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|Figure 1. The 2007 Woodchopper 2 Fire burning in boreal mixedwoods, Yukon-Charley Rivers National Park. Photo courtesy of the National Park Service.|
As of 2014, the scientific literature about fire regimes in Alaskan white spruce communities is scarce. There are few studies that quantify historical fire-return intervals or fire-rotation intervals. These studies use different methods, are limited in scope, and describe different white spruce communities. I used anecdotal and qualitative descriptions to supplement quantitative literature. Descriptions of fire ignition, season, pattern, and size specific to white spruce communities were not found in the literature, so I described characteristics of Alaskan boreal forest in general. Syntheses of fire regimes of associated boreal forests types (i.e., black spruce, quaking aspen and balsam poplar) are available in FEIS. Literature reviews describing some characteristics of fire regimes of white spruce communities [28,85,130,134] were used in this synthesis.Common names are used throughout this synthesis. For a complete list of common and scientific names of species mentioned and links to FEIS species reviews, see Appendix B.
This section summarizes information on fire regimes of Alaskan white spruce communities that was available in the scientific literature as of 2014. Details and documentation of source materials follow this summary.
White spruce is widespread throughout Alaska, particularly in the interior regions. The distribution of white spruce communities is influenced by fire history, elevation, soil drainage, topography, location of permafrost, and climate. White spruce commonly occurs in mixed-hardwood and mixed-conifer communities.
Historically, most fires in the Alaskan boreal forest were caused by lightning in June or July. Historical mean fire-return intervals (MFRIs) in boreal white spruce communities range from about 40 to >250 years (see Table 2), and MFRIs in subboreal white spruce communities are longer. Floodplain, stringer, and treeline white spruce communities may have longer fire-return intervals than other boreal white spruce communities. Ground, surface, and crown fires can occur in white spruce communities, although crowning is generally less frequent than in Alaskan black spruce communities. Most fires are stand-replacing because white spruce is sensitive to fire. Most fires in the Alaskan boreal forest are small, but large fires account for most of the acreage burned.
Current fire regimes in white spruce communities may not differ much from historical regimes because most of the Alaskan boreal forest is sparsely populated and has little road access. However, fire regimes in localized regions may have been influenced by human activity. Human-caused ignitions are increasingly common near settlements, but human-caused fires tend to be small because these areas are also where fires are actively suppressed. Climate change may cause the area burned across Arctic and boreal regions to increase due to longer fire seasons, less effective moisture, and higher ignition rates. Climate change models predict varied effects on white spruce communities; some communities may expand while others may decline.
Appendix A summarizes data generated by LANDFIRE succession modeling for the Biophysical Settings (BpSs) covered in this review. The range of values generated is shown in Table 1.
|Table 1. Modeled fire-return intervals and severities in Alaskan white spruce communities |
|¹Average historical fire-return interval derived from LANDFIRE succession modeling (labeled "MFRI" in LANDFIRE).
²Percentage of fires in 3 fire severity classes, derived from LANDFIRE succession modeling. Replacement-severity fires cause >75% kill or top-kill of the upper canopy layer; mixed-severity fires cause 26%-75%; low-severity fires cause <26% [13,78].
³NA (not applicable) refers to BpS models that did not include fire in simulations.
DISTRIBUTION AND PLANT COMMUNITY COMPOSITION
|Figure 2. Land cover distribution of Alaskan white spruce communities based on the LANDFIRE Biophysical Setting (BpS) data layer . Numbers indicate LANDFIRE map zones. Click on the map for a larger image and zoom in to see details.|
White spruce communities are widely distributed in boreal and subboreal Alaska. Approximately 30% of Alaska's landscape is boreal forest [28,121], and white spruce occupies about 10% of that region [22,28]. Alaskan white spruce communities typically occur on well-drained sites with a deep or absent permafrost layer [85,108,128,129,137]. White spruce communities may generally be distinguished as floodplain, upland, and treeline types . Floodplain white spruce stands are highly productive [100,128] and occur along valley floors and river terraces [123,126,128]. Upland white spruce communities are widespread and generally occur on relatively warm, well-drained, south-facing slopes [46,121,129,133]. Treeline communities at the transition of boreal forest and tundra are often dominated by widely spaced white spruce . In this review, "white spruce stringer communities" refer to a minor stand type that occurs in wet depressions on upland, south-facing slopes , rather than Western North American boreal riparian stringer forest and shrubland as described by NatureServe .
In Alaska, white spruce communities are most common in the boreal interior, between the Brooks Range in the north and the Alaska Range in the south. They also occur in subboreal south-central Alaska on the Kenai Peninsula [16,51,99]. White spruce is less common in coastal regions . Westward, white spruce communities extend to the Seward Peninsula; eastward, they extend throughout much of Canada and into the northeastern United States [37,67].
White spruce dominates or codominates many Alaskan boreal landscapes and often occurs in mixed-conifer and mixed-hardwood stands [85,99,100,133]. At the landscape level, Alaskan boreal white spruce communities form mosaics with quaking aspen, paper birch, balsam poplar, black spruce, and mixedwood stands [124,140]. Stand composition varies due to differences in fire history, elevation, soil drainage, topography, location of permafrost, and climate [129,130]. Several authors do not distinguish among spruce types; consequently, this review uses the term "spruce" when white, black, or mixed spruce communities are not defined in the literature. Understory dominants vary with site but often include willows, alders, prickly rose, and ericaceous shrubs in the shrub layer. Feather mosses, lichens, horsetails and/or herbaceous plants often dominate the ground layer [47,85,99,128].
Appendix A lists the Biophysical Settings (BpSs) covered in this review and summarizes data generated by LANDFIRE's  successional modeling for those BpSs.
NatureServe  identifies the following white spruce forest and woodland types in Alaska. Corresponding BpS types are given in parentheses.
HISTORICAL FIRE REGIMES
Historically, lightning was the main source of ignition in the Alaskan boreal forest [7,56,91]. Although fire records prior to European settlement are lacking [7,48], the concentration of contemporary lightning-caused fires (Figure 3) reflects the historical occurrence of lightning-ignited fire . Summer thunderstorms are common in Alaska [28,31], with almost 90% of the lightning strikes occurring in interior Alaska . Within the interior boreal forest, lightning strike density tends to increase from west to east while precipitation decreases [31,71], which may contribute to a west-to-east increase in fire frequency [28,71].
Lightning-caused fires are rare on the Kenai Peninsula. From 1939 to 1993, there were only 13 documented lightning-caused fires .
|Figure 3. Distribution of lightning-caused fires (left) and human-caused fires (right) from 1957 to 1979. Because remote, lightning-caused fires often went undetected, their occurrence may be greater than shown .|
In interior Alaska between 1986 and 1999, boreal forest had higher lightning strike densities than tundra or shrublands , and between 1992 and 2000, forested areas had higher densities of lightning-caused fires than nonforested areas . Using lightning detection sensors, Kasischke  found a significant correlation between total lightning strikes and the number of lightning-ignited fires in Alaska in the 2000s (r=0.79, P=0.01).
Moderate to strong El Niño episodes generally produce dry thunderstorms with associated lightning activity and warm, dry conditions in the Alaskan interior. These conditions led to 15 out of 17 of the biggest fire years between 1940 and 1998. These 15 years accounted for nearly 63% of the total area burned during this time period .
