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Sources: Unless otherwise indicated, the information in this Research Project Summary comes from the following papers:
Dyrness, C. T. 1965. The effect of logging and slash burning on understory vegetation in the H. J. Andrews Experimental Forest. Res. Note PNW-31. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Forest and Range Experiment Station. 13 p. [3].
Dyrness, C. T. 1973. Early stages of plant succession following logging and burning in the western Cascades of Oregon. Ecology. 54(1): 57-69 [4].
Halpern, C. B. 1989. Early successional patterns of forest species: Interactions of life history traits and disturbance. Ecology. 70(3): 704-720 [6].
Halpern, Charles B. 1987. Twenty-one years of secondary succession in Pseudotsuga forests of the western Cascade Range, Oregon. Corvallis, OR: Oregon State University. 239 p. Dissertation [7].
Halpern, Charles B. 1988. Early successional pathways and the resistance and resilience of forest communities. Ecology. 69(6): 1703-1715 [8].
Halpern, Charles B.; Franklin, Jerry F. 1989. Understory development in Pseudotsuga forests: Multiple paths of succession. In: Ferguson, Dennis E.; Morgan, Penelope; Johnson, Frederic D., compilers. Proceedings--land classifications based on vegetation: Applications for resource management; 1987 November 17-19; Moscow, ID. Gen. Tech. Rep. INT-257. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Research Station: 293-297 [10].
Halpern, Charles B.; Franklin, Jerry F. 1990. Physiognomic development of Pseudotsuga forests in relation to initial structure and disturbance intensity. Journal of Vegetation Science. 1(4): 475-482 [11].
Halpern, Charles B.; Spies, Thomas A. 1995. Plant species diversity in natural and managed forests of the Pacific Northwest. Ecological Applications. 5(4): 913-934 [12].
Lutz, James A.; Halpern, Charles B. 2006. Tree mortality during early forest development: A long-term study of rates, causes, and consequences. Ecological Monographs. 76(2): 257-275 [16].
STUDY LOCATION:
These studies took place primarily in watersheds 1 and 3 of the HJ Andrews Experimental Forest, located along the western slope of the central Cascade Range, 50 miles (80 km) east of Eugene, Oregon. One study [12] also includes information from similar logging prescriptions, but without slash burning, in watershed 10.
SITE DESCRIPTION:
Topography of these ~250-acre (100-ha) watersheds is characteristic of much of the western Cascade Range, with steep dissected slopes (averaging over 50%), and intermittent or perennial stream channels separating watersheds [6,16]. Colluvial soils are primarily volcanic, largely derived from pyroclastic tuffs and breccias with varying influence from andesite and basalt. Soils are shallow to moderately deep, and textures are loamy and moderately stony with generally high porosity and high water-holding capacity [3,6,10,16]. The climate is maritime with mild, wet winters and warm, dry summers [16]. Annual precipitation averages 91 inches (2,302 mm); only 6% falls from June to August [10]. Although snowfall is common, snowpacks rarely persist for more than 1 to 2 weeks [6]. Elevations range from 1,450 to 3,550 feet (442-1,082 m) [16].
PREFIRE PLANT COMMUNITY AND FUELS:
Before clearcutting, all watersheds were dominated by mature and old-growth Douglas-fir (~125 and 300-500 years old, respectively), with western hemlock present in a range of size and age classes [10,12]. Most Douglas-firs were 300 to 500 years old [4]. Understory vegetation on watershed 1 was composed of 6 plant communities arrayed along a gradient of available moisture: California hazelnut-salal and Pacific rhododendron-salal on the driest sites, vine maple-salal on sites with moderate moisture and fertility, vine maple-dwarf Oregon-grape and Oregon goldthread communities on moderately moist sites, and a western sword fern community on moist sites [10], generally along drainage channels and north-facing slopes. Five of these communities (all except the California hazelnut-salal community) were described on watershed 3 [3,4]; California hazelnut was rare on watershed 3.
| Table 1. Characteristics of the 6 understory communities on watersheds 1 and 3 arranged in order of increasing available moisture [8] | |||
| Plant community | Topographic position and aspect | Dominant understory growth form(s) | Dominant understory species |
| California hazelnut-salal* | ridgetops; south-facing upper slopes | tall shrubs | California hazelnut, vine maple, dwarf Oregon-grape |
| Pacific rhododendron-salal | ridgetops; midslope benches | tall shrubs (herbs and low shrubs) | Pacific rhododendron, salal |
| vine maple-salal | mid to upper south-facing slopes | herbs and low shrubs, tall shrubs | salal, vine maple |
| vine maple-dwarf Oregon-grape | mid to lower slopes | tall shrubs | vine maple |
| Oregon goldthread | mid to lower slopes | understory trees | western hemlock |
| western sword fern | bottom and steep, north- to east-facing slopes, seeps | herbs and low shrubs (tall shrubs) | western sword fern, vine maple |
| *The Californian hazelnut-salal community did not occur on watershed 3. | |||
Study sites are classified in the following plant community and probably historically experienced the fire regime described below:
| Table 2. Fire regime information for the vegetation community studied in this Research Project Summary. Fire regime characteristics are taken from the LANDFIRE Rapid Assessment Vegetation Model [15]. This vegetation model was developed by local experts using available literature, as documented in the PDF file linked from the Potential Natural Vegetation Group listed below. | |||||
| Vegetation Community (Potential Natural Vegetation Group | Fire severity* | Fire regime characteristics | |||
| Percent of fires | Mean interval (years) |
Minimum interval (years) |
Maximum interval (years) |
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| Douglas-fir-western hemlock (dry mesic) | Replacement | 25% | 300 | 250 | 500 |
| Mixed | 75% | 100 | 50 | 150 | |
| *Fire Severities: Replacement=Any fire that causes greater than 75% top removal of a vegetation-fuel type, resulting in general replacement of existing vegetation; may or may not cause a lethal effect on the plants. Mixed=Any fire burning more than 5% of an area that does not qualify as a replacement, surface, or low-severity fire; includes mosaic and other fires that are intermediate in effects. Surface or low=Any fire that causes less than 25% upper layer replacement and/or removal in a vegetation-fuel class but burns 5% or more of the area [1,14]. |
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No information was available on prefire fuel characteristics.
PLANT PHENOLOGY:Watershed 3: fall (September 1963), low to high severity [6]
Watershed 10: harvested in fall 1975; large slash was removed without burning [12]
FIRE DESCRIPTION:
Each watershed was clearcut, after which slash was broadcast burned, except on watershed 10, where large slash was removed. In Douglas-fir forests of the Pacific Northwest, logging slash is typically broadcast burned to reduce fuel loads (thus lowering the probability of subsequent fire in the short term) and to control vegetation that would otherwise compete with conifer seedlings for available moisture, light, and nutrients [12].
On watershed 3, three areas of 12, 22, and 27 acres (5, 9, and 11 ha), totaling one-quarter of the watershed, were clearcut during winter 1962 to 1963, and slash was broadcast burned in September 1963. All clearcut units on watershed 3 were planted with 3-year-old Douglas-fir between March and May 1964. The entire 237 acres (96 ha) of watershed 1 was clearcut over 4 years, between fall 1962 and summer 1966, and slash was broadcast burned in October 1966. Douglas-fir was aerially seeded on watershed 1 during October 1967 and October 1968. Because regeneration was poor, 2- and 3-year-old Douglas-fir were planted in April and May 1969 and in April 1971 [6]. Despite aerial seeding and planting, most Douglas-fir regenerated through natural seeding on both watersheds [16].
