Fire and Climatic Change in Temperate Ecosystems of the Western Americas (Ecological Studies)

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There are Many Ways to Edit a. Open the File Manager Log into cPanel. In the Files section, click on the File Manager icon. The File Manager will open in a new tab or window. Shrub associates are numerous. On Kodiak Island, in the Kenai Fjords, and Prince William Sound, Sitka spruce is frequently the dominant canopy tree from sea level to treeline on productive sites. It is the only conifer that occurs on Afognak and Kodiak islands, where its range is actively expanding.

In the southern portion of the Alaskan rainforest, Sitka spruce is linked more closely with limestone substrates and frequently disturbed sites e. Alaskan Pacific maritime western hemlock forest is the dominant forest system along the southeastern portions of the Alaskan coast. It occurs from the northern limit of Douglas-fir in coastal British Columbia, north through southeastern Alaska to Prince William Sound the northwestern limit of western hemlock.

Sites are typically well drained. Sitka spruce sometimes codominates. In the north Yakutat through Prince William Sound , mountain hemlock may also be present, and in southeastern Alaska Glacier Bay to British Columbia yellow-cedar may be present. Ovalleaf huckleberry often dominates the shrub layer. Salmonberry or devil's-club may dominate the shrub layer on disturbed sites.

North Pacific mesic western hemlock-yellow-cedar forest occurs throughout southeastern Alaska at all elevations below the mountain hemlock zone and is most abundant on somewhat poorly to moderately drained slopes. Yellow-cedar and western hemlock codominate, while western redcedar, Sitka spruce, and Pacific silver fir are absent or rare. This system intergrades with mountain hemlock forest, and mountain hemlock may occur in transitional stands. Pacific silver fir has a limited distribution in Alaska, where it is apparently confined to the extreme southern mainland and a few islands and occurs in nearly pure stands or mixed with Sitka spruce and western hemlock.

The associated BpS series is not mapped in Alaska, and more information on this system is available in the FEIS publication Fire regimes of wet-mesic western hemlock forests. Alaskan Pacific maritime Sitka spruce beach ridge occurs along the Gulf of Alaska coast in the following areas: Western hemlock may codominate on older sites.

Devil's-club is usually the most abundant understory shrub. As beach ridges form, they initially support brackish meadows and communities dominated by American dunegrass. The inland portion of these meadows transitions to Sitka spruce forest, which establishes about years after beach ridge formation and may succeed to western hemlock forest. Alaskan Pacific maritime mountain hemlock forest occurs from Kenai Fjords through southeastern Alaska, primarily in the maritime region covered in this synthesis , but also in the subboreal transition on the inland side of the Kenai and Chugach mountains covered in the FEIS synthesis Fire regimes of Alaskan mountain hemlock ecosystems.

In southeastern Alaska, this system occurs from about to 3, feet , m —between the western hemlock and subalpine mountain hemlock dwarf-tree systems—on relatively stable sideslopes and benches with well-drained soils, typically on north-facing and rarely on south-facing slopes. Ovalleaf huckleberry typically dominates the shrub layer.

North Pacific maritime mesic subalpine parkland is very localized in Alaska, occurring in the eastern portion of the panhandle at high elevations at the transition from forest to alpine, forming a subalpine forest-meadow ecotone. As they approach treeline, mountain hemlock stands occur in clumps or patches of mature-height trees interspersed with mesic and wet meadows rich in dwarf-shrubs and forbs.

Krummholz often occurs near the upper elevational limit. Major tree species are mountain hemlock, Pacific silver fir, yellow-cedar, and subalpine fir. Very deep, long-lasting snowpacks limit tree regeneration such that trees establish only in favorable microsites usually adjacent to existing trees or during years with low snowpack.

Riparian and wetland forest and woodland: Floodplains, fans, and estuaries subject to flooding are highly productive biodiversity hotspots [ 38 ]. Coastal wetlands are the richest plant communities of the coastal temperate rainforest zone in Alaska [ 98 , ]. Depending on disturbance frequency and severity, wetlands support plant communities of various composition and ages, including deciduous forests, stands of massive spruce and hemlock, and shrub and herb communities [ 38 ].

Investigations in southeastern Alaska suggest that the development of muskeg vegetation may take to several thousand years [ 99 ]. Muskegs may be a physiographic climax over much of southeastern Alaska, and they may be slowly increasing at the expense of forest cover in some areas [ 65 , 99 , ]. The following descriptions of site and successional relationships in Alaskan Pacific maritime riparian and wetland forests and woodlands are modified from NatureServe [ 95 ] unless otherwise cited.

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Alaskan Pacific maritime floodplain forest and shrubland occurs along the Gulf of Alaska coast and south through southeastern Alaska. It does not occur on Kodiak Island. Frequent flooding, shifting channels, and sediment deposition characterize the system. Vegetation composition and disturbance cycle vary depending on type of input glacial vs.

