central Alaska (see Postsettlement fuels) and are projected to continue. Water limitation not only increases the potential for drier fuels during a longer portion of the fire season, it also constrains the growth and distribution of many tree species, and makes some more susceptible to attack from insects and disease [8,112,124]. These changes have the potential to alter fuel structure and distribution at landscape scales.

Climate change and fire characteristics: Drier fuels alone can increase the likelihood of fire, but annual fire activity is also driven by fire season length, extreme fire weather, and ignition frequency, all of which have been and will likely continue to increase with climate warming (e.g., [40,57,74]). Effects of fires in coastal ecosystems are likely to be very severe for dominant conifers and other plants that are not well adapted to survive fire.

Wildfires may become more common as ignition sources increase. A warmer planet will have a moister atmosphere, a larger number of extreme convective storms, and a greater density of lightning discharges [50,56]. Increased incidence of lightning, especially during extended drought, would certainly increase the likelihood of wildfires in coastal Alaska. Unlike temperature and precipitation, lightning is often very localized and difficult to determine accurately with standard weather stations [4]. Human-caused ignitions are also likely to increase as populations grow [19].

Extreme fire weather could render a greater portion of the landscape susceptible to fire [74], and an increase in annual area burned is likely with longer fire seasons [57]. Modelling by Bachelet et al. [12] projects increases in future area burned along the southern and western coasts of Alaska [67]. In Canadian forests, most area is burned on a small number of days with extreme fire weather. Wang et al. [132] examined the historical and projected future frequency of such extreme fire weather events across 16 fire regime zones in the forested regions of Canada from 1970 to 2090, and found that the number of days with active fire spread in Canadian forests is likely to increase by 35% to 400% by 2050, with the largest proportional increase occurring in coastal and temperate forests. Similar increases in the number of days with active fire spread could occur Alaskan coastal forests. Fire management agencies in coastal and temperate regions may need to adapt their planning and capacity to deal with proportionally larger changes to their fire weather regime compared to the already high fire management capacity found in drier continental regions [132].

While projected increases in global temperatures are unambiguous, projections for other climate variables are less certain, which complicates projections for fire regime characteristics. Because changes in precipitation with climate warming are difficult to predict, projected changes in area burned are less certain in ecosystems where fire activity is driven by precipitation and drought, than in ecosystems where fire activity is driven by temperature. Variations due to complex topography further complicate finer scale projections [88]. Projections vary depending on climate models, emission scenarios, time periods, and the methods used to downscale projections to local and daily scales [57]. Until the uncertainty associated with precipitation projections has been satisfactorily addressed by models, Haughian et al. [57] suggest that precipitation correction factors be considered when modelling future fire regimes.

Management considerations under a changing climate: Alaskan coastal ecosystems have historically experienced little or no fire, and are therefore particularly susceptible to changes in community structure or composition with increases in fire activity. Fire can alter vegetation composition by selecting for more fire-tolerant species, altering regenerative processes, and increasing susceptibility to insect attacks. Increased fire activity, especially in conjunction with changes in other disturbances, can exacerbate and accelerate vegetation changes caused by climate warming and may catalyze relatively rapid changes in ecosystem composition and structure [48,77,112,124]. Fire activity could change faster than many terrestrial species may be able to accommodate [67]. Recognizing thresholds for systems beyond which changes are irreversible will be an important component of forest management in a changing climate [18].

Another anticipated effect of climate change on Alaskan coastal ecosystems concerns the potential release of carbon to the atmosphere via wildfire, and to aquatic ecosystems and the ocean via other processes. Southeastern coastal forests are carbon rich. For example, the total carbon stock in the forest and soils of the Tongass National Forest in southeastern Alaska comprises 8% of that in all forests of the conterminous United States [118]. Intense and severe fires increase carbon losses from the ecosystem [77,109,112,124], and the amount carbon emitted during a wildfire can exceed the amount of carbon accumulated over decades or centuries of tree growth [11]. The total dissolved organic carbon flux from the Tongass is the highest per unit area of any ecosystem in the world. Cumulative annual discharge from Tongass watersheds equals or exceeds that of the Yukon River in an area 1/13th the size. Changes in temperature and precipitation could mobilize carbon stored in forest soils and flush large amounts of carbon, nitrogen, and phosphorus from the forest to the adjacent marine ecosystem with potentially large impacts on marine productivity and the regional carbon balance [56].

Consideration of intermediate and long-term climatic trends is essential for anticipating and planning forest management [38]. Given that the atmospheric concentrations of greenhouse gasses will exacerbate climate change effects for the foreseeable future, adaptive actions to reduce ecosystem vulnerability, increase the capacity to withstand or be resilient to change, and/or accept their transformation to novel systems compatible with likely future conditions are warranted [124]. Landscape managers are tasked with devising regionally-specific but flexible strategies to adapt to a warmer climate, while maintaining the essential values of ecosystem productivity, integrity, and biodiversity [57]. Although uncertainty and gaps in knowledge exist, resources are available to help plan for and address climate change impacts (e.g., [130]). Implementing strategic adaptation actions early may reduce severe impacts and prevent the need for more costly actions in the future [124].

