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Fire regimes of Alaskan alder and willow shrublands

Table of Contents:

Figure 1. Barclay's willow community on the Kenai Peninsula, Alaska.		Figure 2. A Sitka alder/field horsetail stand on a discharge slope on the Kenai Peninsula.
Images courtesy of Mike Gracz, Kenai Watershed Forum, https://cookinletwetlands.info.

• provide up-to-date information to the management community on historical fire regimes and contemporary changes in fuels and fire regimes,
• supplement information on individual species' adaptations and responses to fire provided by FEIS Species Reviews, and
• enable LANDFIRE to incorporate the latest science on historical fire regimes into data revisions and identify regions and plant community types lacking fire history data.

As of 2015, the scientific literature about fire regimes in Alaskan alder and willow shrublands is scarce. Anecdotal and qualitative descriptions are used in this synthesis to supplement the limited quantitative literature. Descriptions of fire ignition, season, pattern, and size specific to alder and willow shrublands were not found in the literature, so this synthesis describes these characteristics for Alaska in general. Literature reviews describing some fire regime characteristics of alder and willow shrublands in Alaska, Canada [99,137,149], and the western United States [37] were used in this synthesis.

Common names are used throughout this synthesis. For a complete list of common and scientific names of species mentioned and for links to FEIS Species Reviews, see Appendix B.

SUMMARY
This section summarizes fire regime information available in the scientific literature and LANDFIRE models as of 2015. Additional details and documentation of source materials follow this summary.

About half of the LANDFIRE BpS models for alder and willow shrublands include fire, and most of these are characterized by long return-interval, stand-replacing fires. Appendix A summarizes data generated for these BpS models and includes links to documents with additional information. Table 1 shows the range of values generated by LANDFIRE modeling.

DISTRIBUTION AND PLANT COMMUNITY COMPOSITION

Figure 3. Land cover distribution of Alaskan alder and willow shrublands based on the LANDFIRE Biophysical Settings (BpS) data layer [95]. Numbers indicate LANDFIRE map zones. LANDFIRE did not map every BpS in this group. Click on the map for a larger image and zoom in to see details.

Biophysical Settings (BpSs) covered in this review occur throughout Alaska. They include shrublands where alder and/or willow are dominant or codominant in late succession. Other nonericaceous species (i.e., arctic dwarf birch, beauverd spirea, devil's club, red elderberry, salmonberry, and sweetgale) and ericaceous species (e.g, bog blueberry and marsh Labrador tea) may be codominant. Shrublands included in this review include riparian and floodplain shrublands, shrub swamps, tundra shrublands, and topoedaphic shrublands such as those in subalpine and alpine zones and on steep, avalanche-prone slopes [132]. Alders and willows occur in seral stages of poplar, white spruce, or black spruce forests after disturbances such as flooding or fire (e.g., [1,39,45,99,108,129,134,138,145,147]). These seral stages are not discussed in this review. For information on fire regimes of seral and other alder and willow shrublands in Alaska, see these Fire Regime Syntheses:

• Fire regimes of Alaskan quaking aspen and balsam poplar communities
• Fire regimes of Alaskan black cottonwood communities
• Fire regimes of Alaskan black spruce communities
• Fire regimes of Alaskan coastal herbaceous communities and active inland dunes
• Fire regimes of Alaskan tundra communities
• Fire regimes of Pacific Maritime ecosystems
• Fire regimes of Alaskan white spruce communities

Shrublands included in this review are typically wet or mesic. Soils range from bare mineral soil to peat. Some sites are underlain with permafrost and are susceptible to cycles of degradation and aggradation that form thermokarsts [133]. Most of the shrublands covered in this review form a complex mosaic of small to large patches [20,30,133].

Appendix A lists the BpSs covered in this review and summarizes data generated by LANDFIRE's [93] successional modeling for those BpSs.

NatureServe [107] identifies the following alder and willow shrublands in Alaska. Corresponding BpS series are given in parentheses.