Although human-ignited fires have greatly outnumbered lightning-ignited fires throughout Alaska in recent decades [2,8,28,70,119] (see CONTEMPORARY CHANGES IN FIRE REGIMES), lightning-caused fires tend to be larger and account for most of the area burned in interior boreal forests [28,30,119]. From 1956 to 1999, over 90% of the area burned in Alaska was ignited by lightning, and most was in the interior .
There is a long history of native Alaskan burning in boreal forests in interior Alaska [9,87,96,97,106]. Gwich'in Athabaskan groups used fire for hunting, signaling, fuel reduction, and habitat enhancement for wildlife, berries, and medicinal herbs [96,97]. Historical and contemporary accounts of native Alaskan burning describe intentional burning in seasons with high precipitation or when conditions were not conducive to extensive fire spread. Before European settlement, human-caused fires were likely concentrated in eastern interior Alaska where native Alaskans used fire as a management tool. Roessler and Packee  argue that intentional and accidental burning by Athabaskan Indians and early explorers has been underestimated in the Tanana River Basin. The actual impact that native Alaskans have had on Alaska's interior remains unclear .
Most wildfires in the Alaskan boreal forest burn during June and July [28,130], but annual variability in the fire season is high [28,71]. The fire season in interior Alaska is defined as occurring between April 1 and September 30; however, fires can occur whenever fuels are not covered with snow and are dry enough to burn . Generally, the peak fire season (i.e., June-July) coincides with periods of high temperatures and lightning activity, and low humidity and precipitation [28,130]. The fire season in interior Alaska is typically short due to mid- to late-summer precipitation; July and August are the wettest months .
"High fire years" (i.e., years where the area burned is >1.5 times the long-term average area burned) are associated with extended warm, dry periods and large fires that burn late in the growing season. Most of the area burned in Alaska is burned during high fire years. In a typical year, most ignitions (76%) and fire spread occur from the beginning of June to mid-July. In high fire years, most area burns in July, and fires may burn into August and September . A study in black spruce stands indicated that fire severity—defined as depth of burn—tended to increase over the course of the fire season . Although white spruce communities were not investigated, this may apply to white spruce types. In general, larger fires in Alaskan boreal forests tend to have higher burn severities (P=0.02), perhaps because they burn into late summer . It seems likely that this pattern would apply to white spruce forests.
Human-caused fires lengthen the fire season [30,74]. Throughout interior Alaska, from 1992 to 2000, 85% of lightning-caused fires began in June and July, when convective thunderstorms were frequent and after soils dried from snowmelt, but before late-July and August rain. Human-caused ignitions occurred most frequently in May and extended the fire season by about 2 months , (see CONTEMPORARY CHANGES IN FIRE REGIMES: Causes of Ignition and Fire Size).
Sources suggest that the Alaskan boreal forest has a fire-return interval or fire-rotation interval in the range of 50 to 200 years [57,121,134,139]. Fire-return intervals of many white spruce communities fall within this range; however, white spruce stringer, treeline, and some floodplain communities may have longer fire-return intervals. Studies that assess fire frequencies among different forest types have found that white spruce forests have longer fire-return intervals than adjacent black spruce and hardwood forest types [28,139]. Fire history studies of Alaskan white spruce communities indicate a range of MFRIs from 40 to 515 years and fire-rotation intervals from 113 to 279 years (Table 2). In the boreal forest, many sites that are capable of becoming white spruce stands often burn before they reach the mature white spruce stage. Consequently many stands are dominated by earlier successional species (e.g., paper birch and quaking aspen) . Fire-return intervals less than 30 or 40 years may result in white spruce recruitment failure because seed production "in quantity" typically begins once stands are that old or older [101,117,141].
|Table 2. Fire frequency information for white spruce types in Alaska. Fire-return intervals/fire-rotation intervals, scope of study, and time period studied applies to white spruce sites only; site type and community description indicate general site characteristics of the study area.|
|a. Historical fire frequencies|
|Location (approximate LANDFIRE map zone)||Fire-return interval or fire-rotation interval (shaded) in years||Scope of study||Time period studied||Site type, linked to discussion in text||Community description|
|Yukon Flats National Wildlife Refuge, northeastern interior (zone 70)||x̄=82* measured with fire scars; x̄=92* measured with tree cohort ages (106)**||40 fires on 27 study sites, white spruce stands occurred on 17 study sites
||1751-1999||boreal forest||mosaic of conifer, hardwood, and mixed stands; white spruce stands common on well-drained sites |
|Porcupine River and Upper Yukon River drainages, northeastern interior (zone 70)||105 (106)**||89,000,000 acres, 371 stands (some dominated by white spruce); 201 of 968 sampled trees were white spruce||~1770-1980||boreal forest||mosaic of white spruce, black spruce, and hardwood stands on gentle slopes, broad floodplains, terraces, and low ridges |
|Western Kenai Peninsula (zone 75)||x̄=515 (626)**||22 sites||~2,500 BP-present||subboreal forest||upland white spruce and Lutz spruce forests |
|Noatak watershed (zone 68)||228-279||1,253,000 acres, 15% of watershed||1956-1983||treeline||tundra with white spruce forest patches |
|Riley Creek, Central Alaska Range (zone 73, border with 74)||~40-60 (156)**||1 ~370-acre site; 6 of 8 terraces contain white spruce||1740-1920s||floodplain||alluvial fans and terraces with mixed stands of white spruce, quaking aspen, and balsam poplar |
|Tanana River floodplain, near Fairbanks (zone 74, near 70 and 73)||70-110 (294)**||1 site with 4 stands (3 with white spruce)||~1800-1993||floodplain||mosaic of open and closed floodplain forest; white spruce and black-white spruce forests dominate meander belts |
|Caribou-Poker Creek Research Watershed, near Fairbanks (zone 70/74 border)||>250 (106)**||4-47 trees sampled on each of 21 sites (some dominated by white spruce)||1750-1999||stringer||white spruce upland stringer forests within paper birch, quaking aspen, and black spruce forest on south-facing slopes |
|Caribou-Poker Creek Research Watershed, near Fairbanks (zone 70/74 border)||>200 (106)**||1 watershed, 6 stringers||
|stringer||upland mature white spruce stringer forest within young birch-spruce matrix on south-facing slope |
|*Standard deviations: x̄=82±38(SD); x̄=92±38(SD).