Permanent plots were established in areas that were clearcut and broadcast burned and classified on the basis of type and severity of soil disturbance and fire severity (see Disturbance type and severity effects). Broadcast burning was patchy, with 50% of the permanent plots burned on watershed 3 and 61% on watershed 1 [7].
Watershed 10 is the smallest watershed (~10 ha), and all of it was clearcut in 1975. Slash >8 inches (20 cm) in diameter or >8 feet (2.4 m) long was removed; no burning was conducted. Douglas-fir seedlings were planted in 1976, 1977, and 1978. Only presence/absence data were collected on this watershed [12].
FIRE EFFECTS ON PLANT COMMUNITY AND FUELS:
Permanent plots were sampled before clearcutting, after clearcutting, and for up to 38 years after broadcast burning. Cover and frequency of vegetation were measured and analyzed to evaluate the interactions between predisturbance plant community and disturbance severity, and their relationship to postdisturbance succession, community composition, and plant species diversity [10]. Early postdisturbance (1-5 years after broadcast burning) changes in plant community composition were initially studied on watershed 3 [3,4], and later on watershed 1. Additional postdisturbance measurements were made annually through 1972 on watershed 3 and through 1973 on watershed 1, and generally biennially thereafter through 1983 [8,10]. Watersheds 1, 3, and 10 were sampled consistently from that time through 1989 [12].
Halpern [8] used detrended correspondence analysis "to examine the successional relationships among 6 understory communities exposed to a gradient of disturbance intensity", using data from permanent plots on watersheds 1 and 3. He used this information to characterize the resistance (i.e., the extent to which a community resists change by disturbance) and resilience (i.e., the rate, manner, or degree to which initial community characteristics are restored following displacement) of prelogging forest communities on these watersheds. In his 1989 publication [6], Halpern examined patterns of species abundance (cover and frequency) and characteristics of species responses to disturbance using data collected on watersheds 1 and 3, supplemented with published accounts of species life history traits and postdisturbance responses.
In these studies, species were classified as "invaders" if they were either not present in the unlogged stand or if they were present in very small quantities in the unlogged stand but restricted to severely disturbed sites, such as windthrow areas. "Residual" species were those present on undisturbed sites before clearcutting, even if only in very small quantities [4]. For consistency, these terms are used throughout this summary and should not be confused with the terms "nonnative" or "invasive" species.
Nonnative species contributed minimally in cover and number to postharvest vegetation. For all but one sampling date, native taxa comprised >95% of total plant cover and >90% of all species. It is likely that nonnative taxa were uncommon because of the general distance of these sites from source populations in adjacent clearcut or disturbed sites. Watersheds 1 and 3 were buffered from most nonnative plant populations by old-growth and young, closed-canopy stands. The minor peak in nonnative plant cover in postdisturbance year 2 (11% and 17% of total cover on watersheds 1 and 3, respectively) coincided with maximal abundance of the nonnative woodland ragwort [6,12]. The causes and consequences of woodland ragwort's ephemeral dominance in these communities are the subject of additional field experiments by Halpern and others (e.g., [9]).
Early postdisturbance patterns on both watersheds reflect a temporary loss of community dominants such as vine maple and dwarf Oregon-grape and the establishment of ruderal annuals, especially woodland ragwort and tall annual willowherb. Subsequent changes were less dramatic and reflect gradual shifts in abundance of persistent invading and residual species [7,10]. Invading herbs dominated early succession, but their importance was relatively short lived. In contrast, residual herbs (e.g., white hawkweed and broadleaf starflower) and subshrubs (e.g., common whipplea, twinflower, and trailing blackberry) were prominent 4 to 5 years after broadcast burning and remained so through the initiation of canopy closure. Most predisturbance dominants in the herb layer on both watersheds regained or surpassed their predisturbance canopy cover by final sampling (20 years after broadcast burning on watershed 1, 24 years after on watershed 3) [7,10,11]. Differences in individual species' responses were related to predisturbance plant community composition, site characteristics, disturbance history, disturbance severity [3].
Three major trends were evident: vegetation stature increased with time, invaders showed greatest abundance and persistence on burned plots and those with the most soil disturbance, and residuals displayed greatest development on undisturbed plots (see Table 5 and Table 6 for descriptions of disturbance classes). Contrasting patterns between watersheds were attributed to differences in predisturbance plant community composition and history of logging and burning. Many spatial and temporal trends in vegetation were explained by the life history traits of species in response to disturbance. Abundance patterns of invading species were correlated with propagule origin, phenology, spatial and temporal variation in disturbance, and potential for vegetative reproduction. Abundance patterns of residual species reflected initial species distributions, resistance to disturbance, regeneration strategies, morphology, and spatial variation in disturbance [7].
Postdisturbance years 1 to 5: Most understory species persisted through disturbance: on watershed 1, 92% survived clearcutting and 82% survived broadcast burning; on watershed 3, 71% were present after clearcutting and after broadcast burning [6,7]. Total cover of shrubs and trees decreased dramatically and remained low in the first few years after clearcutting and broadcast burning. Herbaceous cover initially decreased after clearcutting, but it increased in the short term after broadcast burning [3].
Many residual species persisted, but invading species were common; 11 to 19 species established each year for 2 years after broadcast burning [6,7]. Forty-eight species occurred in undisturbed forest, and 72 species occurred 5 years after clearcutting and broadcast burning. Most new plant species established in the first 3 postdisturbance years. Relatively few prelogging species were absent from plots 5 years after clearcutting and broadcast burning (but see Local species extinctions). All tree species present before clearcutting were present on plots 5 years after broadcast burning, and no additional tree species established during that time. Residual tall shrubs dominated until the 5th year after broadcast burning, when invading shrubs became dominant. In the herb layer, residual herbs and low shrubs dominated during the first 2 years after clearcutting and broadcast burning, after which invading herb cover surpassed that of residual species for 2 years. Cover of residual species that showed a marked decrease after clearcutting and broadcast burning had increased slightly by postfire year 5. Information on annual abundance of individual species and species groups on watershed 3 during the first 5 years after broadcast burning is shown in Table 3 [4].
Postdisturbance years 1 to 20: Clearcutting substantially reduced diversity of forest understory species, and broadcast burning further reduced species numbers. However, losses in total species diversity were short lived. On both watersheds, species richness and heterogeneity exceeded preharvest levels within 1 to 3 years after burning, due to rapid colonization by a diverse, invading flora, and gradual reestablishment or recovery of residual understory species. Thereafter, species richness increased continuously with time, despite partial closure of the tree canopy. Most additional taxa were native, ruderal herbs. Ruderal establishment was most common during the first 2 growing seasons after broadcast burning, although new species continued to establish through final sampling (20 years after disturbance). Residual species regained dominance within 5 to 10 years, and populations of most vascular plant species recovered to predisturbance levels prior to canopy closure [12]. Species richness increased continuously on watershed 1, from 83 species before clearcutting (in 1962) to 138 species 21 years later. Similarly, on watershed 3, species richness increased from 48 to 90, despite a temporary loss of 14 species after clearcutting. Total canopy cover of herb and low shrub species peaked rapidly, at 93% in postdisturbance year 2 on watershed 1 and at 94% in year 6 on watershed 3. Canopy cover of tall shrub and regenerating tree species increased more slowly, but continuously, over time. At final sampling on both sites, tall shrub and tree cover peaked at 76% and 44%, respectively, on watershed 1, and 60% and 63%, respectively, on watershed 3 [6]. More detailed information on floristic and physiognomic changes is presented by Halpern [7].