• Alaska arctic mesic alder shrublands are widespread but uncommon on mountain slopes, hillslopes, and small steep streams throughout arctic Alaska. Patch size is typically small. Soils are mesic but sometimes wet if found adjacent to small streams. Total shrub cover is >25% and dominated by American green alder. Grayleaf willow and diamondleaf willow may be codominant. Low shrub tundra and dwarf shrub systems are commonly adjacent to these shrublands. (16380 series)
• Alaska arctic mesic sedge-willow tundra is common on mountain slopes, hillslopes, drained lake basins, stabilized dunes, and snow beds throughout arctic Alaska. Permafrost is present. Patch size is small to large. Sedges and dwarf and low shrubs are dominant in this system. Total shrub cover is <25% and dominated by willows, especially diamondleaf willow and Richardson's willow, although other willows such as Bebb willow, grayleaf willow, and planeleaf willow may occur. Arctic dwarf birch and bog blueberry may be codominant. The dominant sedges are water sedge, tall cottongrass, and smallawned sedge. More information on mesic sedge communities is available in the Fire Regime Synthesis for Alaskan wet and mesic herbaceous communities. (16830 series)
• Alaska arctic mesic-wet willow shrublands are widespread and common on mesic to wet mountain slopes, hillslopes, flats, and streamsides throughout arctic Alaska. Patch size is small to large and often linear. Total low- and tall-shrub cover is >25% and dominated by willows, especially Alaska willow, diamondleaf willow, and grayleaf willow. Other shrubs, such as barren-ground willow, Chamisso's willow, Bebb willow, planeleaf willow, Richardson's willow, American green alder, arctic dwarf birch, bog blueberry, and marsh Labrador tea may be codominant. Sedges, feather mosses, and lichens may be common. (16390 series)
• Alaska subboreal avalanche slope shrublands are common throughout subboreal Alaska and infrequently in boreal Alaska on steep mountain slopes. Sitka alder is typically dominant. Other shrubs, including red elderberry, willows, and beauverd spirea may be common. These shrublands are often intermixed with bluejoint reedgrass and fireweed communities (16080 series).
• Alaska subboreal mesic subalpine alder shrublands are widespread on upper mountain slopes above treeline throughout south-central and southwestern Alaska. They occur less commonly in boreal Alaska. They often appear as a band of vegetation above treeline and below alpine systems. Sitka alder is typically dominant. Willows, red elderberry, and beauverd spirea may be common. In subboreal Alaska, these shrublands are often intermixed with mesic bluejoint reedgrass or fireweed communities. In boreal Alaska, they are often intermixed with low shrub tundra. (16090 series)
• Aleutian floodplain wetlands occur within active and inactive portions of floodplains. They develop on poorly drained deposits, oxbows, and abandoned channels, and are often interspersed with well-drained floodplain vegetation. Patches are small and often linear. River channel migration, flooding, and other fluvial processes are major disturbances. Wetland succession and species composition vary with water depth, substrate, and nutrient input. These wetlands are part of a complex that includes aquatic beds, marshes, wet meadows, herbaceous peatlands, and mesic-wet willow shrublands. More information on aquatic beds, marshes, wet meadows, and herbaceous peatlands is available in the Fire Regime Synthesis for Alaskan wet and mesic herbaceous communities. (17290 series)
• Aleutian mesic alder-salmonberry shrublands occur on the Alaska Peninsula and Kodiak Island and on the Aleutian Islands west to Unimak Island. They occur on flat to steep slopes at low to mid elevations in valleys and on mountain slopes. The slopes are typically ash-covered, colluvium, or glacial drift. They are dominated by low and tall shrubs with >25% cover. Green alder or salmonberry contribute >50% of the total shrub cover. Green alder may be dominant on recently disturbed sites, wind-sheltered sites, or recent ash deposits, while salmonberry may be dominant on older substrates such as stabilized talus slopes. Other common shrubs include red elderberry, beauverd spirea, devil's club, and tall willows such as Barclay's willow and grayleaf willow. Sitka alder and Siberian alder may be common. The understory is typically sparse and litter cover is high. Sites codominated by tall willows typically occur along streams and at the upper limits of alder growth. Some sites are interspersed with mesic herbaceous meadows. (17180 series)
• Aleutian mesic-wet willow shrublands are relatively uncommon but widespread on the eastern Alaska Peninsula and Kodiak Island. They typically occur in small patches in broad valleys and on mountain slopes. They are frequently found on wet sites in lowlands and on stream- and lakesides. They are often adjacent to peatlands and wet meadows. Most of these shrublands are on wet sites; mesic sites are uncommon. Soils range from mineral to peat. Total low- and tall-shrub cover is >25%, with willows contributing >25% of the total shrub cover. Barclay's willow is dominant. Sitka alder may be codominant. Alaska willow, undergreen willow, grayleaf willow, and diamondleaf willow are important. Sedges may be common. (16400 series)
• Aleutian shrub and herbaceous meadow floodplain systems occur on small active and inactive floodplains and outwash plains. They are widespread on the Aleutian Islands and Alaska Peninsula and form a mosaic with Aleutian floodplain wetlands. The substrate is typically well-drained sand or cobble alluvium, and permafrost is absent. Patches are often linear. This floodplain system includes mesic herbaceous meadows, American dunegrass grasslands, and tall or low willow and/or Sitka alder shrublands. More information on mesic herbaceous meadows is available in the Fire Regime Synthesis for Alaskan wet and mesic herbaceous communities. (17270 series)
• Western North American boreal alpine floodplains include active alpine and subalpine floodplain systems. Frequent river channel migration and associated flooding and fluvial processes are the major disturbances. Soils develop in alluvium and are typically shallow and well drained. Common shrubs include Alaska willow, other willows, arctic dwarf birch, and Sitka alder. Common herbaceous species include dwarf fireweed, lupines, tall bluebells, bitter fleabane, boreal yarrow, and hawksbeards. (16371 and 16372 series)
• Western North American boreal deciduous shrub swamps occur throughout the boreal and subboreal regions of Alaska on poorly drained, fine-textured soils. Depressions with standing water are common throughout the growing season. Soils range from muck to mineral and are relatively nutrient-rich. Some sites have a thin peat layer. Siberian alder and thinleaf alder are typically dominant, but Sitka alder, diamondleaf willow, or Richardson's willow may be dominant or codominant. Common understory species include bluejoint reedgrass, horsetails, purple marshlocks, and mosses. (16240 series)
• Western North American mesic boreal and subboreal scrub birch-willow shrublands occur throughout the boreal and subboreal regions of Alaska on mesic sites on flats, on mid- to upper slopes, and above treeline. Arctic dwarf birch and/or willows are dominant. Bog blueberry, marsh Labrador tea, diamondleaf willow, Barclay's willow, or other willows may be common. Dwarf shrubs such as black crowberry and mountain cranberry may also be common. Feather mosses and lichens may be common. (16101 and 16102 series)