On nonglacial floodplains, red alder or Sitka alder may be common in early-seral stands, and floodplain wetlands are common, but small. On glacial floodplains, wetlands are uncommon near the glacier, a high proportion of the floodplain is in barren and in early seral stages, and mature forest development is minimal. Vegetation on distal outwash plains varies with frequency of flooding and seral stage. Alaskan Pacific maritime poorly drained conifer woodland occurs at low to midelevations on rolling terrain, benches, and gentle slopes from Kenai Fjords through southeastern Alaska. Soils are shallow to deep and usually have a thick organic layer or some peat development.

In some places, stands are a fine mosaic of peatlands and better-drained inclusions. These are low-productivity sites that are intermediate between shore pine or mountain hemlock peatland sites and more productive forest systems. Standing dead trees are common. In the north, paludification on these sites may lead to conversion from mountain hemlock forest to mountain hemlock peatland over long time scales.

Fire and Climatic Change in Temperate Ecosystems of the Western Americas

Overstory trees may include western hemlock, mountain hemlock often alone or with Sitka spruce in the subpolar rainforest zone , western redcedar southern portion of the Alaska distribution only , and yellow-cedar. This system represents a topoedaphic climax that is relatively stable over time.

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

Alaskan Pacific maritime shore pine peatland occurs from Yakutat south through southeastern Alaska. Shore pine does not occur north or west of Yakutat. This ecological system is a mosaic of shore pine, dwarf-shrub, and herbaceous peatland communities. It includes a range of canopy structures and compositions from mixed conifer peatlands on sideslopes and benches with yellow-cedar, mountain hemlock, western hemlock, and shore pine, to well-developed peatlands on flat, rolling, or sloping terrain with scrub shore pine.

Soils are poorly drained with deep organic layers. Upland shrubland and grassland: Alaskan Pacific maritime upland shrubland communities include alpine dwarf shrubland communities that appear to be successionally stable; alder-dominated communities on floodplains and rivers; communities intermediate between marsh and Sitka spruce-western hemlock forest; and avalanche track, steep alpine, and tundra upland communities.

Red alder is successional on disturbed forest sites [ ]. Depending on elevation, soils, and disturbance frequency these patches may succeed to western hemlock or other forest types [ 5 ]. Alaskan Pacific maritime grasslands include a variety of herbaceous vegetation types on sideslopes, rolling hills, and alluvial deposits. Graminoids, forbs, or ferns may dominate these herbaceous meadows. The following Alaskan Pacific maritime upland shrublands and grasslands occur in the area covered by this synthesis.

Corresponding BpS series are given in parentheses after NatureServe's ecological system name. Follow links to see detailed descriptions of vegetation and site characteristics provided by NatureServe [ 95 ]. Alaskan Pacific maritime avalanche slope shrubland Alaskan Pacific maritime subalpine alder-salmonberry shrubland Alaskan Pacific maritime subalpine copperbush shrubland Alaskan Pacific maritime alpine sparse shrub and fell-field Alaskan Pacific maritime alpine dwarf shrubland Alaskan Pacific maritime mesic herbaceous meadow Riparian and wetland nonforest: Wetland succession and species composition vary depending on site conditions such as water depth, substrate characteristics, and nutrient input.

The following Alaskan Pacific maritime shrub and herbaceous wetlands occur in the area covered by this synthesis. The coastal ecosystem in Alaska is, in many respects, a system of long time scales. Tree species are long-lived, intervals between large-scale disturbances are on the scale of centuries to millennia, and decay and decomposition progress equally slowly [ 38 ]. In order to understand how current vegetation and fire regimes developed, fire histories must be long enough to characterize the frequency and range of variability of fire occurrence in the system of interest [ 48 ].

In ecosystems where fire is rare or infrequent and fire-free periods last for centuries to millennia, it is useful to understand past changes in vegetation and fire occurrence over similar time scales.

Paleoecological studies of charcoal records from lake sediments and soil profiles represent a key approach to understanding the natural variability of these fire regimes because they broaden the reference period and are well suited for reconstructing the incidence of past fire and its relationship to changing climate and vegetation, especially in ecosystems with fire-return intervals that exceed the age of the oldest trees [ 48 ].

Knowledge of the time since the last fire is not adequate for understanding historical fire frequency or assigning historical fire-return intervals in these ecosystems. Without long-term data, inferences about historical fire regimes are questionable and not likely to provide insights needed to manage for projected climate changes [ ]. In this synthesis, the "historical" or "presettlement" period refers to the time beginning when the current vegetation types established, about 4, to 2, years ago the late Holocene , and ending with European settlement in the late 18th and early 19th centuries.

The time period beginning with European settlement and leading to the present day is referred to as "contemporary". Many of the dominant species and genera that make up the vegetation in contemporary Alaskan coastal ecosystems have been present for most of the Holocene, and components of the contemporary flora have evolved together for the past several millennia. In coastal areas of both Alaska and British Columbia, temperatures, precipitation, and vegetation composition began to resemble the contemporary record and landscapes about 4, years ago table 1 reviews by [ 48 , 60 ].