Individual species abundance fluctuated during the first 20 years after disturbance. Halpern [6] grouped species according to seral origin (invaders vs. residuals), and the timing, duration, and magnitude of peak abundance (Table 4).
Patterns of invaders: Annual invader species were the first to peak in abundance. Woodland ragwort and tall annual willowherb were the most widespread and had a sharp spike in canopy cover 2 years after broadcast burning on both watersheds. Fireweed established, spread fairly rapidly, and began to dominate the herb layer on both watersheds 2 years after disturbance; it persisted as a major herb through most of the study period. Initial establishment of fireweed was primarily from off-site, wind-dispersed seed, and subsequent spread was more likely due to lateral spread via rhizomes. Minor invading herbs that reached greatest abundance between years 3 and 5 were wind-dispersed Asteraceae including annual agoseris, bull thistle, prickly lettuce, bigflower agoseris, Canada thistle, clustered thistle, and Wright's cudweed. These species as a group were most abundant within lightly and heavily burned microsites and in the vine maple-salal community. Minor invading herbs and shrubs reached peak abundance between postdisturbance years 3 and 9. The tall shrubs snowbrush ceanothus and redstem ceanothus established from buried seed and dominated around 5 to 10 years after disturbance. Scattered ceanothus individuals established after clearcutting on watershed 1; however, most ceanothus established during the first growing season after broadcast burning. Subsequent establishment declined rapidly with time. Snowbrush ceanothus abundance peaked sharply and declined rapidly between years 9 and 11, and declined more slowly thereafter. Invading perennials of various growth forms dominated more than 10 years after broadcast burning. The most abundant of these were thimbleberry and western brackenfern, although Scouler willow and bitter cherry were locally common. Perennials had the greatest longevity of any of the invading species. Both thimbleberry and western brackenfern occurred in predisturbance plots but were classified as invaders because they were rare and restricted to disturbed microsites before clearcutting and burning. These species may have established from the seed bank (thimbleberry and bitter cherry), animal-dispersed seed (thimbleberry and bitter cherry), or wind-dispersed seeds and spores (Scouler willow and western brackenfern). Persistent increases in abundance of thimbleberry and western brackenfern were likely by extensive lateral spread of deeply buried rhizomes ([6] and references therein).
Patterns of residuals: Abundance of predisturbance understory dominants on both watersheds (vine maple, western sword fern, salal, Pacific rhododendron, dwarf Oregon-grape, California hazelnut, western hemlock) was dramatically reduced after clearcutting and broadcast burning. Recovery was continuous but varied among species. Species in this group were dominant within portions of both watersheds around 10 years after broadcast burning. Vegetative recovery was likely the principal means of regeneration; multiple-stemmed, sprouting individuals were common. Together with annual and perennial invaders, trailing blackberry dominated the ground layer as early as 2 to 5 years after broadcast burning, exhibiting the most rapid spread of any forest residual. Trailing blackberry was an inconspicuous member of the predisturbance community, but it remained prominent on both watersheds throughout the 20-year study period. It spread from stems that trailed and climbed over logs, stones, and stumps; among associated herb and low shrub species; and vertically into the crowns of tall shrubs and trees. Trailing blackberry is likely to persist as a dominant component of the ground layer until it is suppressed by canopy closure. Two initially subordinate forest herbs (broadleaf starflower and white hawkweed) and one subshrub (common whipplea) had an early and short period of peak abundance around 5 to 10 years after disturbance. Abundance of other subordinate forest species (Oregon goldthread, evergreen violet, red huckleberry, golden chinkapin, redwood-sorrel, snow raspberry, bigleaf maple, Pacific dogwood) was initially reduced but showed relatively minor long-term changes. Patterns of abundance varied among species. Red huckleberry and golden chinkapin showed gradual, continuous increases in cover on both watersheds. Their postdisturbance distribution generally resembled their predisturbance distribution, with the exception of heavily burned plots. Postdisturbance dynamics of Oregon goldthread and evergreen violet were correlated more with differences in disturbance severity than with predisturbance distributions (see Disturbance type and severity effects). A group of uncommon, but characteristic, forest species (prince's-pine, western redcedar, western rattlesnake plantain, snowqueen, Pacific yew) had minimal postdisturbance recovery. Reestablishment of western redcedar was delayed for 8 to 9 years within the moist Oregon goldthread and western sword fern communities on watershed 3. Its long-term recovery is largely dependent on regeneration by seed. Its greater postdisturbance abundance on watershed 3 was likely due to greater edge effect and seed availability associated with small cutting units. Prince's-pine was severely reduced on burned plots of both watersheds and was completely eliminated from the sample plots of all but one community on watershed 1 [6]. Loss of its long-lived, evergreen leaves may reduce prince's-pine's survival after fire (Antos personal communication cited by [6]). Although it has deep roots and rhizomes, regenerating buds apparently occur only near the soil surface [17].
Local species extinctions: Observations of up to 27 years (the point where tree canopies began to close) following clearcutting with (watersheds 1 and 3) and without broadcast burning (watershed 10) suggest that some species may be locally eliminated by clearcutting and/or broadcast burning. Five of 57 original forest species were absent from sample plots on watershed 1; 4 of 44 from watershed 3; and 7 of 55 from watershed 10. Eleven taxa were lost from at least 2 of the 9 watershed × disturbance class categories representing both unburned and burned conditions: the herbs bride's bonnet, Pacific coralroot, western rattlesnake plantain, whiteveined wintergreen, snowqueen, oneleaf foamflower, and beargrass; the subshrubs prince's-pine and little prince's-pine; and the understory tree, Pacific yew. Most species that were absent after disturbance were lost from heavily burned plots (see Disturbance type and severity effects) [12].
Disturbance type and severity effects: Plots were initially classified using 4 postlogging soil surface disturbance classes defined by Dyrness [2] and 4 slash burning disturbance classes defined by Dyrness and Youngberg [5] (Table 6). For subsequent analyses, plots were subjectively assigned to one of the soil disturbance classes described in Table 6 [7].