Fire ignition
No information is available on specific ignition sources in Alaskan alder and willow communities, so this section describes more general patterns. Historically, lightning was the main source of ignition in Alaska [11,53,102]. Lightning strikes are most common in interior Alaska [34]. Within the interior boreal forest, lightning strike density tends to increase from west to east while precipitation decreases, which may contribute to a west-to-east increase in fire frequency [34,74]. Kasischke and others [73] found a positive correlation between total lightning strikes and the number of lightning-ignited fires in Alaska in the 2000s (r=0.79, P=0.01). Moderate to strong El Niño episodes generally produce dry thunderstorms with associated lightning activity and warm, dry conditions in the Alaskan interior. These conditions led to 15 out of 17 of the biggest fire years between 1940 and 1998 [56]. Because of the maritime climate, fewer lightning-caused fires occur in coastal areas than in interior Alaska [34]. Forested areas have more lightning-caused fires than shrublands or other nonforested areas [33,34].

Some Native Alaskans used wildland fire in the past [99], but the frequency and locations of these fires were not well documented. Historical and contemporary accounts of native Gwich'in Athabaskan groups in eastern interior Alaska describe intentional burning of the landscape when conditions were not conducive to extensive fire spread, such as during wet periods. In central interior Alaska, however, Koyukon Athabaskan groups did not historically use fire to modify the landscape [106].

Human-ignited fires have greatly outnumbered lightning-ignited fires throughout Alaska in recent decades [3,12,29,73,127]. Most of these ignitions are near human settlements [32]. Lightning-caused fires tend to be larger and account for most of the area burned in interior boreal forests [29,33,127]. See Fire regimes and human activity for more information on contemporary changes in ignition patterns.

Fire season
The typical fire season in Alaska starts in May after snowmelt and lasts until late July or August at the start of mid- to late-summer precipitation [44,118]. The peak fire season—from June to mid-July—coincides with high temperatures, intense lightning activity, low humidity, and sparse precipitation. Before June, soils are typically still wet from snowmelt. After mid- to late-summer rains, fuel moisture increases and lightning strike density declines, reducing the number of lightning-caused ignitions [29,134].

Large fire years (i.e., years when area burned is >1.5 times the long-term average) are associated with extended warm, dry periods and burning late in the growing season. In a typical year, most ignitions (76%) and fire spread occur from the beginning of June to mid-July. In large fire years, most fires occur in July, and they may burn into August and September [74].

LANDFIRE modelers stated that in subboreal mesic subalpine alder shrublands [88] and arctic mesic alder shrublands [86], fire spread would be more likely prior to green-up in spring than later in the growing season.

Fire frequency
Little published information is available on fire frequency in Alaskan alder and willow shrublands. Fire is probably less important than other disturbances in alder and willow floodplain wetlands. Flooding, shifting channels, and sediment deposition are the most common disturbances in these communities [84,85,90]. Most communities in Alaska, including alder and willow shrublands, support fire spread during severe fire weather [32,41,111,117,142,150]. Willows and alders along rivers burned during the 2007 Anaktuvuk River tundra fire, which occurred primarily in moist acidic tundra [68]. This fire was associated with record-high temperatures, record-low precipitation, and record-low sea-ice extent, and it marked the first time that the area had burned in at least 5,000 years [59,64].