For thousands of years the natural vegetation along the southern Alaskan coast was dominated by old-growth conifer stands [ 13 ] with large, old trees and multiple canopy layers [ 95 ] in a mosaic with muskegs and other late-successional wetlands and small disturbance gaps [ 66 ]. Rainforest ecosystems established during this time [ 74 ], and Sitka spruce, mountain hemlock, and western hemlock appeared in south-central Alaska about 4, to 3, years ago as a result of a migration northwestward along the Gulf of Alaska that took place as storm tracts strengthened during the late Holocene review by [ ].

These old-growth forests comprise a large carbon pool and support a disproportionately high diversity of plant and animal species, given their small areal coverage [ ]. Old growth is the dominant age class in upland coastal forest, with mostly small canopy gaps formed by topoedaphic variation e. Susceptible sites may experience large-scale windthrow or geomorphic disturbances. The continual small-scale disturbance and renewal pattern in Alaskan coastal forests generates a shifting steady-state mosaic of mature forest with small patches in seral stages [ 35 ].

The different types of disturbances do not occur homogeneously across the forest landscape, and most are confined to, or occur predominantly in, specific site types or landscape positions [ 38 ]. Over most of the coastal landscape, stand-replacing disturbances are rare and stands are very old, with multiple canopy layers of large trees and occasional, small gaps [ 38 , 76 , 95 ]. Most canopy gaps typically result from the death of 10 or fewer trees gapmakers [ 38 ]. Gap and gapmaker characteristics are generally similar among sites [ , ].

Disruptions to the forest floor may be largely restricted to storms with high winds and rain that saturates the soil, making uprooting more probable [ 23 , ]. Ott and Juday [ ] provide details on sizes and characteristics of gaps and gapmakers in a western hemlock association on the Tongass National Forest. Fire was absent to rare historically see Historical fire regimes , and while large-scale windthrow and geomorphic disturbances occur, they are infrequent and generally restricted to susceptible landscapes [ 38 , ].

Low-severity disturbances resulting in small-scale canopy gaps are common; and frequent, small-scale windthrow is the most important natural disturbance shaping old-growth stand structure [ 23 , 35 ]. Flooding, landslides, and avalanches are the primary geomorphic disturbances [ , ]. Snow, ice, frost, drought, insects, fungal pathogens, and mammals may also kill or injure trees [ 38 ]. The role of small-scale disturbance in controlling and maintaining forest structure in coastal temperate rain forests of North America has been well studied e.

Estimates of the average interval between successive gap creation events range from to years in southeastern Alaska [ ] and from to 1, years on the southern coast of British Columbia [ 38 , 75 ]. On study sites in southeastern Alaska, forest turnover time was estimated to range from to years, and averaged years [ ]. The following is a brief discussion of the dominant disturbance processes and related successional relationships influencing forest structure in these settings over the past several millennia.

Wind is the primary disturbance agent in the coastal forests of southeastern Alaska [ 4 , 10 , 66 , , ] and far more prevalent than fire in creating canopy openings in coastal forests in general [ 10 ]. Coastal rainforests are susceptible to wind damage because dominant trees e. Gale-force winds may occur during any month in southeastern Alaska, but the strongest winds usually occur in autumn and winter [ 38 , 54 ].

Windstorms create a variety of opening sizes with a variety of internal remnant structure depending on site characteristics e. In general, large-scale, infrequent blowdowns occur on exposed landscapes during severe storms, and small-scale canopy gaps resulting from the death of one to several trees are common on more protected landscapes and older forests [ 6 , 34 , 38 , 66 , , , ]. Small-scale windthrow is much more common and is considered the most important natural disturbance in southeastern Alaska e. Estimated return intervals for large-scale blowdown on exposed sites range from years [ ] to years [ 38 , 66 , ].

Large-scale blowdown typically results in even-aged stands with a higher Sitka spruce component than the previous stand [ ]. Where large openings are part of the wind regime, the resulting landscape mosaic can be patchy at multiple scales and made up of single-cohort stands, stands with multiple, even-aged cohorts, and all-aged stands maintained primarily through gap dynamics [ 54 , 66 , ]. Because the understory is mostly left intact, regeneration after wind disturbance may follow different pathways than regeneration after mass wasting or fire.

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

Geomorphic disturbances are among the most important naturally occurring, high-severity, stand-replacing events in southeastern Alaska. Due to the steep topography, they are important in shaping both vegetation and physical landscapes in coastal forests. The steep slopes and gullies along the sides of the trough-shaped valleys are susceptible to avalanches and episodic mass wasting.