| Table 5. Descriptions of disturbance classes used for evaluating effects of disturbance severity [7] | |
| Disturbance class | Description |
| Undisturbed | The soil surface appeared similar to that prior to disturbance, with minimal mixing of mineral soil and litter and with no evidence of fire. |
| Disturbed and unburned | Disturbance from logging was evident. Although litter was removed or mixed with mineral soil, there was no evidence of fire. |
| Lightly burned | Surface litter was charred by fire but was not completely removed. |
| Heavily burned | Surface litter was completely consumed by fire, exposing a highly colored mineral soil. |
In the first year after broadcast slash burning, plant cover generally decreased with increases in disturbance severity (Table 6):
| Table 6. Plant cover (%) on watershed 3 following clearcutting and broadcast burning on plots classified by disturbance severity [3] | ||||
| Disturbance class | Trees | Shrubs | Herbs | Total |
| After clearcutting* (1963) | ||||
| Undisturbed | 5.3 | 19.1 | 4.0 | 28.4 |
| Heavy slash and logs | 2.8 | 2.7 | 0.1 | 5.6 |
| Slightly disturbed | 1.0 | 4.2 | 3.8 | 9.0 |
| Deeply disturbed or compacted | 0.4 | 1.1 | 0.1 | 1.6 |
| After broadcast burning (1964) | ||||
| Undisturbed | 7.7 | 14.6 | 19.4 | 41.7 |
| Disturbed-unburned | 0.8 | 3.0 | 15.4 | 19.2 |
| Lightly burned | 1.4 | 2.5 | 2.9 | 6.8 |
| Heavily burned | 0.1 | 0.6 | 0.4 | 1.1 |
| *Postlogging disturbance classes based on those described by Dyrness [2]. Subsequent studies [4,6,7,8,9,10,11,12] do not use this level of discernment among logging disturbance classes and classify plots as either disturbed or undisturbed, effectively grouping the 3 levels of soil disturbance initially described. There is also no distinction between burned plots as to whether the soils were disturbed or undisturbed before burning. | ||||
Differences in disturbance type and severity strongly influenced early succession. Plots where the forest floor remained mostly undisturbed had vegetation that most closely resembled the prelogging understory composition. Residual species often increased in cover following overstory removal, fully occupying the site and excluding invaders. In disturbed-unburned and lightly burned plots, residual shrubs typically dominated [4]. Increasing severity of disturbance shifted early dominance of the ground layer from long-lived residual forest species that are released by canopy removal (e.g., twinflower and common whipplea) to short-lived colonizers (e.g., woodland ragwort and tall annual willowherb) with establishment that is promoted by exposed soil, enhanced nutrient levels, and reduced competition from surviving vegetation [6]. The importance of wind-dispersed annuals in these communities was enhanced by disturbance, because availability of germination sites on mineral soil increased with disturbance severity [8]. The rate and magnitude of recovery of residuals decreased with disturbance intensity. For example, cover of residuals peaked after 6 years on undisturbed and disturbed-unburned plots (114% and 62%, respectively), but only after 13 and 16 years on lightly and heavily burned plots, respectively (60% cover on each type) [7]. Cover of residuals dwarf Oregon-grape, salal, and western sword fern declined with increased disturbance severity, while that of invading shrubs (e.g., ceanothus) generally increased as disturbance severity increased [6]. Shifts from herb to shrub dominance occurred earlier on burned than on unburned plots due to rapid development of invading shrubs with fire-stimulated germination and establishment. Subsequent transitions to tree dominance showed no clear relationship with disturbance severity. Tree cover developed most rapidly on heavily burned plots on watershed 1 and on disturbed-unburned plots on watershed 3 [11].
Measures of resistance and resilience varied dramatically among soil disturbance classes in these communities, because predisturbance vegetation and buried seed responded differently to disturbance severity. Resistance and resilience decline with the loss of aboveground structures and with early establishment and persistence of invading species [8]. Changes in species richness and heterogeneity were correlated with disturbance severity but were transient, largely limited to the first 4 years of succession. On burned plots, residual species richness remained below predisturbance levels for up to 20 years after burning [12].
Abundance patterns for some species were related to disturbance severity, although others, such as trailing blackberry, showed little relationship with disturbance severity or community type. Trailing blackberry appears tolerant of fire and highly competitive for space after canopy removal, although its spread was delayed on heavily burned microsites [6]. Of the 11 species that were eliminated from both burned and unburned plots, some species (prince's-pine, western rattlesnake plantain, wintergreen) appear as sensitive to removal of the tree canopy as to fire. However, relatively few taxa were eliminated by physical soil disturbance or light burning, and many more by severe fire. Several species were lost only from burned plots [12]. See Table 7 for information on individual species' postdisturbance response and life history characteristics.
Undisturbed-unburned and disturbed-unburned plots: On both watersheds composition on undisturbed plots changed relatively little after broadcast burning because sprouting dominants, such as vine maple, salal, redwood-sorrel, and Pacific rhododendron persisted, and few invading annuals established [4,7,8]. Vine maple, red huckleberry, western hemlock, redwood-sorrel, and salal were largely restricted to undisturbed areas. Pacific dogwood, Pacific yew, Pacific twinflower, western sword fern, and trailing blackberry were not restricted to undisturbed areas, but they occurred there in largest quantities [4]. Residual shrubs spread dramatically and maintained high cover on undisturbed plots through final sampling, although residual herbs maintained dominance through initiation of canopy closure [11]. The prominence of residuals on undisturbed plots reflected early expansion of trailing blackberry and twinflower populations, as well as continuous, 2- to 3-fold recovery of salal and western sword fern [7]. The only invading shrub on undisturbed plots was snowbrush ceanothus. Increased insolation following canopy removal was apparently sufficient to stimulate germination of a few snowbrush ceanothus seeds on some unburned plots [4,11]. Fireweed was absent or nearly so from undisturbed plots on watershed 3 [3]. On undisturbed plots in the western sword fern community on watershed 1, fireweed was prominent in early succession due to rhizomatous spread during the 4 years of staggered harvesting prior to broadcast burning. This expansion was followed by a dramatic but temporary release of the residual trailing subshrub common whipplea and subsequent recovery of western sword fern. On undisturbed plots in Oregon goldthread communities, surviving western hemlock quickly reestablished a dense understory tree canopy, largely preempting annual and perennial invaders [8].
Areas in watershed 3 with soil disturbed by clearcutting but not burned supported a variety of both residual and invader species during the first 5 postdisturbance years. Both residual and invading herbs showed a preference for disturbed-unburned plots. Broadleaf starflower and common whipplea were the principal species contributing to peak residual herb cover [7]. Thimbleberry characterized disturbed-unburned plots, germinating from a seed bank or from newly deposited seed [7]. Whitebark raspberry showed a marked preference for disturbed-unburned areas, while snowbrush ceanothus cover was lowest on these plots [4]. Dwarf Oregon-grape, western sword fern, and stickywilly occurred to a limited extent on disturbed-unburned plots [3]. Pacific rhododendron was absent from disturbed-unburned plots both before and after clearcutting and burning [4].
Burned plots: Major changes in composition occurred on both lightly and heavily burned plots and were characterized by a temporary loss of residuals and widespread establishment of invading annuals. Cover of many residuals was initially and persistently reduced on burned plots. Although residual community dominants such as vine maple, salal, and western sword fern may survive and sprout after fire, aboveground structures may be absent for several years after fire; recovery of these species was typically slower on burned than on unburned plots in these studies [4,8]. Herbaceous invaders common to both lightly and heavily burned plots included fireweed, fringed willowherb, tall annual willowherb, bull thistle, Wright's cudweed, and woodland ragwort; fireweed was the most important herb in both burn classes [4]. In all plant communities, burned plots were initially dominated by invading annuals such as woodland ragwort and tall annual willowherb [8]. Two years after broadcast burning, invading annuals reached their maximum abundance [8,10]. Invading perennials such as fireweed and snowbrush ceanothus were also common on burned plots [4]. Residual herbs regenerated relatively slowly on burned plots [11], and residual herb cover was reduced on early- to midseral burned plots in both watersheds [7]. The most important residual species in both lightly and heavily burned plots were sweetscented bedstraw, white hawkweed, broadleaf starflower, and trailing blackberry [4]. Abundance of species with strategies of aboveground perennation (e.g., twinflower and common whipplea) increased on unburned plots, but they were largely replaced by species establishing from seed (e.g., white hawkweed) on burned plots. Fire stimulates germination of snowbrush ceanothus and redstem ceanothus seeds, and these species dominated both lightly and heavily burned plots [7]. Early establishment and rapid spread of ceanothus resulted in an earlier transition to shrub dominance on burned plots. On watershed 1 transitions from herb to shrub dominance occurred earlier on burned (11 to 12 years) than on unburned plots (15 years). On watershed 3, a shrub phase occurred only on burned plots, 18 to 19 years after disturbance [11]. Several species were lost only from burned plots, including northern maidenhair, Pacific rhododendron, Pacific yew, western redcedar, and big huckleberry [12]. Successional trajectories and individual species' response varied among plant communities (see Patterns among understory plant communities).