**The MFRIs predicted by LANDFIRE models for similar vegetation types are shown in parentheses. See Appendix A for further information on BpS groups.
|b. Paleoecological fire frequencies|
|Location (approximate LANDFIRE map zone)||Fire-return interval (years)***||Scope of study||Time period studied***||Site type (link to discussion)||Community description during time period studied (contemporary community description)|
|Paradox, Portage, and Arrow Lakes, Kenai lowlands, Kenai Peninsula (zone 75)||x̄=81||3 lakes, 4 sediment cores||8,500-4,600 BP||subboreal forest (Ancient fire regimes)||white spruce-birch (white spruce and birch on well-drained soils and slopes, and black spruce on poorly drained lowland flats) |
|Ruppert, Code, and Wild Tussock Lakes ; Ruppert, Xindi, and Wild Tussock Lakes , south-central Brooks Range (northern edge of zone 71)||x̄=131-238; composite MFRI=227 
|4 lakes, 2 sediment cores from each||8,500-5,300 BP||treeline (Ancient fire regimes)||white spruce forest-tundra treeline with birch and/or alder (discontinuous black spruce-dominated forest surrounds lakes) [59,60]|
|Grizzly Lake, Copper River Basin (zone 74)||x̄=386 and 201||1 lake, 6 sediment cores||6,800-5,500 BP and 3,900-2,750 BP||boreal forest (Ancient fire regimes)||south-facing boreal forest dominated by white spruce (white spruce around lake, slopes, and treeline; black spruce extensive in lowlands) |
|Chokasna and Moose Lakes, Wrangell-St. Elias National Park and Preserve, south-central Alaska (border of zones 74/77)||x̄=190->500||2 lakes, 3 sediment cores||7,000-2,000 BP||boreal forest (Ancient fire regimes)||closed white spruce, black spruce, and birch forests (white spruce-hardwoods dominate 1 lake; black spruce dominates other lake) |
|Dune Lake, interior Alaska, near Fairbanks (border of zones 73/74)||Fires rare (no charcoal peaks), but infrequent, small, low intensity fires possible||1 lake||10,000-5,500 BP||boreal forest (Ancient fire regimes)||birch-white spruce-alder communities (open white spruce-hardwoods on well-drained, south-facing dune slopes, young hardwoods on recently burned area) [88,91]|
|Low Lake and Farewell Lake, Alaska Range (zone 73)||x̄=>400||2 lakes||8,500-6,600 BP||boreal forest (Ancient fire regimes)||forest-tundra ecotone transition to closed white spruce-dominated forest (no description of contemporary community) |
|***Paleoecological fire frequencies and time periods are reported for periods of white spruce dominance or codominance.|
Boreal forest: Because white spruce communities occur within a matrix of other forest types (e.g., black spruce, quaking aspen, paper birch), fire frequencies are difficult to distinguish from and strongly influenced by neighboring forest types. MFRIs calculated for boreal white spruce stands (not including floodplain stands) range from 82 to 105 years. Although no substantial differences in fire regimes among white spruce types have been reported , wet stringer and treeline communities appear to be less fire-prone than other boreal white spruce communities. However, the scarcity of fire history studies makes comparisons among community types difficult. See those sections for additional details.
Boreal white spruce forests tend to have less frequent fire than adjacent forest types in interior Alaska [28,32,98,139]. A large-scale analysis of 371 stand ages in the approximately 89,000,000 acre (36,000,000 ha) Porcupine and Upper Yukon river drainages of eastern Alaska, found hardwood stands had the shortest fire-rotation interval (26 years), followed by black spruce (36 years), then white spruce (113 years); and fire-return intervals were estimated to be 30, 43, and 105 years for hardwood, black spruce, and white spruce stands, respectively. Stands over 200 years old were omitted from analysis (3 stands) . Another fire history study of 27 sites in the same area found that the MFRI in white spruce/quaking aspen stands (82 years) was not significantly different than that in black spruce stands (67 years) . High fire frequency in black spruce communities is often attributed to the species' high flammability [20,59,63,88,90,98]. White spruce stands are also flammable under some conditions, but they are less so than black spruce [20,90].
Lightning-caused fires are rare in Wrangell-St. Elias National Park and Preserve in southeastern Alaska and adjacent Kluane National Park, Yukon Territory, areas dominated by boreal white spruce and quaking aspen. Between 1957 and 1979, there was a 0.04 probability of fire in the Wrangell Mountains for a forested area the size of Kluane National Park (unpublished data from the Bureau of Land Management, Alaska, in ). These parks are south of where most lightning-caused fires occur in the region (Figure 3). Fire scars and stand ages in predominantly white spruce forests in Kluane National Park indicated MFRIs ranged from 113 to 238 years. Study sites were established along fire boundaries in order to locate fire scars, so these MFRIs may be shorter than if study sites were randomly located. Human-caused fires were included in this study, which suggests that these MFRIs are shorter than if only lightning-caused fires were included. The author speculates that MFRIs would increase to 200 to 300 years if human-caused fires were excluded .
Subboreal forest: Most historical and paleological evidence suggests that subboreal white spruce communities on the Kenai Peninsula burned infrequently [16,18,103]. Soil charcoal data from the western Kenai Peninsula suggest that upland white spruce and Lutz spruce forests have not burned for an average of 600 years. Across 22 sites, time-since-fire ranged from 90 to ~1,500 years. Over the last ~2,500 years, the mean fire-return interval was 515 ± 355, with intervals ranging from 105 to 1,642 years . This long fire-return interval contrasts with the relatively short interval found from another study in the northern part of the study area. Lake sediment charcoal analyses at Paradox Lake indicated the MFRI from the early Holocene (13,500 BP) to present ranged from 128 to 77 years; the MFRI was 81 years when white spruce and birch forest dominated the landscape during the middle Holocene (8,500-4,600 BP) . Berg and Anderson  acknowledge that the MFRI estimate of 130 years at Paradox Lake over the last 4,600 years differs substantially from their estimate of 285 years at the same location. They suggest this difference is due to different sampling methodologies.
A fire history study of white spruce stands in the Kenai National Wildlife Refuge (KNWR) indicated that 1 fire in the 1880s, burned approximately 60% of the 257,000-acre (104,000-ha) study area. Based on the mean age of the oldest trees, the estimated time period studied was 234 years. Although widespread presence of charcoal indicated that almost the entire area had burned at some time in the past, the dominance of a single fire suggests that 234 years is too short to describe the fire frequency of white spruce forests in the KNWR. However, the authors calculated a fire cycle of 229 years while acknowledging its probable imprecision . The cause of the 1880s fire is not indicated, but human-caused fires were common during this settlement and gold prospecting period . The 195,858-acre (79,260 ha) Funny River Fire in 2014  burned much of the same area as the 1880s fire, suggesting that, under the right weather conditions, these forests in the central Kenai Lowlands are capable of fueling large fires after approximately 130 years (Gracz 2014, personal communication ). The Funny River Fire and the 1880s fire were large; however, unsuppressed human-caused fires often fail to burn substantial areas of white spruce forest in this area. In fact, a nearby 1994 active crown fire burning in black spruce under relatively dry conditions stopped when it reached white spruce forest .
Although quantitative fire history studies of mountain hemlock-Lutz spruce forests on the Kenai Peninsula were not available at the time of this writing (2014), other evidence suggests that these forests had long fire-return intervals historically. The cool, wet climate and rare lightning occurrence in spruce-mountain hemlock forests make wildfire infrequent . The Kenai Peninsula is wetter than interior boreal forests, yet drier than the coastal rainforest, suggesting that these forests may have fire-return intervals that are longer than in the interior but shorter than in the coastal rainforests. It takes at least 300 to 400 years for mature mountain hemlock to dominate the forest, and prior to the settlement period (late 1800s), most of the forests were recorded to be in late successional stages. This suggests that stand replacing fires had not occurred in a very long time prior to the settlement period. Although no fire scars were observed, subsurface soil charcoal from 5 different sites was dated at 3,010, 2,430, 1,540, 1,290, and 570 years BP, indicating infrequent fire occurrence .
Spruce beetle outbreaks that cause extensive white spruce mortality occur on average every 50 years on the Kenai Peninsula, but no association was found between fire activity and past outbreaks .