On the gentler slopes near the valley bottoms, more continuous fluvial processes dominate [ 38 , ]. Successional patterns on landslide or mudflow surfaces are similar to those that occur on glacial deposits, with spruce and alder dominating early succession. Forests on avalanche slopes are usually distinguished from nearby old-growth forest by the presence of pure, even-aged, early-successional species such as Sitka spruce with alder and salmonberry mixtures.

These forests succeed to western or mountain hemlock in about to years if not subject to additional avalanches [ ]. Floods, triggered by rainstorms, rain on snow, or rapid snowmelt, can cause varying degrees of tree mortality [ 38 ]. During large floods and mudslides in riparian habitats, a lens of material is deposited and forms a new terrace, on which red alder and Sitka spruce establish over several hundred years. After the initial to years of stand development, gap processes become important in creating fine-scale structure.

Riparian stands along large streams and rivers are most likely to have stand-replacement floods [ ]. Western hemlock and Sitka spruce have few insect pests in southeastern Alaska, likely due to the cool climate, although western black-headed budworm and hemlock sawfly outbreaks occasionally cause widespread defoliation, particularly to western hemlock figure 3. Western spruce budworm and western hemlock looper have done little damage in southeastern Alaska.

The spruce beetle occurs at endemic levels in old-growth stands but rarely reaches epidemic proportions in southeastern Alaska. Dwarf mistletoes and heart rot fungi are common in old-growth, Alaskan Pacific maritime forests and perpetuate western hemlock dominance [ 55 ]. Historically from the mids to the lates , spruce beetle outbreaks occurred every 30 to 50 years and have affected 3.

Spruce beetle outbreaks frequently follow windstorms or fires because trees that are blown over, broken by wind, or scorched by fire are ideal breeding sites for beetles [ ]. In south-central Alaska, a spruce beetle outbreak of unprecedented magnitude and size caused a massive die-off of mature white, Lutz, and Sitka spruce forests on the Kenai Peninsula. See Postsettlement fuels for more information. Little information is available regarding postfire succession in Alaskan Pacific maritime ecosystems; however, fire is likely to favor species such as Sitka spruce, western redcedar, and Pacific silver fir by reducing thick organic layers and exposing patches of mineral soil, increasing nutrient availability, and removing advanced regeneration of western hemlock [ 38 , , ].

Sitka spruce usually regenerates on mineral or mixed-soil microsites, and western hemlock usually regenerates on organic substrates, regardless of the frequency and intensity of disturbance [ 34 ]. Successional patterns after stand-replacing disturbances in western hemlock-Sitka spruce forests vary due to differences in soils, microclimate, and disturbance type [ 6 ]. For example, even-aged stands grew faster when they originated after wildfire or logging than after blowdown; the authors speculate that this was due to higher soil temperatures leading to increased decomposition and greater nutrient availability [ ].

Alaback [ 6 ] did not find differences in understory successional patterns between stands originating after fire and those originating after logging. A general model of succession after in order of most to least common large-scale disturbances clearcutting, blowdown, or fire begins with a shrub stage in which residual shrubs such as blueberry, huckleberry, salmonberry, currant, menziesia, devil's-club, red elderberry grow quickly and dominate the site.

Tree seedlings establish at about the same time and follow a pattern similar to that of the woody shrub species [ 6 , 55 , ]. Production of shrubs and herbs increases linearly with time, for up to about 20 years, and understory biomass peaks 15 to 25 years after canopy removal [ 6 ]. Within about 8 to 10 years tree saplings begin to overtop the shrub layer [ 55 , ], and shrub and herb abundance declines as forest canopies close after about 25 to 35 years. During this successional stage, clumps of rhizomatous ferns such as western oakfern and spreading woodfern occur on both decaying wood and the forest floor, and extensive carpets of mosses begin to form.

Bryophytes and ferns begin to dominate the understory around 50 to 60 years after canopy removal. Moss biomass peaks around to years after canopy removal, and then declines as shrub and herb components increase during the last stages of successional development.

Biomass of tree seedlings, mostly concentrated on well-decayed logs and stumps, also increases during the final successional stages [ 6 ]. Historically, western hemlock and Sitka spruce forests on low-elevation, upland sites were typically uneven-aged due to the predominance of small-scale disturbances, but developed even-aged stands after rare, stand-replacing disturbance [ 34 ]. After a stand-replacing disturbance and subsequent regeneration, forests may remain even-aged for up to years before gradually becoming uneven-aged [ 55 , ].

Old-growth forests are generally more structurally heterogeneous than any other age class [ 6 ], although in southeastern Alaska, tree species composition and stand structure were similar among even- and uneven-aged stands [ 34 ]. The age at which forests become old growth differs with site and forest type. It seems reasonable, however, that at least years is required [ ]. In many cases successional trajectories are very long, with directional change continuing to occur to years after disturbance [ ].

The ideal combination of lightning, fuels, and climate suitable for wildfires was historically very rare in wet coastal temperate rain forests [ 31 ]. The mild maritime climate allows for abundant biomass fuel production; however, temperate rainforests only rarely experience fire-conducive conditions [ ].