Burn severity: Lightly burned plots showed profound changes in community composition, and recovery of original community composition on watershed 1 was slower on lightly burned than on unburned plots. Cover of many residuals was initially and persistently reduced, and all communities were initially dominated by invading annuals such as woodland ragwort and tall annual willowherb. Subsequent successional trajectories among plant communities diverged (see Patterns among understory plant communities) [8]. More species occurred on lightly burned plots than on heavily burned plots during the first 5 postdisturbance years on watershed 3. Residual shrubs and trees such as bigleaf maple, vine maple, and Pacific rhododendron were more abundant in lightly burned areas than in heavily burned areas. Some invading herbs occurred on lightly burned plots but were absent from heavily burned plots (Table 7) [4].
Total cover on heavily burned plots consistently lagged behind cover for other plots; substantially lower tree cover on heavily burned plots was especially striking [4]. Although populations of most vascular plant species recovered to predisturbance levels prior to canopy closure, species diversity remained depressed for more than 2 decades on heavily burned plots, and some species experienced local extinctions [12]. Dwarf Oregon-grape, western sword fern, and stickywilly were absent from heavily burned plots in the first postdisturbance season on watershed 3 [3]. Very few shrub and tree species were present on heavily burned plots 5 seasons after burning [4]. Heavily burned plots in all communities were persistently dominated by invaders: initially by annuals (woodland ragwort, tall annual willowherb, Siberian springbeauty), secondarily by biennials and small-statured perennials (agoseris, western pearly everlasting, whitebark raspberry), and subsequently by tall, woody perennials (snowbrush ceanothus, redstem ceanothus, thimbleberry). Some subordinate forest herbs increased on heavily burned plots, but understory dominants were greatly reduced (western sword fern) or eliminated (western hemlock) [7,8]. Germination and establishment of white hawkweed were promoted on heavily burned plots. Postdisturbance distribution of red huckleberry and golden chinkapin generally resembled their predisturbance distribution, except on heavily burned plots [6].
Snowbrush ceanothus was the most important shrub on both lightly and heavily burned plots; it and fireweed were most abundant on heavily burned plots during the first 5 postdisturbance years on watershed 3 [4]. The importance of both ceanothus species increased with intensity of disturbance. Peak cover on heavily burned plots was 3 to 5 times as great as that on undisturbed plots. On watershed 1, snowbrush ceanothus abundance was positively correlated with burn severity. However, on watershed 3 its abundance was similar on lightly and heavily burned plots (see Watershed-level patterns) [6]. Ceanothus establishment generally increases with burn severity ([11] and references therein).
Patterns among understory plant communities: After clearcutting and broadcast burning, all understory communities were initially dominated by invading annuals such as woodland ragwort and tall annual willowherb, but subsequent successional trajectories among plant communities diverged. For example, an otherwise uncommon suite of annual (e.g., Canadian horseweed) and perennial species (American vetch, hairy manzanita) invaded the vine maple-salal community, whereas other uncommon perennials (e.g., field horsetail, fringed willowherb, currant) colonized the Oregon goldthread community [8].
Predisturbance community composition and structure influenced postdisturbance succession more than invading species, which seemed more sensitive to timing or severity of disturbance [7]. Some residual species were most abundant or largely restricted to particular community types. For example, broadleaf starflower was associated with peak herb cover on watershed 1 but was largely restricted to the vine maple-dwarf Oregon-grape, Oregon goldthread, and western sword fern communities [7]. Responses of invading shrubs were more complex, particularly on watershed 3, where greatest development was on initially shrub-dominated, mesic to dry sites. Although the dominant snowbrush ceanothus may possess an affinity for mesic sites, disturbance history and environment interact in determining its distribution and abundance (see Watershed-level patterns) [11].
The driest and wettest understory plant communities were easiest to recognize during the first few years after disturbance. Many of the drier sites that supported Pacific rhododendron-salal or vine maple-salal understory communities before clearcutting were dominated by snowbrush ceanothus 5 years after broadcast burning on heavily burned sites. Wet sites were often dominated by closed stands of ferns, forbs, and low shrubs 5 years after broadcast burning. Some invaders (snowbrush ceanothus, bigflower agoseris, and Wright's cudweed) were restricted to sites that supported more xeric communities prior to clearcutting, and others (whitebark raspberry and western pearly everlasting) occurred on plots characteristic of more mesic communities [4].
Subsequent rates of succession and growth-form recovery were not related to available soil moisture. Recovery of understory dominants was comparable among relatively xeric, mesic, and moist communities. This is due to the capacity for most forest species in these communities to persist through disturbance, sprout from root crowns or rhizomes, and spread vegetatively, despite altered environmental conditions [11].
Differences among plant communities largely reflected the disturbance tolerance of component herbs and shrubs, and successional changes were closely related to predisturbance community structure and life form dominance. Initially herb-rich communities maintained herb dominance through initiation of canopy closure; shrub-dominated types generally regained shrub dominance after an initial herb phase; and tree-dominated sites progressed most rapidly toward canopy closure. In herb-rich sites on watershed 3 (vine maple-salal and western sword fern communities), a transitional shrub phase was absent because herbs dominated for most of the study period. The western sword fern community on watershed 3 was typified by a lush herbaceous flora, which showed dramatic increases in subordinate forest herbs in response to low levels of disturbance (50% of plots were unburned) [11]. In communities with well-developed shrub layers (California hazelnut-salal, Pacific rhododendron-salal, vine maple-salal, and vine maple-dwarf Oregon-grape), shrubs became codominant earlier than in herb-rich types (western sword fern). However, because residual shrubs were subordinate to invaders for most of the period prior to canopy closure, transitions to shrub dominance were largely tied to the relative growth of invader shrubs. The dominant shrubs in undisturbed forests, vine maple and Pacific rhododendron, are moderately resistant to fire, but their postdisturbance regeneration is slower than that of invaders such as ceanothus. Therefore, except on initially shrub-dominated sites that were undisturbed and unburned, residual shrubs contributed minimally to structure and changes in early succession. In shrub-dominated communities on watershed 3 (Pacific rhododendron-salal and vine maple-dwarf Oregon-grape), herb dominance was relatively brief. In an extreme example on watershed 1, unusually high establishment and spread of invader shrubs in the vine maple-salal type (>100% cover) promoted very rapid transition to shrub dominance. In the tree-dominated but otherwise depauperate Oregon goldthread community, shrubs displayed minimal importance. Canopy closure occurred most rapidly in this community, and tree cover averaged 81% on watershed 1 and 114% on watershed 3 in postdisturbance years 20 and 24, respectively [11].