Treeline: Alaskan white spruce treeline communities probably have infrequent fire. Some treeline sites, such as the forest-alpine tundra zone on the south slope of the Brooks Range, have no evidence of fire . In the 1,253,000-acre (507,000-ha) Mission Lowlands region of the white spruce-tundra ecotone of the Noatak River watershed, the fire-rotation interval calculated using fire data from 1956 to 1983 ranged from 228 to 279 years . While this interval may better represent the tundra because it dominates the watershed, there was no evidence of forested stands burning more frequently. The authors noted a tundra fire that burned to the margin of white spruce stands but barely spread into them, probably because white spruce forest fuels had higher moisture content. The frequency and extent of wildfire in the Noatak watershed may be greater than in other tundra regions of arctic Alaska, even though the conditions that cause fires rarely occur in the Noatak watershed. In most years, there is only a 1- to 3-day period when the combination of dry weather followed by lightning strikes makes ignition possible . In a review of treeline habitats, Van Ballenberghe  states that "wildfire frequencies in treeline environments appear to be low compared to adjacent lowlands". This may be because mountains have cool temperatures, high humidity, and cloudy conditions, which may prevent heat build-up and thunderhead formation .
While most of the literature describes treeline communities has having infrequent fire, fire frequency at treeline sites likely varies by site and plant community. Viereck  states that "on the Seward Peninsula, northwestern Alaska, and the south slope of the Brooks Range, fire occurs very frequently in treeline areas". No data are given to support this statement. Viereck himself acknowledges a lack of studies in Alaskan treeline communities and indicates that fire history in these communities needs additional study. It is possible that he was referring to black spruce-lichen woodlands below treeline and not white spruce treeline stands. Spruce-lichen woodlands below treeline may have relatively high fire frequencies due to highly flammable fruticose lichens and dwarf ericaceous shrubs [5,27]. On the south slope of the central Brooks Range near Walker Lake, little sign of fire was found in white spruce forest at treeline or in stands along rivers [27,49], whereas most spruce-lichen woodlands below treeline had signs of fire . Fire-scarred trees and stand age distributions suggest that some black and white spruce-lichen woodlands burned twice in the past 150 years, but the absence of trees with multiple scars precludes quantifying a precise fire-return interval. The author speculates that fire frequency in spruce-lichen woodlands at Walker Lake may be lower than areas of continuous spruce-lichen woodlands in Canada, due to vegetation discontinuity and topographic variation at Walker Lake .
In white spruce and black spruce treeline stands in the southern Brooks Range, postfire recruitment dynamics suggest that white spruce would replace black spruce if fire-return intervals were sufficiently long. Following a fire in the early 1900s, black spruce had high recruitment for <30 years, while white spruce recruitment has been consistently high since the fire, and the majority of seedlings in the stands at the time of study were white spruce. This suggests that white spruce will become increasingly dominant in these stands . In theory, black spruce requires periodic fires to maintain dominance; however, modeling of black spruce suggests that it is stable in the absence of fire and under long fire-return intervals due to low adult mortality rates. In fact, models suggest that fire-return intervals <350 years may destabilize black spruce populations and fire-return intervals <100 years would be associated with rapid population declines. This may be due to black spruce's slow growth and low seed production at its northern limit (i.e., fires occurring before trees mature and produce sufficient viable seeds). However, the models did not include interactions with white spruce, and the authors indicate that white spruce may become dominant in stands with long fire-return intervals because it recruits more successfully in unburned conditions .
Floodplain: Several authors describe Alaskan floodplain white spruce forests as having small, infrequent fires or infer that fires are not ecologically important on floodplain sites (e.g., [57,121,130]). Heinselman  stated that of all boreal forest regions in Canada and interior Alaska, "the longest fire cycles were probably in white spruce forests on the floodplains of major rivers where cycles may have been as long as 200 to 300 years. Perhaps some floodplain forests never burned". Compared with nearby black spruce forests, white spruce floodplain forests likely experience less frequent fire. In the Yukon Flats, muskeg and black spruce forest is highly prone to fire (MFRI = 50-60 years) (personal communication in ), whereas white spruce floodplain forests are less prone to fire because they are protected by river channels, sloughs, and oxbow lakes . Additionally, these forests typically have high moisture conditions on the forest floor, and alders and willows—which are not flammable during the growing season—often occur in the understory . Along the Mackenzie River, Northwest Territories, floodplain white spruce forests were "resistant" to fire; some stands had not burned in 300 years [57,109].
Two studies suggest that fires occur in white spruce floodplain forests more frequently than otherwise assumed. Along 1 site on the Tanana River floodplain, fire scars, tree age distributions, and buried charcoal suggest relatively frequent fires. White spruce and black spruce age distributions indicated that the interval between fires ranged from 70 to 110 years over the past 200 years. The authors suggest that fires are widespread on floodplains, except on islands . On the mixed hardwood-white spruce floodplain of Riley Creek, near the entrance to Denali National Park and Preserve in the Central Alaska Range, results of a small-scale study (~370 acres (150 ha)) suggest that fires occurred at 40- to 60-year intervals during the 150 years prior to active fire suppression . Compared to the Tanana River, Riley Creek is a lower volume river with a much narrower floodplain (i.e., it is not considered a "large river" in the LANDFIRE BpS 16150 series). Based on tree age distributions and fire scars along a chronosequence of geomorphic surface ages, the authors infer a fire regime of frequently recurring, low-intensity, surface fires that maintain sprouting species (i.e., quaking aspen) on upper terraces. Although fires were inferred to be lightning-caused, the location of the study site (i.e., adjacent to Denali National Park and Preserve headquarters and the railroad) suggests that fires may have been influenced by settlement and railroad construction toward the end of the study period .
|Figure 4. Aerial view of the Tanana River floodplain. White spruce floodplain forests are the patches of dark vegetation. Photo courtesy of Trish Wurtz, USDA.|
Stringer: Fires may occur less frequently in white spruce upland stringer forests than surrounding young forests. Two studies in the Caribou-Poker Creek Research Watershed near Fairbanks found that stand-replacing fires were absent from white spruce stringer forests for over 200 years [43,104]. The estimated fire-return interval for surrounding birch-spruce forests was approximately 65 years , and those for nearby black spruce stands were 102 and 146 years . Stringer forests were located in depressions along rills or swales with diffuse springs. Quirk and Sykes  concluded that these sites were less susceptible to fire than surrounding forests due to higher soil moisture and sheltered topography. They suggest that "white spruce stringers may be stable islands characterized by a moister physiography among a less stable and drier matrix of young birch-spruce". While the conclusion that stringers are less fire-prone than the surrounding young birch-spruce matrix due to moister physiography may be accurate, the authors assume a "long-term cycling" of fires that burn every 40 to 60 years in the birch-spruce matrix; they do not provide evidence of this, however. Fastie and others  note that low intensity surface fires may occur with higher frequency than indicated by these studies but remain undetected because they may not kill or scar trees and do not promote tree recruitment.