However, alder and willow communities often remain unburned within fire perimeters because of wet soils and vegetation [109,111,141]. On a gentle hillslope near Imuruk Lake on the Seward Peninsula, willow shrublands usually remained unburned within the fire perimeter, except where low willows occupied narrow drainageways along the hillslope [111]. Wein [141] examined the characteristics of 50 tundra fires and found that alders and willows along streams were typically left unburned within fire perimeters. Dwarf willow-birch, dwarf ericaceous-lichen, cottongrass, and sedge meadow communities burned during dry years [141].

Vegetation flammability: Vegetation flammability may influence fire frequency in alder and willow shrublands. Alders and willows are generally considered to be of low flammability. Racine [111] considered upland willow shrublands and low willows along drainageways to have a moderate probability of burning, while riparian tall willow shrublands had a low probability of burning. Among plant communities within the area of the 1977 Seward Peninsula tundra fires, tall willow, sedge willow, wet sedge-grass, and open mat and cushion communities had the lowest probabilities of burning, while sedge tussock tundra and sedge tussock-ericaceous, sedge tussock-mixed, and birch-ericaceous shrublands had the highest probabilities of burning [111].

Alders and willows and other nonericaceous shrubs are generally of lower flammability than ericaceous shrubs [125]. Sylvester and Wein [125] studied the fuel characteristics of common arctic tundra and forest-tundra species and genera near Inuvik and ranked them in order of decreasing flammability as follows: dead leaves of graminoids (tussock cottongrass and bluejoint reedgrass)>evergreen ericaceous shrubs (northern Labrador tea and black crowberry)>fruticose lichens>dwarf deciduous woody shrubs (dwarf birch, bog blueberry, and grayleaf willow)=sphagnum>fireweed [125]. Thus, alder and willow communities with graminoids and ericaceous shrubs may have a higher probability of burning than alder and willow communities without them.

In boreal forest regions, shrublands are generally less flammable than coniferous forests [27,33,48]. DeWilde and Chapin [33] categorized 4 fuel types in interior Alaska and ranked them in order of decreasing flammability as follows: boreal spruce>mixed hardwood/spruce >>open tundra, shrub/grass=boreal lichen. Boreal forest was considered the most flammable because of its fine twigs and needles, high resin content, low moisture content, and ladder-like structure that carries fire into the canopy. The "open tundra, shrub/grass" fuel type consisted of treeless or nearly treeless vegetation types dominated by graminoids and shrubs, which the authors noted can dry rapidly and support fire spread under severe fire weather [32,33]. Alder, willow, and other kinds of shrublands in interior Alaska are not typically considered as flammable as boreal forest due to their relatively high live fuel moistures and relatively smaller fuel loads than boreal forests [78].

Flammability of alder and willow shrublands in Alaska increases during warm, dry weather [52,78]. In interior Alaska, shrublands accounted for >30% of the total burned area in the warm and dry years of 2002, 2005, and 2007. Low precipitation, high temperature, and low relative humidity during fires were positively associated with both percent of shrubland area burned per fire and total shrubland area burned (P<0.05 for both variables) [78]. However, researchers noted that alders and willows are less flammable during the growing season than prior to green-up [19].

Firebreaks: Alder and willow shrublands (e.g., [80,83,84,85,86,89,90,91]) are frequently associated with open water, where fires do not spread [4,14,23,117,143]. Many alder and willow shrublands (e.g., [80,83,84,85,87,89,90,91]) are also associated with landscape features that can form firebreaks, including wetlands, riparian areas, talus and boulder fields, and avalanche tracks [2,8,37,123,131]. Fires often stop at edges of alder and willow communities because soils and vegetation have relatively high moisture content, for example where tussock tundra intergrades with moist alder and willow communities [50,67,96,112,141,143]. In a study of 50 lighting- and human-caused fires occurring in arctic tundra from 1954 to 1973 west of Hudson Bay, white spruce, willow, and alder located along stream channels survived fires in the tundra-forest ecotone because their wet understories did not carry fire [141]. During the Buttes Gap prescribed burn that started on 9 July 1989 and burned for 3 days, all forest and woodland types burned (white spruce forest, black spruce forest, white spruce woodland, and mixed white spruce-paper birch woodland), while “even after vigorous ignition attempts” none of the tall willow scrub types (closed tall diamondleaf willow scrub, open tall diamondleaf willow scrub, open tall Alaska willow scrub, and open tall littleleaf willow scrub) burned because the vegetation was too wet [46]. LANDFIRE modelers indicated that wet sedge-willow communities are unlikely to carry fire from adjacent tussock tundra because they are too wet [81].

Some landscape features that typically act as firebreaks in conifer forests, such as avalanche tracks dominated by alders and other deciduous vegetation, may cease to do so during severe fire weather [7,55,103]. LANDFIRE modelers indicated that boreal deciduous shrub swamps typically act as firebreaks but could burn during severe fire weather [83].