Resistance and resilience differed among understory plant communities. Western sword fern and Pacific rhododendron-salal communities had high resistance and resilience. Initial understories in these communities were rich and structurally complex, with combined herb and shrub cover averaging 86% and 102%, respectively. Although invaders briefly dominated the postdisturbance flora in both communities, residuals were also abundant. Surviving herbs and shrubs served as centers for vegetative spread, promoting relatively rapid community recovery [8]. The Oregon goldthread community had low resistance and resilience; invaders and initially subordinate residuals dominated after even low-severity disturbance, and a lack of residuals resulted in poor long-term recovery. Predisturbance community composition was not very diverse and consisted primarily of dense subcanopies of western hemlock, with combined shrub and herb cover averaging only 14% [8,10].
Successional trajectories for the moist western sword fern community remained relatively distinct. Dominance rapidly shifted from annual (e.g., Siberian springbeauty) and perennial (e.g., fireweed, western pearly everlasting, thimbleberry) invaders to initially subordinate (e.g., common whipplea, sweetscented bedstraw, redwood-sorrel, white insideout flower) and dominant (e.g., western sword fern) residual forest herbs [7,8]. Snowbrush ceanothus showed limited development in the western sword fern community. Early successional trajectories in dry (Pacific rhododendron-salal and vine maple-salal) and mesic (vine maple-dwarf Oregon-grape and Oregon goldthread) communities in watershed 3 were similar, reflecting the broad ecological amplitude and widespread establishment of herbaceous invaders and a similar floristic composition among these communities [8]. Annuals such as tall annual willowherb and woodland ragwort characterized dry to mesic communities in early succession [7]. Successional trajectories subsequently diverged as abundance of invading herbs declined and initial understory dominants recovered [8].
Watershed-level patterns: Postdisturbance changes in vegetation structure differed between watersheds. On watershed 1 herbs dominated for 11 years, after which shrubs became codominant. In contrast, on watershed 3 herbs dominated for 18 years, after which trees became codominant; a distinct, transitional shrub phase was absent on watershed 3 [11]. At both the plot and site levels, dominant understory species recovered more slowly and invaders were more common on watershed 1 than on watershed 3 [11,12]. Temporal patterns of fireweed establishment and spread also differed between watersheds [12]. These trends may reflect watershed-level differences in disturbance intensity, seed availability, local fluctuations in weather [11], and timing and duration of disturbances [12].
On both watersheds long-term herb dominance was due to early establishment and spread of invading herbs, persistent release of residual herbs, and comparatively slower development of shrubs [11]. Total herb and low shrub cover increased rapidly on watershed 1, and peaked at 93% within 2 growing seasons after broadcast burning. This was likely due to the staggered harvesting on watershed 1 that allowed rapid and widespread establishment of winter annuals. Total herb and low shrub cover increased more slowly on watershed 3 and peaked at 94% in year 6. Principal colonizers on watershed 3 were also annuals, but fireweed was equally abundant. Trailing blackberry was the dominant residual, although trailing subshrubs (e.g., twinflower and common whipplea) and tuberous herbs (e.g., broadleaf starflower) also increased [7]. Subsequent dominance by shrubs was either of short duration (on watershed 1) or absent (on watershed 3), depending on the long-term dynamics of invading shrubs. Specifically, on watershed 1 invading shrubs exhibited continuous population growth for 17 years, then declined abruptly. On watershed 3, however, cover of invaders increased for only 9 years, limiting the importance of shrubs prior to canopy closure [11].
A comparison of early changes in diversity on watershed 1 and 3 illustrates the importance of timing of multiple disturbance events on the resulting richness of invading flora. At both the plot and site levels, invaders such as tall annual willowherb were far more common after burning on watershed 1 than on watershed 3. They were apparently favored by the protracted, staggered harvesting during which local populations established and spread prior to burning [7,12]. For example, fireweed establishment and spread differed between watersheds. On watershed 1 fireweed individuals that established during staggered harvesting sprouted from rhizomes after broadcast burning; on watershed 3, fireweed establishment was not substantial [6]. On watershed 1, abundance of invaders (especially woodland ragwort, tall annual willowherb, and ceanothus) varied directly with disturbance severity. Abundance of these species was similar among disturbed and burned soil classes on watershed 3 [7].
While loss of aboveground structures and establishment of invading annuals was directly related to disturbance severity on both watersheds, longer-term changes in species composition varied with disturbance severity on watershed 1 but not on watershed 3. Historical and stochastic factors (e.g., heterogeneity of the seed bank and the effect of local frost) may modify the influence of disturbance severity on community response, as evidenced by a different ceanothus response between watersheds. Both the magnitude and duration of dominance of snowbrush ceanothus and redstem ceanothus increased with soil disturbance on watershed 1. At final sampling in 1983, total ceanothus cover ranged from 11% on undisturbed plots to 57% on heavily burned plots. About 5 to 10 years after broadcast burning, both disturbed and burned plots on watershed 1 were dominated by invading shrubs; thimbleberry characterized disturbed-unburned plots and snowbrush ceanothus and redstem ceanothus characterized lightly and heavily burned plots [8,10]. Although ceanothus shrubs generally showed the same affinities for disturbed and burned plots on watershed 3, they did not dominate these plots as they did on watershed 1. Instead, widespread recovery of residual shrubs occurred on all plots in watershed 3 [8].
Variation in available seed and local weather conditions among watersheds affected the establishment and longevity of invading shrubs, and may therefore have indirectly determined the duration of the shrub phase. Succession from herb to shrub dominance was closely tied to the performance of invading shrubs (largely ceanothus). Originating from buried seeds, ceanothus typically develops more quickly than residual shrubs, despite the ability of residuals to sprout from root crowns and/or rhizomes [11]. Relatively rapid growth of invaders on watershed 1 resulted in a shift from herb to shrub dominance 14 years after burning. In contrast, a discrete shrub phase was absent on watershed 3 due to poor establishment of redstem ceanothus and to early, frost-induced senescence of evergreen snowbrush ceanothus, the dominant woody invader [6,7]. On watershed 3 snowbrush ceanothus establishment was similar on lightly and heavily burned plots, whereas on watershed 1 its abundance correlated with burn severity. This pattern may reflect local difference in historical distribution (therefore seed bank contents). It may also reflect competitive interactions with cooccurring shrub species such as Scouler willow. The relative scarcity of redstem ceanothus on watershed 3 may similarly suggest variation in its historical distribution. Differences in the dynamics of these ceanothus species on the 2 watersheds reflect the complex nature of their distribution and persistence after severe disturbances [6].
Complete recovery of initial understory dominants was limited to undisturbed plots on watershed 1 but was widespread among disturbed and burned plots on watershed 3 [8]. The poorer overall recovery of dominant understory taxa such as dwarf Oregon-grape, salal, and western sword fern on watershed 1 may reflect watershed-level differences in disturbance severity, site characteristics, or competition from invading plants [11]. Proportionally more plots were burned on watershed 1 than on watershed 3 (61% vs. 50%, [7]), and environmental conditions (e.g., temperature and moisture) were probably harsher on watershed 1 than on watershed 3 due to the greater area of disturbance relative to forest perimeter. The magnitude and duration of competition from invading herbs and tall shrubs were probably greater on watershed 1 than on watershed 3, further slowing the recovery of residual taxa [11].