Ancient fire regimes: Paleoecological studies of ancient fire regimes indicate that climate, lightning occurrence, and vegetation flammability are major drivers of fire regimes. Sediment-charcoal records indicate that warm, dry periods are associated with higher fire frequencies , increased biomass burned , higher fire severity , and larger annual area burned ; however, in some instances, vegetation flammability may override climatic factors [20,59,60,63,72]. In Alaska, more frequent fires were sometimes associated with cooler and wetter climatic conditions [59,60,62,72,89,90]. Researchers concluded that this counterintuitive increase in fire frequency was associated with the spread of black spruce, because black spruce stands are more flammable and support fire spread more easily than white spruce, hardwood, or tundra vegetation [28,60,63,72,88,90,114]. Fire-return intervals were unchanged in the mid-Holocene when deciduous woodland transitioned to white spruce forest-tundra despite cooler summer temperatures and higher relative moisture. The authors suggest that the cooler, wetter climate was countered by an increase in landscape flammability resulting from the replacement of poplar by white spruce . Other studies report shorter MFRIs during wet conditions and attributed this to large-scale climatic patterns, which may have contributed to increased lightning strikes and ignitions [89,90]. The relative importance of climatic conditions and vegetation type/fuel availability may vary over time and among regions (e.g., [11,91,118]).
Paleoecological data suggest a variety of fire regimes were present when white spruce woodlands (~10,500-7,000 BP) dominated the landscape, but the overall importance of fire for interior Alaska and the Copper Plateau was low during this time period [28,88,91]. However, complete absence of fire is unlikely. Charcoal was found at 3 depths within the birch-white spruce strata in Dune Lake, interior Alaska. These few charcoal particles suggest that small, low intensity, infrequent fires may have occurred . Paleoclimate data, climate analogs, and model simulations suggest that the regional climate during the white spruce woodlands period was warmer and drier than present, but becoming cooler and wetter leading into the boreal forest period (7,000 BP-present). Fire frequency generally increased during the boreal forest period along with the replacement of white spruce woodland with black spruce forest (reviewed in ).
Estimated MFRIs during periods of white spruce dominance over the past ~10,000 to 2,000 years ranged from 81 years on the Kenai Peninsula  to >500 years in Wrangell-St. Elias National Park and Preserve, south-central Alaska  (Table 2b). The high variability in MFRIs in ancient white spruce communities shown in this table may be attributed to differences in vegetation types, geographic and topographic locations, local climate, lightning occurrence, and sampling methodologies.
Alaskan white spruce communities typically experience stand-replacing ground, surface, and crown fires. In general, white spruce stands experience less frequent crowning than black spruce stands because white spruce trees have fewer ladder fuels (higher canopy base height) and lower resin content in the needles than black spruce [47,98,119]. However, during extended dry periods, white spruce stands will burn with characteristics similar to black spruce , and closed white spruce forests often experience high intensity crown fires or severe, stand-replacing surface fires . Intense surface fires may occur where feather moss-lichen understories are well developed . Ground fires may consume or damage vegetation throughout most of the burned area .
Fire behavior: Typical surface fire behavior varies among white spruce communities. Open forests and woodlands generally experience higher rates of spread and longer flames than closed forests (Table 3). In open forests, fires spread primarily in shrubs and litter, while in closed forests, fires spread in feather mosses, litter, and duff . Rates of spread and flame lengths may be greater in upland than in riparian areas because riparian areas typically have higher fuel moisture . Immature white spruce stands in both riparian and upland sites may experience fire behavior similar to that in closed black spruce stands, although crown fire is less likely due to the higher canopy base in white spruce stands . The closed black spruce fire behavior fuel model (FBFM) Timber-Understory 3 (TU3) is characterized by high rates of spread and moderate flame lengths . Fires in spruce-hardwood stands generally burn less intensely than in pure white spruce stands. Fires may creep in the surface fuels with less frequent crowning than in pure white spruce stands .
Fire behavior often differs between white spruce and black spruce forests. On the Kenai Lowlands, fires burning in black spruce often stop when they reach white spruce forests (Gracz 2014, personal communication ). Two fires were described as "spreading continuously" through black spruce stands, but stopped at the edges of the small hills and ridges dominated by white spruce, paper birch, and quaking aspen .
|Table 3. Fire behavior characteristics of white spruce vegetation types* [23,112] .|
|Vegetation type||Primary fire carrier||FBFM||Rate of spread||Flame length|
|Closed white spruce forest||feather mosses, litter, duff||TU1||low||low|
|Open white spruce forest||shrub, litter||TU5/TU4||moderate||moderate|
|White spruce woodland||feather mosses, shrub||TU5||moderate||moderate|
|Spruce-paper birch-aspen||leaf litter||TL6||moderate||low|
|White spruce-paper birch-quaking aspen||leaf litter, herbaceous plants||TU1||low||low|
|*Cella and others  crosswalked the FBFM40 with Alaskan vegetation types. Rate of spread and flame length descriptions are from the standard fire behavior fuel models .|
Fire severity in Alaskan white spruce communities is measured by canopy mortality, remotely-sensed satellite imagery, and forest floor consumption. Canopy mortality in Alaskan white spruce stands tends to be high. Fire severity studies that use remotely-sensed imagery often find a mosaic of fire severities, mostly moderate and high. Studies that report forest floor consumption as an index of fire severity show patchy patterns of consumption. Fire severity in boreal forests is generally a function of vegetation flammability, topography, weather, and elevation.
Fires in white spruce stands tend to burn with enough intensity to be stand-replacing [47,53,54,92,103,110]. White spruce trees are susceptible to fire mortality due to their thin bark and shallow roots [61,85,117]. Consequently, even low-intensity fires may kill white spruce [53,54]. In closed white spruce-balsam poplar alluvial stands of the Bear Creek Fire in interior Alaska, the fire killed all the trees but left the canopy intact. "Many fine fuels (needles, leaves, small twigs) remain on shrubs and low tree branches, indicating this was not the result of an intense (hot) fire. Evidently the flame front passed through the area quickly, yet the fire continued to smolder in the dry duff, finally consuming it". In stands composed of approximately equal amounts of black and white spruce with 50% canopy closure, the same fire "burned both severely and intensely, crowning and killing all trees" . Fires in black and white spruce-reindeer lichen woodlands on the south slope of the central Brooks Range are typically patchy and create areas of high, moderate, and low fire severity, although few white spruce survive .
Fire severity evaluated with remotely-sensed imagery is not typically reported for white spruce stands in mixed forests because severity is typically assessed over entire fires rather than by vegetation type. Remotely-sensed fire severity indices, such as the differenced Normalized Burn Ratio (dNBR), refer to fire-caused changes to the soil, vegetation, and fuels , although some studies suggest that dNBR is disproportionately sensitive to changes in the vegetation canopy, and that it does not adequately distinguish between moderate and high burn severity in the soil in Alaskan boreal forests [12,95]. The Taylor Complex Wildfire, which had a prefire plant community of mixed white spruce, black spruce, quaking aspen, and paper birch, resulted in mostly high fire severity, but produced a mosaic of unburned patches and patches that ranged from low to high severity. Satellite imagery of the 1,181,840-acre (478,274 ha) fire showed that 58% of the mosaic was high severity, 30% was moderate severity, 5% was low severity, and 7% of the area was unburned. The maximum high-severity patch size was 317,560 acres (128,510 ha). Exposed soil ranged from ~18% on high-severity patches to ~4% on low-severity patches . On the Eureka Creek Fire, satellite imagery showed that fire severity was predominantly moderate (47%) and high (40.9%) in the closed coniferous forest (white and black spruce) and predominately moderate (60.2%) for open coniferous forests .