Haufler et al. [56] list and describe a large number of initiatives, programs, and organizations focused on management adaptations for climate change, and they outline a strategic plan for adapting management with regard to climate change in south-central and southeastern Alaska. They describe several landscape conservation strategies, including (but not limited to) monitoring key indicators of climate change, establishing model watersheds to support monitoring and adaptive management, assessing risks to identify changes that can be mitigated, and mitigating and adapting to anticipated and observed effects [56]. Examples of planned or ongoing adaptation efforts in the northern Pacific coastal region are described by Tillmann and Glick [124]. Sommers et al. [121] provide some specific suggestions so managers can use both available and projected information about fire history and climate change to better understand potential fire regimes in the face of climate change, and use this information to help shape fire and fuel management decisions in the 21st Century. Haughian et al. [57] stress the importance of a greater emphasis on risk analysis over productivity, as well as managing for flexibility and resilience.

LIMITATIONS OF INFORMATION

Detailed knowledge of historical fire regime characteristics in Alaskan coastal ecosystems is lacking, likely due to the historical rarity of fire; few fire studies have been conducted in or around Alaskan rainforest zones because fire is so rare in these communities. Contemporary changes in fire patterns, fuel characteristics, and climate suggest that fire activity is likely to increase in these ecosystems and may lead to broadscale changes in plant community composition, structure, and distribution. While it is reasonable to expect that vegetation composition, structure, and productivity will change with changing climate and disturbances, future trends in climate, weather events, species range shifts, and tree mortality are difficult to project with confidence. Regional differences in topography and vegetation types strongly influence disturbance regimes and will continue to do so under changing climates [57]. A better characterization of climate variability, seasonal patterns, and frequency of severe events—rather than long-term trends in climate means—is needed to better understand potential ecological responses. Because global climate models do not account for local variability, fine-scale monitoring is important for evaluating assumptions based on larger-scale models [118].

The direct and causal relationships between climate, weather, fuel moisture content, ignition probability, and fire behavior make plausible estimates of climate change impacts on wildfire possible [57] and, since the early 1990s, researchers have been using projected changes in temperature and precipitation derived from global climate models to explore the potential influence of climate change on disturbance regimes, especially fire regimes (e.g., [42,67,78,79]). However, the nature of these changes and their effects on individual ecosystems are difficult to discern with precision, due to the complex and non-linear interactions between weather, vegetation, and people [41], as well as fire-climate-vegetation feedbacks that could have a further warming effect on global climate (e.g., through fire-related emissions) and affect other natural disturbances that kill or defoliate trees. The result of interactions between these phenomena and fire activity will be very difficult to predict. For models projecting the future of species distributions, especially that of plants, methods are needed that explicitly integrate the effects of fire activity on vegetation, in addition to species range changes based on plant-climate relationships alone [67]. To better understand the implications of potential increases in the frequency and scale of fire in Alaskan coastal ecosystems, more information is needed regarding the consequences of fire and disease on successional trajectories of contemporary forest types [118].

Long-term paleoecological data might provide insights necessary to understand forest response to projected climate changes [121,137] and can help to clarify the links between fire, fuel, and climate through periods of substantial, and sometimes rapid, climate change [48,85,121,137]. In particular, times of rapid change or extreme climate anomalies (e.g., the last glacial-interglacial transition (~12,000-10,000 years ago) [84], the Medieval Warm Period (~950-1250), and the Little Ice Age (~1650-1890)) may offer insights into potential future changes [4,121]. Fire reconstructions that span millennia (i.e., charcoal records from lake sediments and soil profiles) help demonstrate the historical range of variability in fire activity over the duration of a vegetation type, and they can offer insights into the role of large-scale climate changes, the influence of prehistoric human activity, and the causes and consequences of major reorganizations of vegetation [136,137]. These insights may help managers anticipate potential effects of changing climates on disturbance regimes and forest succession [32,121], so they can plan management in that context [48]. Applications of paleoecological data to future forecasts are complicated because climate-vegetation combinations in the future may not resemble those of any time in the past. Nonetheless, an understanding of the processes that produced the variability observed in paleoecological records may be used to test the mechanistic models used for predicting future variations in fire [48].

Table A1: Summary of modeled fire regime information for Biophysical Settings covered in this synthesis