The overall development of tree cover was surprisingly parallel among watersheds, despite different histories of seeding and planting. Transitions to tree dominance occurred after 20 years on both sites. Understory tree cover surpassed predisturbance levels within 11 to 13 years. Tree canopies were largely dominated by residual species (Douglas-fir, and to a lesser extent, western hemlock). Greater development of western hemlock on watershed 3 may reflect less burning on that watershed. Denser recruitment may derive from heavier seed rain and a more benign environment–both a function of the proportionally greater forest perimeter of the smaller clearcuts. The parallel development of total cover and differential success of Douglas-fir and western hemlock between sites illustrate that a common pattern of canopy closure may have multiple origins [11].
Tree development and mortality: Fourteen tree species were present 14 to 38 years (1979-2001) after clearcutting and broadcast burning, but only 7 were common enough to warrant summary statistics, which included estimates of stem density, basal area, and bole biomass for live and dead stems. These 7 species accounted for >97% of all stems and >96% of total biomass at each sampling date. Species richness increased initially, then remained constant at 3 to 4 species per plot through final sampling. Douglas-fir was dominant, accounting for 50% to 58% of stems and 57% to 82% of total aboveground tree biomass. Densities of the 4 hardwood species peaked early: Pacific dogwood and bigleaf maple at 18 to 21 years, golden chinkapin and bitter cherry at 22 to 25 years. Densities of the 2 shade-tolerant conifers, western hemlock and western redcedar, peaked later (29-32 years) but declined during the last measurement interval. Nevertheless, at 35 to 38 years, western hemlock accounted for 21% of stems [16].
Despite declines in density, most tree species showed continuous increases in bole biomass, albeit at varying rates. Mean tree density peaked at >3,000 stems/ha (≥1.4 m tall) after 22 to 25 years, then declined rapidly. At final sampling (35-38 years), densities were comparable to those at initial measurement, 14 years after disturbance. Bole biomass increased continuously to >100 Mg/ha. Stem density prior to canopy closure was positively related to cumulative mortality, and environmental variables were weakly related [16].
Although "suppression" was the dominant cause of mortality (applied to small, dead stems in subordinate canopy positions that had previously exhibited slow growth and showed no sign of physical damage or pathogen activity), mechanical damage yielded greater loss of biomass and greater structural heterogeneity through creation of windthrow gaps. Suppression mortality occurred in >80% of plots and was >2.5 times as frequent as mechanical damage (uprooting, stem snap, and crushing). However, biomass lost to mortality via mechanical damage was nearly 4 times that lost to suppression, a result of episodic storms that created windthrow patches, with some plots losing 30% to 50% of biomass. Of 7,146 recorded incidents of mortality, suppression was the most frequent cause (51%), mechanical damage accounted for 18%, and 26% could not be assigned a cause. None of the remaining causes of mortality exceeded 2% of stems. Suppression killed many small-diameter stems and was broadly distributed across both watersheds (82% of plots); mechanical damage killed fewer, but considerably larger, stems. Pathogen mortality accounted for 4% of total dead biomass and typically removed the largest trees. Rates of stem loss peaked between 25 and 32 years. Mechanical damage occurred in 56% of plots, with 86% of stems and 98% of biomass lost during 2 of the 5 sampling intervals and reflecting the effects of major winter storms preceding the 1991 and 2001 measurements. Mortality attributable to slope failure was also episodic and localized [16].
Total annual mortality increased from 1.0% to 5.3% of stems over the study period and was highly variable among species. Although mortality rates were highest for sprouting hardwoods (reaching 9.7% in Pacific dogwood), biomass of most hardwood species increased through canopy closure as dominant stems achieved large sizes. Hardwoods showed steep increases in mortality, peaking 18 to 25 years after disturbance. The highest mortality rate among the hardwoods was for Pacific dogwood (exceeding 9.6% over 7 years), and the lowest was for bigleaf maple (maximum of 5.4%). Mortality of Douglas-fir increased more slowly, exceeding 5% after 25 to 28 years. Mortality of shade-tolerant conifers (western hemlock and western redcedar) was consistently low until the last measurement interval (29-38 years), when rates increased to about 4%. Shade-tolerant conifers, typically assumed to be absent or to play a minor role in early forest development, accounted for 26% of stems after 38 years [16].
Causes of mortality also differed among species. Among hardwoods, suppression was the dominant cause of mortality. For Douglas-fir, mortality from suppression was 3 times more frequent than from mechanical damage, but mechanical damage resulted in 4 times the loss of biomass. Mortality of the shade-tolerant conifers was caused mainly by mechanical damage (Table 8) [16]:
| Table 8. Tree mortality attributed to suppression (Supp), mechanical damage (Mech), and unknown (Unkn) for the 7 common tree species over the study period (1979/1980-2001) [16] | |||||||
| Species | n* |
Stems (%) |
Bole biomass (%) |
||||
| Supp | Mech | Unkn | Supp | Mech | Unkn | ||
| bitter cherry | 710 | 42.9 | 6.5 | 49.3 | 51.1 | 12.5 | 33.9 |
| golden chinkapin | 578 | 48.5 | 2.6 | 48.4 | 49.5 | 13.2 | 35.6 |
| Pacific dogwood** | 117 | 36.5 | 9.7 | 47.8 | 25.3 | 9.0 | 48.8 |
| bigleaf maple | 316 | 26.3 | 13.6 | 58.4 | 11.0 | 10.5 | 78.1 |
| Douglas-fir*** | 4,399 | 59.3 | 18.8 | 18.5 | 12.5 | 58.0 | 22.6 |
| western redcedar | 144 | 21.1 | 67.7 | 11.3 | 2.6 | 94.5 | 2.9 |
| western hemlock | 804 | 29.1 | 51.5 | 13.2 | 6.1 | 75.6 | 15.8 |
| *Number of tagged stems that died. **A pathogen (dogwood anthracnose) was responsible for mortality of 6% of Pacific dogwood stems and 16.3% of biomass. ***Root rot pathogens were responsible for mortality of 1% of Douglas-fir stems and 4.8% of biomass. |
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FIRE MANAGEMENT IMPLICATIONS:
Forest management effects can be grouped into 2 broad categories: 1) initial effects of disturbances associated with logging and site preparation, including slash burning, and 2) effects of subsequent activities that directly or indirectly alter rates or patterns of succession. For most understory species in these systems, the initial effects of clearcutting and broadcast slash burning are probably no more severe than those of wildfire. Populations of most understory species can recover to original levels prior to canopy closure. However, epiphytic species that depend on the tree canopy and species that are sensitive to fire may require centuries to recover. Complete canopy removal (e.g., clearcutting) minimizes the probability that strictly shade-loving plants will survive; for some understory species, removal of the tree canopy is sufficient to induce local extinction, and additional species are lost after slash burning (see Local species extinctions). The environmental requirements of shade-loving species, rather than the direct effects of disturbance, may limit their persistence. Consequently, it may not help to eliminate slash burning or use less severe, spring burns during site preparation. Retention of large canopy trees in aggregates or islands may ameliorate groundlayer environments enough to provide refugia and a source for recolonization. Tree clumps are also critical for maintaining local populations of epiphytes. Such clumps may need to be retained through several rotations because of the limited dispersal and slow growth of epiphytic species. Although initial survival and early rates of recovery are closely tied to fire severity, it is more likely that frequency of disturbance limits the long-term persistence of some populations. Thus, a system of frequent, short-rotation harvests may reduce their populations [12].