Unlike interior Alaska [ 46 , ], fire has historically been very rare in southern coastal Alaska [ 2 , 6 , 9 , 10 , 16 , 66 , 81 , 98 , , ] and northern coastal British Columbia [ 13 , 38 , ] due to higher amounts of rainfall, wet conditions year-round, lower summer temperatures, and low incidence of lightning. Exceptions may occur where rainforest transitions into subboreal forest on the Kenai Peninsula [ 9 , ] and in areas of severe rain shadow such as Lynn Canal north of Juneau [ , ].

As perhumid rainforests intergrade with drier forest types farther south central and southern British Columbia, Washington, Oregon , increasingly more evidence of periodic fire is evident on the landscape [ 4 ], such as a greater abundance of Douglas-fir. The model of episodic, stand-replacing fires developed for coastal Douglas-fir forests has sometimes been applied to the wetter, coastal rainforests, despite higher annual precipitation, less seasonality of precipitation, different stand structure, and different dominant species [ 74 ].

However, multiaged stand structures, dominance of late-successional, fire-sensitive species, and evidence from paleological charcoal studies in coastal forests farther south indicate that stand-replacing fires have been extremely rare in temperate rainforests over the past several thousand years. This rarity of fire has supported the development of ecosystems characterized by very large, old trees and the ubiquity of late-seral species and structures at stand and landscape scales [ 74 ].

While it is generally agreed that fire has not been a major disturbance factor in these forests for several millennia, the historical role of fire is not fully resolved [ 10 ], perhaps because stand-replacing fires that have occurred since European settlement have influenced our perception of fire in these forests see Postsettlement fires. However, these fires are outside the historical range of variability for coastal rainforests, most of them were associated with logging [ 31 ], and statistics derived from recent fire history do not likely reflect presettlement dynamics [ 38 ].

The lack of fire applies not only to the forests, but also to the other community types in the coastal landscape, including riparian, wetland, shrubland, and alpine systems [ 81 ]. The nonforest BpSs that occur in the area covered by this synthesis occur as patches within the larger matrix of coniferous forest and, due to their small patch size, they would likely have burned with a similar frequency as the surrounding forest.

In other words, they were historically unlikely to burn. Very little information was found in the literature that discussed fire in these communities. The southeastern variant of the mountain hemlock BpS is included in this synthesis because, similar to lower elevation Sitka spruce and western hemlock forests in southeastern Alaska, it has a nonfire history.

The FEIS synthesis Fire regimes of Alaskan mountain hemlock ecosystems covers the northern variant of Alaskan mountain hemlock ecosystems in south-central Alaska, which are more likely to have burned historically. Modelers of Alaskan Pacific maritime western hemlock forests in southeastern Alaska note that fire plays some role in inland areas near Haines, Skagway, and generally north of Lynn Canal, where the climate is drier and more continental [ 69 ]; however, contemporary fires are most likely anthropogenic in origin and not representative of the longer record.

The conditions necessary for ignition and fire spread are more likely to occur in coastal forests of southern coastal British Columbia and the Pacific Northwest. Some information from these regions is referenced in this synthesis to provide additional context; however, for more detailed information on fire regimes in those ecosystems, see the FEIS publication Fire regimes of Pacific Northwest coastal forests. The mild, moist climate in coastal Alaska leads to substantial biomass production and slow decomposition, and therefore ample fuel availability over the long term decades to centuries , but also poor combustibility in the short term the fire season.

Weather anomalies, topography, plant community composition, and disturbances e. Quantities and types of fuels: Quantitative information on fuels in Alaskan coastal forests is limited. Although coarse woody debris generally accounts for most of the persistent surface fuel loads in coastal temperate rainforests [ 31 ], contemporary wildfires e. Most of these forest types also have large quantities of biomass and downed wood; however, mature Alaskan Pacific maritime Sitka spruce beach ridge forests usually have very little downed wood or snags [ 95 ].

Larson [ 73 ] collected data on downed woody fuel in 11 forest types and 19 plant associations in southeastern Alaska. For the 78 samples in predominantly Sitka spruce and western hemlock stands, downed wood weights averaged Downed wood weights ranged from 0. The author stated that "downed woody material is not much of a problem in southeast Alaska, as the nature of the nearly fireproof rain forest prevents fire hazard from becoming severe. Stand structure and species composition of coastal forests may both reflect a history of fire and influence their susceptibility to fire.

A review by Dorner and Wong [ 38 ] regarding disturbance regimes in coastal British Columbia suggests a potential link between high western redcedar cover and fire incidence in low productivity bog woodlands and western redcedar-western hemlock stands in the northern part of the province. They suggest that western redcedar is an indicator of past and potential fires because fire seems to favor western redcedar regeneration and persistence over associated tree species, and it is more flammable than those species.