A negative feedback loop may occur where fire increases the probability of avalanches, which may create and maintain firebreaks, thereby restricting the spread of subsequent fires [124].

Adjacency: The fire regimes of alder and willow shrublands in Alaska are likely influenced by fire regimes in adjacent communities. LANDFIRE modelers indicated that fires usually spread into arctic mesic alder shrublands from adjacent communities [80]; fires are likely to spread into the edges of subboreal avalanche slope shrublands from neighboring communities [87]; and fires are likely to be more frequent in mesic scrub birch-willow shrublands if adjacent vegetation is flammable [82,92].

In interior Alaska, alder and willow shrublands occur within a matrix of other boreal vegetation types (e.g., coniferous forest, deciduous forest, and grasslands) [107,132]. Wildfires are common in boreal forests [73], and they usually are the source of fires that occur in adjacent communities [33,34].

Ancient times: Studies of ancient fire regimes in Alaskan boreal forests show several apparent vegetation and climate-driven fluctuations in fire frequencies. At the end of the Ice Age (~13,000 BP), lowland areas on the Kenai Peninsula that are now black spruce taiga were herbaceous tundra. As the climate warmed during the early Holocene, these areas succeeded to willow-alder shrub tundra. Fire-return intervals averaged 138 years during this type shift. As climate warming continued during the mid-Holocene, fire-return intervals shortened as poplar-willow and then white spruce-birch gained dominance. Black spruce became dominant during the Little Ice Age, and fire-return intervals lengthened to an average of 130 years (Table 2) [6].

Other analyses of sediment cores in Alaskan boreal forest found that fire became more frequent when black spruce dominance increased with climate cooling in the late Holocene [21,57,58,61,62,71,100,101]. In the south-central Brooks Range, fire became more frequent with the transition from sedge-willow tundra to arctic dwarf birch and/or dwarf birch tundra about 14,300 to 13,300 years BP. This early-postglacial birch shrub tundra is thought to have lacked alder, been more productive, and have greater shrub cover than modern birch shrub tundra, leading to more frequent and severe fires than in the modern birch shrub tundra. Fire-return intervals increased with a shift to poplar woodlands that established about 10,500 years BP (Table 3) [58]. The low flammability and/or fuel loads of the early-Holocene poplar, alder, and white spruce forests/woodlands probably limited fire occurrence even though the regional climate was conducive to burning [63].

A paleological study on Alaska's North Slope found little evidence of fire in the 5,000 years prior to the Anaktuvuk River Fire in 2007. A lack of burnable biomass and an overall cooler summer climate compared with other regions in Alaska likely limited fire on the North Slope [59,64].

Fire type and severity
Little published information was available regarding fire type and severity in Alaskan alder and willow shrublands as of this writing (2015). Some authors observed high-severity fire in Alaskan alder and willow shrublands. According to Hardy and Franks [52], alder-willow fuel types tend to burn "hot", suggesting that high-severity, stand-replacing crown fires may occur in these types. During the 1977 Bear Creek Fire near Farewell, Alaska, open alder-birch-willow/grass shrublands burned with high-severity. These communities occurred on well-drained, cool sites on glacial moraines. After the fire cover of mineral soil was 50%, dead wood was 11%, unburned litter was 2%, and scorched organic soil was 18% [51]. LANDFIRE modelers indicated that fires in boreal deciduous shrub swamps are usually stand-replacing [83].

However, some observations suggest that Alaskan willow communities can also burn with moderate or low severity. Fire severity was low in an open grayleaf willow-tall cottongrass community on a short, poorly drained slope along the Noatak River in Alaska [113]. In a late July to mid-August 17-mile² (44 km²) wildfire in arctic tundra in the Kokolik River Watershed on the North Slope of Alaska, irregularly distributed areas of low relief were either moderately burned, averaging 30% live plant cover immediately after fire, or lightly burned, with 40% live pant cover. Vegetation in these areas consisted of wet water sedge-tall cottongrass-diamondleaf willow communities in shallow drainage depressions. In contrast, all areas of raised relief, which consisted of tussock cottongrass tundra, were severely burned and averaged <10% live vascular plant cover immediately after fire. The high severity of the fire in the latter community was attributed to dry soils and more fuels (litter and standing dead vegetation). Burned willows and other shrubs were completely top-killed [66].

Hydrology likely influences burn severity in alder and willow shrublands. Fire is generally less severe on poorly drained willow tundra communities than on well-drained sites with birch and ericaceous shrub tundra [111].

Repeated, severe fires in boreal white spruce and black spruce forests may result in replacement by shrub or herb communities, such as arctic dwarf birch-willow shrublands, bluejoint reedgrass-sedge meadows, or fireweed-grass communities [76,99,132].