Table A2—Common and scientific names of plant species that commonly occur in Alaskan Pacific maritime ecosystems or were mentioned in this synthesis. Follow links to FEIS Species Reviews for additional information on those species.
Common name Scientific name
Trees
balsam poplar Populus balsamifera subsp. balsamifera
black cottonwood Populus balsamifera subsp. trichocarpa
black spruce Picea mariana
coast Douglas-fir Pseudotsuga menziesii var. menziesii
cypress Cupressaceae
fir Abies spp.
hemlock Tsuga spp.
Kenai birch Betula papyrifera var. kenaica
Lutz spruce Picea × lutzii
mountain hemlock Tsuga mertensiana
netleaf willow Salix reticulata
Pacific silver fir Abies amabilis
paper birch Betula papyrifera
red alder Alnus rubra
resin birch Betula neoalaskana
shore pine Pinus contorta var. contorta
Sitka spruce Picea sitchensis
spruce Picea spp.
subalpine fir Abies lasiocarpa
western hemlock Tsuga heterophylla
western redcedar Thuja plicata
yellow-cedar Callitropsis nootkatensis
Shrubs
Alaska bellheather Harrimanella stelleriana
Alaska willow Salix alaxensis
alder Alnus spp.
Aleutian mountainheath Phyllodoce aleutica
alpine laurel Kalmia microphylla
arctic willow Salix arctica
alpine azalea Loiseleuria procumbens
Barclay's willow Salix barclayi
bog cranberry Vaccinium oxycoccos
blackberries Rubus spp.
black crowberry Empetrum nigrum
blueberries Vaccinium spp.
bog blueberry Vaccinium uliginosum
bog laurel Kalmia polifolia
bog rosemary Andromeda polifolia
boreal sagebrush Artemisia norvegica
copperbush Elliottia pyroliflora
currant Ribes spp.
devil's-club Oplopanax horridus
dwarf bilberry Vaccinium caespitosum
dwarf mistletoe Arceuthobium spp.
hemlock dwarf mistletoe Arceuthobium tsugense
highbush cranberry Viburnum edule
huckleberries Vaccinium spp.
Labrador tea Ledum spp.
Lapland cornel Cornus suecica
laurel Kalmia spp.
menziesia Menziesia ferruginea
mountain cranberry Vaccinium vitis-idaea
netleaf willow Salix reticulata
ovalleaf huckleberry Vaccinium ovalifolium
partridgefoot Luetkea pectinata
red elderberry Sambucus racemosa
redosier dogwood Cornus sericea
salal Gaultheria shallon
salmonberry Rubus spectabilis
Sitka alder Alnus viridis subsp. sinuata
sprouting leaf willow Salix stolonifera
sweetgale Myrica gale
thimbleberry Rubus parviflorus
trailing black currant Ribes laxiflorum
western moss heather Cassiope mertensiana
white arctic mountain heather Cassiope tetragona
yellow mountainheath Phyllodoce glanduliflora
Forbs
Alaska Indian paintbrush Castilleja unalaschcensis
arctic raspberry Rubus arcticus
arctic sweet coltsfoot Petasites frigidus var. frigidus
American skunkcabbage Lysichiton americanus
buckbean Menyanthes trifoliata
bunchberry Cornus canadensis
buttercup Ranunculus spp.
calthaleaf avens Geum calthifolium
Canadian burnet Sanguisorba canadensis
common cow parsnip Heracleum maximum
darkthroat shootingstar Dodecatheon pulchellum
deercabbage Nephrophyllidium crista-galli
felwort Swertia perennis
fireleaf leptarrhena Leptarrhena pyrolifolia
fireweed Chamerion angustifolium
fringed grass of Parnassus Parnassia fimbriata
goldthread Coptis spp.
great burnet Sanguisorba officinalis
green false hellebore Veratrum viride
larkspurleaf monkshood Aconitum delphiniifolium
marsh marigold Caltha spp.
Menzies' burnet Sanguisorba menziesii
mountain Indian paintbrush Castilleja parviflora
narcissus anemone Anemone narcissiflora
Nootka lupine Lupinus nootkatensis
pioneer violet Viola glabella
purple marshlocks Comarum palustre
purple monkeyflower Mimulus lewisii
round-leaved sundew Drosera rotundifolia
saxifrage Saxifraga spp.
scentbottle Platanthera dilatata
seep monkeyflower Mimulus guttatus
Sitka valerian Valeriana sitchensis
violet Viola spp.
woolly geranium Geranium erianthum
Graminoids
American dunegrass Leymus mollis
bluejoint reedgrass Calamagrostis canadensis
cottongrass Eriophorum spp.
fewflower sedge Carex pauciflora
grassyslope arctic sedge Carex anthoxanthea
little green sedge Carex viridula subsp. viridula
livid sedge Carex livida
longawn sedge Carex macrochaeta
Lyngbye's sedge Carex lyngbyei
manyflower sedge Carex pluriflora
Mertens' rush Juncus mertensianus
red cottongrass Eriophorum russeolum
rock sedge Carex saxatilis
sedges Cyperaceae
tall cottongrass Eriophorum angustifolium
tufted bulrush Trichophorum caespitosum
water sedge Carex aquatilis var. dives
Ferns and fern allies
deer fern Blechnum spicant
field horsetail Equisetum arvense
lady fern Athyrium filix-femina
long beechfern Phegopteris connectilis
queen's-veil maiden fern Thelypteris quelpaertensis
spreading woodfern Dryopteris expansa
variegated scouringrush Equisetum variegatum
variegated sedge Carex stylosa
water horsetail Equisetum fluviatile
western oakfern Gymnocarpium dryopteris
Bryophytes
dark sphagnum Sphagnum fuscum
giant calliergon moss Calliergon giganteum
sphagnum Sphagnum spp.
scaly sphagnum Sphagnum squarrosum
streamside sphagnum Sphagnum riparium

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