It is likely that silvicultural practices designed to create structurally diverse environments for wildlife will also create microhabitats that ensure persistence of canopy-dependent and disturbance-sensitive plant species. The practice of removing most merchantable wood during clearcutting contrasts sharply with patterns created by natural disturbances. Wildfires and windstorms produce much of the structural complexity found on the forest floor. Logs and tip-up mounds can persist for centuries. Coarse woody debris serves as a critical substrate for establishment of some species (e.g., western hemlock). Woody debris also provides structural and functional elements that increase the spatial heterogeneity of moisture and nutrients. Long-term reductions in coarse woody debris may reduce plant species diversity during all stages of succession. Thus, silvicultural prescriptions need to incorporate both the retention of large structural elements (live trees, snags, and logs) from current stands and the production of similar structure for future rotations. Hardwoods such as red alder (Alnus rubra), bigleaf maple, and golden chinkapin contribute to the floristic and structural diversity of these forests. Practices that preclude or delay the development of old-growth characteristics may result in long-term loss of species diversity [12].
| Common name | Scientific name |
| Trees | |
| bigleaf maple | Acer macrophyllum |
| bitter cherry | Prunus emarginata |
| coast Douglas-fir | Pseudotsuga menziesii var. menziesii |
| golden chinkapin | Chrysolepis chrysophylla (Castanopsis chrysophylla)* |
| Pacific dogwood | Cornus nuttallii |
| Pacific yew | Taxus brevifolia |
| Scouler willow | Salix scouleriana |
| western hemlock | Tsuga heterophylla |
| western redcedar | Thuja plicata |
| Shrubs | |
| big huckleberry | Vaccinium membranaceum |
| blue elderberry | Sambucus nigra subsp. cerulea (Sambucus cerulea)* |
| California hazelnut | Corylus cornuta subsp. californica (C. c. var. californica)* |
| Cascade barberry | Mahonia nervosa |
| common snowberry | Symphoricarpos albus |
| oceanspray | Holodiscus discolor |
| creeping snowberry | Symphoricarpos mollis var. hesperius |
| currant | Ribes spp. |
| deerbrush | Ceanothus integerrimus |
| dwarf rose | Rosa gymnocarpa |
| hairy manzanita | Arctostaphylos columbiana |
| Pacific rhododendron | Rhododendron macrophyllum |
| red huckleberry | Vaccinium parvifolium |
| redflower currant | Ribes sanguineum |
| redstem ceanothus | Ceanothus sanguineus |
| salal | Gaultheria shallon |
| Saskatoon serviceberry | Amelanchier alnifolia |
| snow raspberry | Rubus nivalis |
| snowbrush ceanothus | Ceanothus velutinus (C. v. var. laevigatus)* |
| thimbleberry | Rubus parviflorus |
| trailing blackberry | Rubus ursinus |
| vine maple | Acer circinatum |
| willow | Salix sp. |
| Forbs and subshrubs | |
| agoseris | Agoseris spp. |
| American vetch | Vicia americana |
| annual agoseris | Agoseris heterophylla |
| beargrass | Xerophyllum tenax |
| big deervetch | Lotus crassifolius |
| bigflower agoseris | Agoseris grandiflora |
| bigflower tellima | Tellima grandiflora |
| broadleaf starflower | Trientalis borealis subsp. latifolia (Trientalis latifolia)* |
| bull thistle† | Cirsium vulgare |
| California spikenard | Aralia californica |
| Canada thistle† | Cirsium arvense |
| Canadian horseweed | Conyza canadensis |
| chaparral willowherb | Epilobium minutum |
| claspleaf twistedstalk | Streptopus amplexifolius |
| clustered thistle | Cirsium brevistylum |
| Columbian windflower | Anemone deltoidea |
| common whipplea | Whipplea modesta |
| evergreen violet | Viola sempervirens |
| field woodrush | Luzula campestris |
| fireweed | Chamerion angustifolium (Epilobium angustifolium)* |
| fringed willowherb | Epilobium ciliatum subsp. watsonii (Epilobium watsonii)* |
| largeflower fairybells | Prosartes smithii (Disporum smithii)* |
| leafy pea | Lathyrus polyphyllus |
| Little Mountain thimbleweed | Anemone lyallii |
| little prince's-pine | Chimaphila menziesii |
| miner's-lettuce | Claytonia perfoliata (Montia perfoliata)* |
| oneleaf foamflower | Tiarella trifoliata var. unifoliata (Tiarella unifoliata)* |
| Oregon drops of gold | Prosartes hookeri var. oregana (Disporum hookeri var. oreganum)* |
| Oregon goldthread | Coptis laciniata |
| Pacific bleeding heart | Dicentra formosa |
| Pacific coralroot | Corallorhiza mertensiana |
| Pacific trillium | Trillium ovatum |
| Pacific twinflower | Linnaea borealis subsp. longiflora |
| pale bellflower | Campanula scouleri |
| pink wintergreen | Pyrola asarifolia |
| prickly lettuce† | Lactuca serriola |
| prince's-pine | Chimaphila umbellata |
| bride's bonnet | Clintonia uniflora |
| red baneberry | Actaea rubra |
| redwood-sorrel | Oxalis oregana |
| Siberian springbeauty | Claytonia sibirica var. sibirica (Montia sibirica)* |
| small enchanter's nightshade | Circaea alpina |
| snowqueen | Synthyris reniformis |
| spiny sowthistle† | Sonchus asper |
| spreading dogbane | Apocynum androsaemifolium |
| starry false lily of the valley | Maianthemum stellatum (Smilacina stellata)* |
| stickywilly | Galium aparine |
| strawberry | Fragaria sp. |
| sweetscented bedstraw | Galium triflorum |
| tall annual willowherb | Epilobium brachycarpum (Epilobium paniculatum)* |
| thistle | Cirsium spp. |
| threeleaf foamflower | Tiarella trifoliata |
| toughleaf iris | Iris tenax |
| twinflower | Linnaea borealis |
| vanilla-leaf | Achlys triphylla |
| variableleaf collomia | Collomia heterophylla |
| western fescue | Festuca occidentalis |
| western pearly everlasting | Anaphalis margaritacea |
| western prince's-pine | Chimaphila umbellata var. occidentalis |
| western rattlesnake plantain | Goodyera oblongifolia |
| white hawkweed | Hieracium albiflorum |
| white insideout flower | Vancouveria hexandra |
| whitebark raspberry | Rubus leucodermis |
| whiteveined wintergreen | Pyrola picta |
| British Columbia wildginger | Asarum caudatum |
| woodland madia | Madia madioides |
| woodland ragwort† | Senecio sylvaticus |
| Wright's cudweed | Pseudognaphalium canescens subsp. microcephalum, P. c. subsp. thermale (Gnaphalium microcephalum, G. m. var. thermale)* |
| Graminoids | |
| brome | Bromus spp. |
| common velvetgrass† | Holcus lanatus |
| fringed brome | Bromus ciliatus |
| redtop† | Agrostis gigantea (Agrostis alba var. palustris)* |
| smoothstem sedge | Carex laeviculmis |
| Ferns and fern allies | |
| deer fern | Blechnum spicant |
| field horsetail | Equisetum arvense |
| common ladyfern | Athyrium filix-femina |
| northern maidenhair | Adiantum pedatum |
| western brackenfern | Pteridium aquilinum |
| western sword fern | Polystichum munitum |
| *For species that have undergone scientific name changes, names in parentheses are those used in the research paper.
†Nonnative species. |
|
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