Fire Effects: Both alders and willows are well adapted to survive and reproduce after fire. While alder and willow seeds are unlikely to survive burning [135,137], they are light and well dispersed by wind and water, and they germinate on areas where moist mineral soil is exposed, such as severely burned areas [137,147]. Alder seeds are usually dispersed in fall and germinate better following cold stratification [137,149]. Thus, they usually establish during the second growing season after fire. Willow seed dispersal times vary with species, so the species of willow that establish after fire depends on the time of year [137,149]. Alaska willow is often the first willow species to disperse seeds in late spring. Pacific willow and littletree willow disperse seeds in summer, while grayleaf willow and barren-ground willow disperse seeds in fall. The seeds of the fall-dispersing species may germinate in fall, but better germination occurs after cold stratification. Often the earliest seeds to germinate in spring are from fall-dispersing species [108,149].

Most alders and willows typically sprout soon after top-kill by fire (e.g., [136,137,147]), although postfire sprouting varies among species [45,137] and with plant age [99]. Alders are more susceptible to mortality by fire than willows, so alder sprouts tend to be less abundant than willow sprouts [137].

Fire severity can affect the mode of alder and willow postfire regeneration. Following low-severity fires, most alders and willows recover quickly, sending up new shoots from undamaged root crowns. Few if any seedlings establish after low-severity fire because organic soil layers are only partially consumed, which prevents seedling establishment. Following severe fires, however, the primary mode of regeneration is seedling establishment. Severe fires that burn deep into organic soils kill plants but expose mineral soil, which provides an excellent seedbed for alders and willows [137,148]. On Nimrod Hill on the Seward Peninsula, the greatest willow seedling establishment occurred in areas where burning was severe [114].

Fuels: Alder and willow shrublands range from open to very dense [10,23,108,144]. Among alder and willow communities, those on floodplains probably have the most abundant fuels. Indeed, Neiland and Viereck [108] stated that the productivity of willow-alder thickets in floodplains may be greater than in any other taiga ecosystem. Johnson and Vogel (1966 cited in [150]) reported total aboveground biomass production of 54,350 kg/ha in a 28-year-old alder-willow stand in the Yukon Flats. Among all the tundra types, standing crop and net annual production are greatest in tall shrub tundra, which is dominated by Alaska willow, grayleaf willow, and Richardson's willow [10]. See Barbour and Billings [10] for more information on plant production in Arctic ecosystems.

Even when sufficient biomass is available for burning, alder and willow communities are often too wet to burn [23,46,141]. However, warm, dry, windy weather may promote vegetation flammability and fire spread in these communities [59]. In tundra ecosystems of Alaska, >95% of the variability in annual area burned over 60 years was explained by interannual variations in growing-season temperature and precipitation [59,64].

Fire spread: Hardy and Franks [52] considered rate of fire spread in willow-alder types to be moderate in valley bottoms, on wet benchlands, and on north-facing slopes; and high on dry benchlands and south-facing slopes.

Fire pattern
Because of variation in topography, plant community composition, soil and fuel moisture, and weather, fires in Alaska typically produce a mosaic of unburned, lightly burned, and severely burned areas (e.g., [5,38,110,111,119,130,140,141,142]). Monitoring Trends in Burn Severity data from 2004 (a large fire year) and 2006 (a small fire year) indicated that 20% and 66% of the area within fire perimeters in Alaska did not burn, respectively, indicating the prevalence of mosaic fires in both large and small fire years. An analysis of the burned and unburned fuel types within fire perimeters from 2004 showed deciduous and nonforested fuel types, such as shrublands and grasslands, burned less often than mature spruce fuel types within the boreal forest ecoregion of interior Alaska. The occurrence of deciduous and nonforested fuel types was lower within fire perimeters (40% of the area) than within the ecoregion (53%). The occurrence of mature spruce fuel types was higher within fire perimeters (47%) than within the ecoregion (39%). Within the fire perimeters, however, there were only small differences between the fraction of fuel types that burned and those that did not, indicating that during large fire years, fire burns evenly across these fuel types [73].

In general, fires in riparian [9,37] and tundra [111,141] communities are patchy because fuels are patchy. A patchy fire occurred on the Seward Peninsula in northwestern Alaska on a gentle hillslope near Imuruk Lake, where unburned tall willow shrublands, wet sedge-grass tundra, and open mat and cushion tundra lay within a matrix of burned sedge tussock tundra and sedge tussock-ericaceous, sedge tussock-mixed, and birch-ericaceous shrublands [111].

Fire size
The published literature reports little information on fire size in Alaskan alder and willow shrublands. However, it is likely that general patterns regarding fire size in Alaska apply to these systems. In general, fire sizes in Alaska range from very small to very large [29,33,47,137] and vary regionally [47]. Approximately 60% to 80% of all fires in Alaska are <12 acres (5 ha) [11], although large fires account for most of the total area burned [11,33,42,73,74]. From 1957 to 1979, average fire size ranged from as little as 9 acres (4 ha) in the Pacific Border Ranges province of southeastern Alaska to as much as 25,831 acres (10,453 ha) in the Alaska-Aleutian province of south-central Alaska, which encompasses the Alaska and Aleutian ranges (Table 4) [47].