In Alaskan boreal forests, canopy mortality is often complete, whereas burn severity in the soil and forest floor may be highly variable [38,65,107]. Prefire organic layer depth and moisture content influence burn severity patterns. Deeper and drier organic layers encourage more intense surface fires and longer, deeper burning ground fires, which result in more complete consumption . In white spruce stands of the Rosie Creek Fire near Fairbanks, forest floor fire severity varied; stands lost between 5% and 76% of their prefire organic matter . In mesic white spruce-quaking aspen stands in Alberta that burned during a single day of high intensity fire that top-killed all of the sampled trees in the interior of the fire, the amount of forest floor combustion and exposed mineral soil (<0.8 inch (2 cm) humus) was highly variable. High variation in mineral soil exposure occurred both among stands and within stands; mineral soil exposure ranged from 0% to 100% among 10.8-feet² (1 m²) plots. On average, postfire organic layer depth was 40% less than on unburned sites outside of the burned area, and exposed mineral soil occurred on approximately 35% of the burned area . In the Gilles Creek Fire, near Fairbanks, the depth of burn ranged from 0.3 to 11.9 inches (0.8-30.3 cm), with a mean depth of 7.6 inches (19.2 cm) in black spruce stands and 4.5 inches (11.5 cm) in white spruce-quaking aspen stands. Prefire organic soil horizons were deeper in black spruce than white spruce-quaking aspen stands, but they were also wetter and less dense. This contributed to lower mean consumption of soil organic matter in black spruce stands (53%) than in white spruce-quaking aspen stands (66%) .
Research on forest floor burn severity in well-drained, permafrost-free, black spruce/feather moss stands found that postfire forest floors were comprised of small-scale mosaics of unburned, scorched, lightly burned, moderately burned, and heavily burned patches. The average decrease in forest-floor thickness across 7 black spruce/feather moss stands ranged from 27.4% to 63.1% . Although black spruce sites generally have lower temperatures and higher moisture content in the forest floor and soil than white spruce sites , the black spruce/feather moss stands in this study occurred on well-drained and permafrost-free sites, which may be similar to white spruce sites. Consequently, fire severity trends observed in this study may also apply to some white spruce/feather moss sites.
Vegetation flammability influences fire behavior and severity. Although white spruce is generally considered less flammable than black spruce [20,90], white spruce forest may burn just as severely as black spruce forest. In the Gilles Creek Fire, total combustion (aboveground and belowground combined) was 19% higher in white spruce-quaking aspen stands than in black spruce stands . Across 9 fires in 4 Alaskan National Parks, white spruce and mixed-white spruce upland forests had higher remotely-sensed mean fire severities than other vegetation types (i.e., black spruce, deciduous, or tundra). Most of the white spruce stands occurred on steep slopes and had >25% canopy cover before the fire, which may have contributed to high fire severities .
Results from 24 remotely-sensed fires in interior Alaska indicate that vegetation type is correlated with fire severity on flat landscapes but not on topographically complex landscapes [33,35]. On the Eureka Creek Fire, partially within the Yukon-Charley Rivers National Preserve, remotely-sensed fire severity was lower in deciduous forests (quaking aspen and paper birch) than in coniferous forests (black and white spruce). Some of the deciduous forest and shrub stands may have acted as barriers to fire spread , due to low flammability and lack of ladder fuels. Within the Gilles Creek Fire, deciduous vegetation made up a large proportion of the low severity and unburned patches within the fire, and may have stopped fire spread from black and white spruce stands .
Although vegetation type and flammability often influence where fires occur, during large fire years (>1,160,000 acres (470,000 ha) burned) different vegetation types burn in equal proportions within fire perimeters . This is probably because fuels are dry and flammable over large, contiguous areas [34,45].
Fire severity in boreal forests depends upon topography, fuel characteristics, weather, and elevation. Fire severity becomes more variable as topography becomes more complex [33,35]. For instance, in spruce sites, fires are generally more severe on rocky slopes or ridges than in valleys, and south and west-facing slopes typically have more exposed mineral soil after fire than north and east-facing slopes . Tree-level and stand-level fuel characteristics also influence fire severity. Lutz  noted that burning may be "deepest and most intense under trees". This may be because cone scales often form deep accumulations of fuel under trees, and because tree crowns intercept much of the precipitation, leaving fuels drier beneath them. Large fires tend to burn more severely than smaller fires, probably because large fires tend to occur late in the summer during years of low late-summer precipitation . Low elevation sites tend to have higher fire severities than high elevation sites, probably because higher elevation sites (i.e., above treeline) have less fuel. The Eureka Creek Fire burned upslope and then probably burned out within the alpine shrub-tundra zone above treeline .
Fire severity and season of fire affect successional pathways in white spruce sites. Postfire succession following stand-replacing fire on white spruce sites is typically characterized by white spruce gradually replacing herb, shrub, and hardwood stages after 100 to 150 years . Because fires in the Alaskan boreal forest typically occur before white spruce seed is ripe, seed may not be available for regeneration except from outside the fire perimeter or from unburned patches within [130,134]. Consequently, white spruce may not establish immediately after stand-replacing fires during a typical fire season. However, if surface or ground fires occur after white spruce seed is ripe, white spruce may have high postfire recruitment [53,54], and an even-aged white spruce stand may develop without the intervening hardwood stage [47,130,131,138]. For instance, 1 growing season after the August 1977 Bear Creek Fire in interior Alaska killed all the trees but left the canopy intact, white spruce seedlings were abundant (12,000 seedlings/acre (30,000 seedlings/ha)). The author speculates that a thin layer of fallen needles may have created a mulching effect over mineral soil, which may have stabilized seedbed moisture [53,54]. Low-severity surface fires in white spruce stands "usually do not cause a radical change in forest composition"; however, the proportion of quaking aspen, birch, and balsam poplar may increase as that of white spruce decreases .
|Figure 5. Mosaic of fire severity in spruce-hardwoods in Yukon-Charley Rivers National Park. Photo courtesy of the National Park Service.|
Fires typically do not burn uniformly in the Alaskan boreal forest, so they produce a mosaic of unburned, lightly burned, and severely burned areas (e.g., [3,38,104,115,125,135]). Topography, plant community composition, soil and fuel moisture, and weather influence burn patterns [3,135]. For instance, fire patterns may be more homogenous on flat landscapes than on hilly or mountainous landscapes , and differences in flammability among plant communities create different fire behavior [40,50]. Monitoring Trends in Burn Severity data from 2004 (high fire year) and 2006 (low fire year) indicate that 20% and 66%, of the area within fire perimeters did not burn, respectively, indicating the prevalence of mosaic fires in both high and low fire years .
Fire sizes in the Alaskan boreal forest range from small to very large [28,30,48]. Approximately 60% to 80% of all fires in Alaska are <12 acres (5 ha) , although large fires account for the most of the total area burned [7,30,44,70,71]. From 1957 to 1979, the average fire size in the Alaskan boreal forest was 2,110 acres (850 ha) in the northern plateaus and 4,720 acres (1,910 ha) in western Alaska physiographic provinces . Large, lightning-caused fires that burn about 62,000 to 124,000 acres (25,000-50,000 ha) are common . In severe fire years, individual fires in the boreal forest tend to be large—often burning about 124,000 to >500,000 acres (50,000->200,000 ha); in contrast, in unusually wet years, the area burned may be negligible .