Low-productivity bog woodlands of Haida Gwaii are usually fire resistant, but they may become conducive to burning in unusually dry weather because they have a large component of western redcedar, a large number of snags, and peaty soils [ 38 ]. In general, fuel flammability in wet coastal temperate rainforests is low due to high moisture content [ 31 , ].

With some local exceptions, the climate of southeastern Alaska has historically been too consistently wet to support major fires. The summer drought that affects coniferous forests farther south is not as apparent southeastern Alaska, and extended periods without precipitation are rare [ 6 ]. Even during years of extreme drought, high moisture content in fuels has historically prevented ignition and fire spread [ 31 ]. In southeastern Alaska, wildfires that began during dry summers in the 20th century usually died out upon reaching old-growth stands, where understories tend to be humid [ 35 ].

These types of ground fires can burn for several days [ 90 ] and char or consume tree roots see Fire type and severity. Coarse woody surface fuels have high bulk density and typically remain moist beneath moss and herbs in the shade of multiple canopy layers [ 31 , ]. Historically, truly exceptional droughts or successive years of drought were needed for fires to carry in coastal communities in the northwestern North America [ 4 ].

For example, Agee and Flewelling [ 3 ] estimated that it would take more than 4 weeks with no significant precipitation in a year with below-average rainfall which recur at decadal intervals to create flammable conditions in the Olympic Mountains of Washington [ 74 ]. Years of high fire activity in the Pacific Northwest were most likely to occur when persistent high pressure ridges formed along the Pacific Coast, reducing precipitation and allowing fuels to dry for extended periods.

Under these conditions, dry winds could spread fire where fuels were available [ 31 ]. Climate oscillations 21 climate oscillation indices used in this study showed no significant relationships to area burned in this zone [ 89 ]. Topography influences fuel moisture and fire spread at both regional and local scales in northern Pacific coastal areas. At a regional scale, the drier climate in the rainshadow of mountain ranges is more conducive to fires, a pattern apparent around Lynn Canal north of Juneau [ , ] and on the mainland coast of British Columbia, Vancouver Island, and Haida Gwaii [ 38 ].

At local scales, topography determines fuel exposure to irradiance and wind and indirectly influences fuel moisture, making certain parts of the landscape more susceptible to fire. For example, southerly aspects may be more flammable than northerly aspects and valley bottom areas. In coastal forests of the Clayoquot Valley, British Columbia, fires were more than 25 times more likely to occur on south-facing slopes than on less susceptible landscape positions [ 38 , 47 ]. Within stands, subtle differences in microtopography can limit the spread of fire and restrict fire size in the wet coastal temperate rainforest [ 31 ].

Fire season and ignition sources: Conditions in Alaskan maritime ecosystems were historically too moist to burn year-round. However, contemporary fires occurred in south-central Alaska between April and October of some years, with most occurring in May, June, or July [ ]; and fire risk is greatest in June and July in southeastern Alaska [ 82 ]. In coastal areas, fires have occurred from April through August e. On the Kenai Peninsula, a brief dry period in June can cause low fuel moisture, making it the most favorable time for fires [ 36 ].

Fire danger indices tend to decline in late August and early September [ ]. Lightning is generally infrequent in cold, rainy coastal climates, where it is associated with anomalously warm, dry, seasonal weather [ 31 ]; however, global climate changes may impact the frequency and distribution of lightning in coastal climates see Climate change and fire regimes.

For example, in May —a period of high temperatures and dry conditions—lighting storms in southeastern Alaska generated hundreds of lightning strikes, several of which ignited wildfires on the Tongass National Forest [ 62 ]. The probability of lightning is greater in south-central than in southeastern Alaska [ 46 ], and it increases from north to south along the coast from southeastern Alaska through British Columbia [ 10 , 55 ]. Lightning-caused fires occur less frequently in south-central Alaska than in all other parts of the state except the Southeast and Arctic regions.

From to , 61 lightning-caused fires occurred in south-central Alaska, ranging in size from 1 to 5, acres 0. Most contemporary fires in south-central Alaska are human-caused and occur around population centers, along transportation routes, and in high-use recreational areas [ ]. Lightning was historically very rare in southeastern Alaska, striking near major towns only once every 1 to 2 decades [ 4 ], and lightning-caused fires were rare even during very dry summers [ 10 , 55 , ]. Two lightning-caused fires were recorded in Southeast Alaska during the 20th century prior to [ 55 ], and 24 lightning-caused fires were recorded in southeastern Alaska between and [ 82 ].

However, most contemporary fires are human-caused [ 55 , , ], often due to campfires of recreationists [ 82 , 90 , 91 ] see Postsettlement fires. Fire records from to in British Columbia indicate that fires were least common on the northern coast. Only four lightning-caused fires were reported for all of Haida Gwaii, and 63 lightning-caused fires were recorded on the northern coastal mainland [ ]. Before Europeans came, relatively small and widely dispersed populations of Native Alaskans made little change in the forest, except in the immediate vicinity of villages and camps.