Figure 3. Percent of each physiographic province in Alaska burned from 1957 to 1979. USDI, BLM image from Gabriel and Tande [47]

When and if fire occurs in Alaskan alder and willow shrublands, it is likely to occur during large fire years (i.e., years when area burned is >1.5 times the long-term average), when dry and warm weather occurs for extended periods. In Alaska, large fires typically occur episodically. Since the 1860s, large fire years occurred 17% of the time and accounted for 68% of the area burned throughout Alaska [73]. From 1950 to 1999, the average fire size during large fire years was 50,200 acres (20,300 ha), while the average fire size was 19,300 acres (7,800 ha) in all other years [74]. Large fire years occurred every 4 years on average [74], although the frequency of large fire years and extreme fire events has varied since the 1940s [73].

In general, fire size in Alaska is primarily driven by summer weather and climate patterns [13,29,36,43,73]. Large fires occur when extended periods of warm and dry weather (≥10 days) occur across the landscape [43]. This weather pattern—often caused by surface-blocking high-pressure systems—dries out fuels across the landscape and increases fire danger [36,43]. In interior Alaska, more shrubland area burned during warm and dry weather [78].

Climate pattern indices such as the Eastern Pacific (EP), Arctic Oscillation (AO), Pacific Decadal Oscillation (PDO) and El Niño/Southern Oscillation (ENSO) influence fire weather at intra-annual, interannual (AO and ENSO), and decadal (PDO) time scales; these indices are correlated with fire occurrence and size [29,36,43,56]. From 1940 to 1998, 15 of the 17 largest fire years occurred during a moderate to strong El Niño episode. These 15 years account for nearly 63% of the total area burned during that period [56].

Fire size may also be driven in part by vegetation type. DeWilde [32] found fewer large fires in open fuel types (open tundra, shrub/grass and boreal lichen) than forested fuel types (boreal spruce and mixed hardwood/spruce), although small fires occurred frequently in these fuel types.

Fire regimes and human activity
Fire regimes in the sparsely vegetated regions of Arctic and interior Alaska may not differ much from historical fire regimes because they have not been altered by human activities [24,32,33,35,54,134]. However, fire regimes in localized areas, such as near human settlements or along roads, may be influenced by activities such as logging, human ignitions, and fire suppression. In populated areas in interior Alaska designated for fire suppression, less area burns and more ignitions are human-caused than in remote areas [32]. From 1992 to 2000, on the 17% of land designated for full fire suppression, area burned decreased by 50% relative to areas designated for modified or no suppression, despite 50 times greater density of fires. While fire suppression reduced the area burned in all fuel types (i.e., boreal spruce, mixed hardwood-spruce, grassland and shrubland tundra, and boreal lichen), it was somewhat more effective in nonforest vegetation [33]. In interior Alaska, total ignition rates were higher near rivers than uplands. Human-caused ignitions were mostly within 0.6 mile (1 km) of rivers, even though lightning was the major cause of fires near rivers [22].

Humans ignite most wildfires in Alaska; however, human-caused fires tend to burn less area than lightning-caused fires [3,12,32,33,49,73,127]. Over the past 2 decades in Alaska, there were nearly twice as many human-caused as lightning-caused fires [73]. However, within the interior, human-caused fires were rarely >100 acres (40 ha) and therefore accounted for only 4.6% of the total area burned from 1992 to 2001 [33]. Human-caused fires tend to be small because they typically occur outside of the fire season in populated areas where they are easily suppressed, and they typically occur in only moderately flammable vegetation such as mixed hardwood-spruce and grassland and shrubland tundra [22,33].

Lightning-ignited fires occur from May to August [33,44,118] (see Fire season). Human-caused fires lengthen the fire season [32,33,79]. In the Fairbanks region, human-caused fires begin in March and continue through October, 2 months before and after the lightning-fire season [32].

Fire regimes and climate change
Warmer temperatures at northern latitudes are expected to reduce snow and ice cover and permafrost; decrease surface albedo; lengthen the snow-free season; and alter vegetation communities. These changes are likely to lengthen fire seasons, increase fire frequency, severity, and area burned, and contribute to vegetation changes (e.g., [3,14,17,25,26,29,40,65,72,73,120]). DeGrange and others [31] predicted changes in cover of ecotypes in northwestern Alaska during 2010 to 2100 based on rates of change during the past 30 to 50 years and projected future air temperatures (Table 5). Results indicated that nearly all ecotypes (56 of 60) are likely to undergo some change in area. Some ecotypes with alders and willows may decrease in area, while others may increase [31].