Large fires typically occur episodically. Since the 1860s, large fire years occurred 17% of the time and accounted for 68% of the area burned throughout Alaska . From 1950 to 1999, the average fire size during high fire years was 50,200 acres (20,300 ha), whereas the average fire size was 19,300 acres (7,800 ha) in smaller fire years . Many large fires burn beyond the typical fire season, sometimes into September . These fires may begin as multiple small fires that merge together into a single large fire . Over the last 50 years, high fires years occurred, on average, every 4 years , although the frequency of large fire years and extreme fire events has varied since the 1940s .
Fire size is primarily driven by summer weather and climate patterns [10,28,34,45,70]. Large fires occur when extended periods of warm and dry weather (10-15 days) occur across the landscape. This weather pattern—often caused by surface-blocking high-pressure systems—dries out the fuels across the landscape and increases the fire danger [34,45]. Climate pattern indices such as the Eastern Pacific (EP), Arctic Oscillation (AO), Pacific Decadal Oscillation (PDO) and El Niño/Southern Oscillation (ENSO) influence fire weather at intra-annual, interannual (AO and ENSO), and decadal (PDO) time scales; these indices are correlated with fire occurrence and size [28,34,45,58]. From 1940 to 1998, 15 of the 17 largest fire years occurred during a moderate to strong El Niño episode. These 15 years account for nearly 63% of the total area burned during that 58 year time period .
On the Kenai Peninsula, only 2 large fires (>24,700 acres (10,000 ha)) have been recorded in white spruce or white spruce-mixed communities as of 2014. One fire burned sometime between 1883 and 1895  and the other in 2014 (Gracz 2014, personal communication ).
Lightning-caused fires burn more area than human-caused fires . See Causes of ignition and fire size for more information about contemporary human-caused fires.
Fire suppression in the Alaska began in 1939 [32,130] but was probably not widespread or effective due to limited resources and lack of fire detection [28,139]. In 1984, the Alaska Interagency Fire Management Plan (AIFMP) prioritized suppression efforts across the state. Approximately 65% of the land was designated to have a "natural" fire regime, where wildfires are allowed to burn unimpeded; 17% of the land was designated for full suppression, due to its proximity to communities and roads; and 16% of the land was designated for modified suppression [26,30,32].
Except in populated areas, the impact of fire suppression in interior Alaska may be relatively minor. On the Yukon Flats National Wildlife Refuge, fire-return intervals during the 1939 to 1984 suppression era (MFRI = 94±35 years) were virtually identical to those after 1984 (94±38 years) . The authors suggest that no difference was found between the 2 time periods due to the short time period that fires were actively suppressed in interior Alaska coupled with long MFRIs relative to the suppression period and high interannual variability. In a similar area, the fire-rotation interval for white spruce stands was 113 years when calculated by both including and excluding suppression era stand ages, which suggests that fire exclusion had had minimal impact on white spruce stands. However, fire exclusion did alter fire-rotation intervals in other vegetation types . Suppression efforts have reduced the area burned in populated areas of interior Alaska . From 1992 to 2000, on the 17% of land designated for full suppression, area burned decreased by 50% relative to areas designated for modified or no suppression, despite 50 times greater density of fires. While fire suppression reduced the area burned in all fuel types, it was somewhat more effective in non-forest vegetation. Fire suppression is effective in interior Alaska partly because managers focus their efforts on a relatively small portion of land .
Causes of ignition and fire size
Humans ignite most wildfires in Alaska; however, human-caused fires tend to burn less area than lightning-caused fires [2,8,29,30,51,70,119]. Over the past 2 decades, nearly twice as many fires in Alaska were human-caused compared to lightning-caused . However, within the interior, human-ignited fires were rarely >100 acres (40 ha) and therefore accounted for only 4.6% of the total area burned from 1992 to 2001 . Most of the human-ignited fires that were >100 acres (40 ha) occurred in forested fuel types, including mixed hardwood-white spruce forests . Human-caused fires tend to be small because they typically occur outside of the fire season (i.e., during suboptimal burning conditions), in populated areas where they are easily suppressed, and in vegetation that does not support large fires [21,30].
Human-caused fires extend the length of the fire season. In the Fairbanks region, human-caused fires begin in March and continue through October, 2 months before and after the lightning-fire season . Human-caused fires are most frequent in May, after snowmelt when dead grasses and litter are dry and before deciduous shrubs and trees leaf-out [8,30,98]. While early season fires had only a small effect on area burned for interior Alaska between 1992 and 2001, mixed hardwood-white spruce forests are particularly fire-prone before the leaves emerge on the hardwoods .
Change in fire frequency
On the Kenai Peninsula, mountain hemlock-white spruce and mountain hemlock-Lutz spruce forests probably had longer fire-return intervals before European settlement . Prior to the settlement period of the late 1800's, the majority of the coniferous forests on the Kenai Peninsula were reported to be in late successional stages, whereas the present landscape mosaic is comprised of forests in early successional stages. Undisturbed, mature forests lack evidence of fire scars, and subsurface soil charcoal in these forests is old. Subsurface charcoal was found at 5 of 6 undisturbed sites; charcoal from each site was attributed to 1 date per site. In the 1 site dominated by Lutz spruce, subsurface charcoal was 1,540±40 years old. In the 4 sites dominated by mountain hemlock, subsurface charcoal was about 600 to 3,000 years old, suggesting low incidence of fire before settlement .
Unintentional and prescribed fires may have increased fire frequency in some areas of white and Lutz spruce forest on the Kenai Peninsula. These forests probably had very long fire-return intervals (MFRI=~515 years) historically . Between 1977 and 1997, more than 10,000 acres (4,000 ha) of Lutz spruce-hardwood forests were burned to improve winter range for moose in the Kenai Mountains of the Chugach National Forest .
High latitudes are disproportionately affected by climate change [28,41]; the 2 to 3 °C warming measured in high latitudes since the 1950s is about 5 times greater than the global mean [41,64]. Warmer temperatures are expected to reduce snow, ice, and permafrost cover; decrease surface albedo; lengthen the snow-free season; and alter vegetation communities. These changes are likely to contribute to longer fire seasons and increased fire frequency, severity, and area burned (e.g., [2,11,14,25,28,41,64,69,70,116]). Over the last decade, fire size and the frequency of large fire years have increased in Alaskan boreal forest due to climate warming. Four of the 11 largest fire years on record since 1940 have occurred between 2002 and 2009 . Paleorecords from 14 lakes in the Yukon Flats ecoregion indicate that fire frequency has been higher in recent decades than at any other time since the establishment of modern boreal forests 3,000 year ago .
Although the response of fire regimes to climate change is complex, the area burned across Arctic and boreal regions will likely increase with the lengthening fire season, decreasing effective moisture and increasing ignition rates . The increase in area burned and predicted increases in fire frequency and severity in Alaskan boreal forests may convert coniferous forest (including white spruce forests) to early successional hardwood forests [14,72,111], which may subsequently lower fire frequency due to their lower flammability [14,72].
Kelly and others  predict that the recent, unprecedented fire activity may be short-lived, and fire activity may become similar to that of the Medieval Climate Anomaly (MCA) (1,000-500 years ago). During the MCA, the climate was warm and dry, which caused high biomass burning. However, because severe fires promoted less-flammable deciduous vegetation, fire was less frequent than in recent decades. Euskirchen and others  reviewed climate change and its relationship with biogeophysical and biogeochemical feedbacks, vegetation dynamics, and fire regimes in Alaskan boreal forests as of 2010. For more information about how climate change may affect fire regimes in Alaskan boreal forests and treeline communities, see the citations listed in Appendix C.