Native Alaskans used fire for a variety of purposes such as cooking, warming, making canoes, and drying or smoking fish and meat, and fires may have occasionally escaped and burned the surrounding forests during infrequent periods of dry weather [ 55 ]. In the perhumid rainforest region, people who used fire were typically those whose settlements were in rainshadow climates.

Fire was used to fell large trees and to clear burial grounds, but the effects were localized [ ]. Intentional low-severity burning by First Nations peoples to enhance berry production has been documented in coastal British Columbia.

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Several western redcedar stands in the central and northern coastal areas are thought to have originated after fires that ignited when western redcedar logs were burned out to make canoes [ 38 ]. Fire use by tribes in the coastal wet forests farther south was also likely limited [ 25 ]. Only one fire history study including any Alaskan Pacific maritime ecosystems [ ] was available as of Few adaptations clearly related to fire are seen in coastal Alaskan vegetation, and evidence of past fires e. The cool, wet climate and low incidence of lightning support long fire-free intervals.

Historically, exceptional droughts or successive years of drought were needed to create conditions conducive to large, severe fires in ecosystems along the northwestern coast of North America [ 4 ]. Several, mostly small and mostly human-caused fires have been recorded in southern coastal Alaska since European settlement and extensive logging began see Postsettlement fires.

While lightning is more common and fires are generally more frequent in south-central Alaska than in southeastern Alaska, fires mostly occur in forests dominated by Lutz and white spruce that are transitional between coastal rainforests and subboreal spruce forests. Some evidence also suggests a history of infrequent fires in the northern variant of Alaskan Pacific maritime mountain hemlock forests, which occur in south-central Alaska. A single fire history study that examined some coastal rainforest stands on the Kenai Peninsula found that evidence of past fires e.

Most Sitka spruce stands showed no evidence of past fires [ ]. Anecdotal evidence from the forest zone of the Kenai Mountains i. It is not clear in which forest types these soil pits were located. Estimated fire frequencies for forests on the Kenai Peninsula that are adjacent to Alaskan Pacific maritime ecosystems indicate that fire is rare in adjacent forests as well [ 19 , ].

In white spruce-Lutz spruce forests across the central and southwestern Kenai Peninsula BpS , radiocarbon dating of soil charcoal samples taken from 22 stands suggests that time-since-fire ranged from 90 to 1, years. On average, these forests had not burned for over years [ 19 ].

See Contemporary changes in fuels and fire regimes for more information. Bluejoint reedgrass communities on the Kenai Peninsula possibly BpS may have been maintained by fire. In , Hanson [ 52 ] anecdotally described fire frequency and effects in upland grasslands at about feet m elevation northwest of Homer: No fire history studies of ecosystems in southeastern Alaska have been published, although some authors cite evidence of past fires such as charcoal fragments in peat layers [ 98 ] and large areas of even-aged stands e.

Nonetheless, paleological studies in similar coastal temperate rainforests on Vancouver Island and southern coastal British Columba e. Neiland [ 98 ] suggests that many forests in southeastern Alaska show scattered signs of past fires, such as charcoal fragments in the upper peat and soil layers.

However, she admits that these were difficult to discern with certainty because the thick, almost continuously wet, organic layers made it difficult to distinguish whether discolorations on dead wood were caused by fire or moisture. She stated that low-severity fire may have been more common than has been realized because the wet, mild climate rapidly obliterates indications of past fires [ 98 ].

Harris and Farr [ 55 ] also noted that the rapid buildup of organic matter can hide charcoal under the forest floor and make it difficult to determine whether even-aged stands originated after fire. Stand structure over most of southeastern Alaska was historically multiaged, although even-aged stands occur in several areas. The origin of some of these stands has been attributed to fires during the past few centuries [ 55 , ].

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For example, even-aged stands ranging from to over years old on Mitkof Island, Chichagof Island, Revillagigedo Island, and on the west coast of Prince of Wales and nearby islands were attributed to extensive wildfires in , , and between and [ 55 , ]. The authors suggest that the wildfires were human-caused [ 55 ]. In his study of understory succession in Sitka spruce-western hemlock forests in southeastern Alaska, Alaback [ 6 ] stated that 13 of the 60 stands studied had originated after fire.

Based on ages of codominant trees, these stands ranged from 71 to years old, suggesting that fires occurred around the following years: Others have reported fires in coastal rainforests on Vancouver Island and coastal mainland British Columbia during similar time frames e. See the section on Postsettlement fires in this synthesis, and the FEIS fire regime publications on Pacific Northwest coastal forests and Pacific Northwest mountain hemlock for additional details.

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

Charcoal is common in the soils of rainforests in coastal British Columbia, indicating that they burned at some undetermined point in the past; however, temporal interpretation of this charcoal has been limited review by [ 74 ]. Examination of soil profiles in Sitka spruce-western hemlock and western hemlock-western redcedar forests on Graham Island in Haida Gwaii revealed charcoal deposited at varying depths.