Paleorecords from 14 lakes in the Yukon Flats ecoregion indicate that fire frequency has been higher in recent decades than at any other time during the past 3,000 years [75]. Over the last decade, fire size and the frequency of large fire years have increased in Alaskan boreal forest due to climate warming. Four of the 11 largest fire years on record since 1940 occurred between 2002 and 2009 [73]. The largest Alaskan North Slope fire on record, the Anaktuvuk River Fire, occurred in 2007 [64].

Although the response of fire regimes to climate change is complex, the area burned across Arctic and boreal regions will likely increase with lengthening fire season, increasing moisture stress, and more human ignitions [58,73]. In Arctic tundra, where lowlands and poorly drained sites are abundant and permafrost is continuous [137], climate warming has increased and may continue to increase the height and/or cover of green alder, willow, and shrub birch (arctic dwarf birch and dwarf birch) in tundra systems. It has also caused boreal forest to expand into tundra at treeline. Predicted increases in the size and frequency of tundra fires and anthropogenic disturbances are likely to drive rapid proliferation of these shrubs in the Arctic [24,25,28,98,105,113,116,121,122,126,128,139].

Shrubland expansion may impact future fire regimes in the Arctic. Paleological studies suggest that increased abundance of scrub birch could increase fire frequency, while increased abundance of alder and willow may decrease fire frequency [6,58,60] (see Ancient times). Beck and others [16,17] showed that predicted increases in fire frequency and severity in Alaska may convert black spruce forests to relatively less flammable communities comprised of deciduous shrubs and trees. If the deciduous cover is long-lasting, it could reduce future fire frequency in the boreal forest region [16,17].

In poorly drained lowlands in the discontinuous permafrost zone, thawing of permafrost and subsequent development of thermokarsts may convert some spruce forests and woodlands to ponds or wetlands. Ponds and wetlands absorb more radiation and thus are likely to accelerate rates of thaw and landscape change [25]. Increases in thermokarsts will likely contribute to the spread of alders and willows [97]. Forty-two percent of Alaska's boreal region is comprised of lowlands, of which 13% are susceptible to thermokarst development (Jorgenson and others 2007 cited in [69]). Researchers estimated that >42% of their 652,269-acre (263,964 ha) study area on the Tanana Flats in central Alaska had thermokarst development. Because of thermokarst development, area of fen meadows increased from 31% in 1949 to 40% in 1995, while lowland paper birch forest area decreased from 23% in 1949 to 15% in 1995. Through the process of paludification, these fen meadows may succeed to sageleaf willow and sweetgale shrublands [70].

In lowlands underlain by gravel, permafrost is uncommon, and climate warming may result in drying of lakes, ponds, and wetlands. Trees and shrubs, especially willows and shrub birches, establish in the newly dried areas, resulting in further drying [18,25]. Decreased size and number of ponds and wetlands have been reported in both interior [104,115] and south-central [18,77] Alaska since the 1950s. Researchers examined >10,000 closed-basin ponds throughout Alaska. They found reductions in the area and number of shallow, closed-basin ponds since the 1950s for all boreal regions but not for the Arctic Coastal Plain region [115]. In the permafrost-free Kenai Peninsula, aerial photography from 1950 to 1996 revealed a 67% reduction in wetland area (including muskegs, kettle ponds, closed- and open-basin lakes) for 3 subregions [77]. A study on the Kenai Peninsula showed that on 11 herbaceous wetland sites, shrubland area increased 11.5%/decade from 1951 to 1968 and 13.7%/decade from 1968 to 1996, while wetland area covered by herbs decreased, and wetland area covered by trees increased [18]. Establishment of shrubs in herbaceous wetlands may reduce albedo and increase atmospheric heating (e.g., [24,26]), causing wetlands that may have served as firebreaks in the past to become "fuel bridges" comprised of shrublands and forests [14,18]. Continuity of fuels across the landscape in both uplands and wetlands may facilitate increased rates of fire spread and much larger areas being burned [18,78].

Invasive species may increase in importance with climate warming in Alaska. On the Kenai Peninsula, green alder may become more susceptible to damage caused by nonnative green and woolly alder sawflies, which defoliate green alder [146].

LIMITATIONS OF INFORMATION
Information on fire history in Alaskan alder and willow shrublands is scarce. More research is needed on fire regimes of these Alaskan shrublands [78]. Alaskan fire records only date back to the 1940s [39], so information on Alaska's recent fire history is also incomplete. Paleological studies of fire history have small sample sizes and may not have modern climate analogs (e.g., the forest-tundra period described in [58]), which complicates the application of ancient fire frequencies to models of modern-day fire frequencies. A lack of long-term fire records and fire history research makes it difficult to project effects of climate change [31].

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