White spruce communities and associated fire regimes may have varied and complex responses to climate change. The distribution of white spruce may expand into black spruce communities with climate warming and permafrost degradation [108,137]. In interior Alaska, upland white spruce dispersed into typical black spruce habitat after fire and was not constrained by seed dispersal, establishment, or juvenile growth; however, its ability to persist depends on continued permafrost degradation in postfire environments . In Denali National Park and Preserve, extensive field sampling and tree occupancy models suggest that white spruce distribution may increase under climate warming into sites formerly underlain by permafrost . Some studies predict that increases in fire extent, frequency, and severity could facilitate a shift from coniferous forests to early successional hardwood forest [14,72,111], although this was not observed in Denali National Park and Preserve .
Several studies suggest that many white spruce communities may be negatively affected by a warming and drying climate [6,22,24,66,81,82,94,136]. In interior Alaska, white spruce trees grow best in the coolest, wettest years . A synthesis of white spruce growth across 25 sites in interior Alaska showed only 5 sites with consistently positive growth responses to warming temperatures; 19 sites had consistently negative responses, and 1 site had a mixed response. Four out of the 5 sites where white spruce had positive responses to warmer temperatures were located in the coolest, wettest regions of the Brooks and Alaska mountain ranges. This suggests that white spruce growth may benefit from warmer temperatures only in the coolest, wettest areas, whereas it is likely limited by warmer temperatures in most areas . White spruce trees may have positive growth responses to warming temperatures until they reach a temperature threshold above which water stress becomes limiting and growth declines rapidly [81,94,136]. At treeline sites in interior Alaska, the temperature threshold at which white spruce growth declines was estimated to be 11 to 12 °C mean July temperature . Modeling suggests that white spruce is limited by moisture once the average growing season temperature increases by 2 °C .
White spruce treeline communities may have mixed responses to climate warming. Models predict that the current white spruce-tundra ecotone will convert to white spruce forest, and white spruce will spread into previously treeless tundra [15,108,111]. Field studies have found recent, widespread advances of white spruce into the alpine tundra likely due to climate warming . Studies of white spruce treeline dynamics indicate both positive and negative growth responses to climate warming. Some trees respond positively to earlier spring thaw, whereas others respond negatively to warmer summer temperatures [66,136]. White spruce response to climate warming may depend on local site conditions. Trees in warm, well-drained sites with low soil moisture may be at greater risk from a warmer climate [66,108]. In addition, trees in sites where drought stress stems from excessive evaporative demand rather than a lack of soil moisture may also be at risk, even if the sites have high soil moisture . Where black and white spruce grow together at treeline, increasing fire frequency may favor black spruce recruitment over white spruce .
|Figure 6. Approximate location of 5 of the 8 fire history studies of Alaskan white spruce communities (in red polygon). Map based on the LANDFIRE Biophysical Setting (BpS) data layer .|
Fire history in Alaska is incomplete because documentary fire records only date back to the 1940s , and records may be missing or incomplete . Fire history is difficult to reconstruct in white spruce communities because fires are often stand replacing, so fire-scarred trees are rare [27,32,39,139]. The few fire history studies that date tree rings and fire scars are limited in spatial extent (e.g., [92,93]). Paleoecological studies of fire history lack annual precision , use different methodologies , and may not have modern climate analogs (e.g., forest-tundra period in ), which complicates the interpretation of ancient fire frequencies. The lack of long-term records and scarcity of fire history studies in Alaskan white spruce communities means that our understanding of historical fire regimes in these communities is incomplete.
In interior Alaska, upland white spruce communities occur in pure and mixed stands within a matrix of other boreal forest types (i.e., black spruce, quaking aspen, paper birch). Consequently, fire regimes of white spruce communities may be difficult to distinguish from, and strongly influenced by, neighboring forest types. Additionally, white spruce may be codominant on sites that are typically dominated by other species, such as boreal mesic black spruce forests (e.g., LANDFIRE 16041 BpS series ). For descriptions of fire regimes in communities where white spruce may be codominant, see the fire regime syntheses on Alaskan black spruce and Alaskan quaking aspen and balsam poplar communities.
Most literature describes fire regime characteristics of white spruce types in relation to other forest types rather than quantifying characteristics specific to white spruce communities. For instance, white spruce is generally described as less flammable than black spruce but more flammable than hardwoods (e.g., [20,90]) and having fire-return intervals longer than black spruce types. Similarly, fire behavior in white spruce stands is generally described as experiencing less frequent crowning than black spruce [47,98,119], but descriptions of fire behavior and fire type in white spruce communities are lacking. Descriptions of fire pattern and size specific to white spruce communities were not found in the literature, so the patterns described in this synthesis apply to Alaskan boreal forest in general. There is very little information to support generalizations about white spruce communities, and the variations in these communities limit the usefulness of generalizations for management.
A few studies suggest that some white spruce communities (e.g., floodplain or stringer) may experience short-interval, low-intensity fires in addition to long-interval, stand-replacing fires [28,43,93]. However, low-intensity fires are difficult to detect in fire history studies because they may not scar trees and do not promote tree recruitment. The fuel models used by fire managers suggest that surface fire behavior in white spruce communities is typically characterized by slow rates of spread and low flame lengths, lending support to the idea that white spruce communities experience low-intensity fires. Perhaps records of recent fires in white spruce communities would contribute to this understanding.
Considerations for LANDFIRE
Although few studies describe fire regimes in white spruce treeline communities and fire history varies by site, fire frequencies on treeline sites may be lower than modeled by LANDFIRE. LANDFIRE BpS models of the Western North American Boreal Treeline White Spruce Woodland-Boreal (16011 BpS series e.g., ) yield a MFRI of 104 years, which may be more frequent than documented by the scarce literature. In interior Alaska, fire frequencies are generally lower at higher elevations due to lower productivity (i.e., less fuel) and less conducive fire weather [71,120]. Some studies found very little evidence of fire in white spruce treeline communities (e.g., [27,49]). Paleoecological data indicate that fire-return intervals for white spruce forest-tundra were >200 years in the south-central Brooks Range, when the climate was warmer and dryer than it is today [59,60].
APPENDIX B: Common and scientific names of plant and lichen species and links to FEIS reviews
|These species are common in Alaskan white spruce communities and referred to in this review. Follow the links to FEIS reviews for further information.|
|Common name||Scientific name|
|balsam poplar||Populus balsamifera subsp. balsamifera|
|black spruce||Picea mariana|
|paper birch||Betula papyrifera|
|Lutz spruce||Picea × lutzii (P. glauca × P. sitchensis)|
|mountain hemlock||Tsuga mertensiana|
|quaking aspen||Populus tremuloides|
|white spruce||Picea glauca|
|dwarf birch||Betula nana|
|green alder||Alnus viridis|
|prickly rose||Rosa acicularis|
|russet buffaloberry||Shepherdia canadensis|
|Sitka alder||Alnus viridis subsp. sinuata|
|swamp red currant||Ribes triste|
|bluejoint reedgrass||Calamagrostis canadensis|
|reindeer lichens||Cladonia (Cladina) spp.|
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