At one site abundant pieces of charcoal in the lower depths of the organic horizon indicated that a fire occurred sometime before the present stand established about years prior , and numerous pieces of charcoal between depths of 27 and 47 inches 69 and cm in the mineral soil suggest a fire "in the distant past". At another site, buried charcoal layers occurred at 37 and 52 inches 94 and cm. Traces of charcoal occurred on the soil surface in several locations in conjunction with reddish-brown mineral soil, suggesting more recent fires [ 33 ].

Other evidence of fire in coastal British Columbia comes mainly from the distant past on the southern coast, with one more recent example from the central coast. Examination of aerial photos taken in and along the central coast of British Columbia revealed only one patch that was created by fire during the past years. The patch was 18, acres 7, ha and was dominated by Douglas-fir [ ], a different forest type than those covered in this synthesis.

Paleological studies from western hemlock and mountain hemlock forests in southern British Columbia show that fires are rare events in coastal temperate rainforests on millennial time scales e. Some sites on Vancouver Island showed no evidence of ever having burned during the past 6, years longer than the time that temperate rainforest has existed in the region , and many sites show evidence of only one to three fires during that period [ 74 ]. Charcoal evidence from the west side of southern Vancouver Island suggests fire-return intervals of approximately 3, years [ 26 ].

In coastal western hemlock forests in Clayoquot Valley the median time since last fire ranged from to 4, years. For more information on these and other fire history studies in and around seasonal temperate rainforests, see the FEIS publications on Fire Regimes of Pacific Northwest coastal forests and Pacific Northwest mountain hemlock communities. Fire type and severity: Some accounts suggest that rare, stand-replacing fires occurred in Alaskan Pacific maritime ecosystems during the reference period e. Evidence of small, mixed-severity fires is difficult to detect retrospectively, so if these types of fires occurred historically, they may not have been documented.

Conversely, several even-aged stands in southeastern Alaska are thought to have originated after stand-replacing fires during the presettlement and settlement eras [ 55 , ]. On sites where Sitka spruce codominated with Kenai birch in south-central Alaska, even-aged stand structure and charred stumps suggested some history of stand-replacing fire, but no additional data were collected [ ]. Some generalized accounts of fire regimes in coastal western hemlock-Sitka spruce forests describe infrequent, high-severity, stand-replacing fires e.

However, this seems to apply more to contemporary forest fires in the seasonal temperate rainforest zone and to drier forest types, where Douglas-fir occurs. Contemporary fire observations in southeastern Alaska indicate that low-intensity ground fires can occur in Sitka-spruce-western hemlock forests, and that the effects of these fires can be severe, although size of high-severity patches is typically small e.

Ground fires occur when the forest floor dries sufficiently to ignite and carry fire see Fuel characteristics. These are typically low-intensity fires with no visible flames [ 90 ] but can be of high severity in areas where they consume the nutrient-rich duff layer and char or girdle the anchoring root systems of Sitka spruce and western hemlock, causing them to topple over [ 82 , 90 , 91 ]. Soils can become unstable and susceptible to erosion when the surface organic layers are consumed, and regeneration may be delayed by 5 to 10 years [ 82 ].

The Bird Island fire, for example, left an area of "burnt roots, bare rocks, and ash" [ 91 ], and 2 years after a wildfire near Juneau, the site was characterized as "a tangled mass of root wads, downed trees, exposed bedrock and a few green shoots of fireweed" [ 82 ]. A growing body of evidence suggests mixed-severity fire regimes were historically more common than previously documented in North America e.

Fire and Climatic Change in Temperate Ecosystems of the Western Americas - Google Книги

A literature review by Dorner and Wong [ 38 ] suggests that historical fire regimes in coastal western hemlock forests in British Columbia were characterized by "a combination infrequent, high-severity crown and low-severity surface fires causing partial or complete stand replacement". A review and analysis by Daniels and Gray [ 31 ] suggests a regime of rare, low- and mixed-severity fires in coastal rainforests of British Columbia, resulting in complex stands with multiple age cohorts.

Cumulatively, mixed-severity fire regimes result in complex landscapes comprised of sites that last burned at a range of fire severities, as well as sites that have not burned for long periods [ 83 ]. Documenting spatially and temporally mixed-severity fire regimes is data intensive, and much of the necessary evidence has been lost where these forests have been altered by logging and urban development [ 31 ].

In general, forest types that historically burned infrequently fire-return intervals longer than about years may still be within the historical range of variability—in terms of fuel loads, stand structures, and species composition [ 45 ]—in areas that have not been altered by logging or other anthropogenic disturbances.

Fire regimes in coastal forests—where fire has been rare for millennia—have not been altered by fire exclusion; however, landscapes dominated by early successional forest resulting from massive timber harvests are far outside the historical range of variability.

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