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Fire regimes of Alaskan wet and mesic herbaceous systems

Table of Contents:

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

Literature reviews describing fire regime characteristics of some wet and mesic herbaceous systems in North America were used in this review [27,41,80,114,117].

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

LANDFIRE models do not include fire in freshwater marshes, herbaceous peatlands, and herbaceous floodplain wetlands. They do include fire in some wet and mesic meadows. Appendix A summarizes data generated by LANDFIRE succession modeling for the Biophysical Settings models covered in this review. The range of values generated for fire regime characteristics in Alaskan wet and mesic herbaceous systems is:

DISTRIBUTION AND PLANT COMMUNITY COMPOSITION

Figure 3. Land cover distribution of Alaskan wet and mesic herbaceous systems based on the LANDFIRE Biophysical Setting (BpS) data layer [76]. Numbers indicate LANDFIRE map zones. Click on the map for a larger image and zoom in to see details.

Biophysical Settings (BpSs) covered in this review include freshwater marshes, herbaceous peatlands (bogs and fens), wet and mesic meadows, and herbaceous floodplain wetlands. These communities occur throughout Alaska. Most communities covered in this review are dominated by nontussock-forming graminoids, but some are dominated by forbs, ferns, horsetails, and/or mosses, especially sphagnum [106]. Shrubs and/or trees may be present in some communities, but shrubs typically have <25% cover and trees <10% cover [106]. Wet herbaceous communities are more common in Alaska than mesic herbaceous communities [106]. For information on fire regimes of other wet and mesic herbaceous systems in Alaska, see these Fire Regime Syntheses:

• Fire regimes of Alaskan tundra communities
• Fire regimes of Alaskan coastal communities
• Fire regimes of Pacific maritime communities

Appendix A lists the BpSs covered in this review and summarizes data generated by LANDFIRE's [74] successional modeling for those BpSs.

NatureServe [84] identifies the following wet and mesic herbaceous systems in Alaska. Corresponding BpS series are given in parentheses.

• Alaska arctic pendantgrass freshwater marshes have standing water with >10% cover of emergent herbaceous vegetation, primarily pendantgrass. These marshes occur in small patches throughout arctic Alaska, typically on the margins of ponds and lakes. Most are semipermanently flooded, but some have seasonal flooding, and the water is typically >4 inches (10 cm) deep. They are also found on small to large floodplains in oxbows, wet depressions, low-lying areas, and abandoned channels. Soils are muck or mineral and water is nutrient-rich. On floodplains, permafrost is absent (16970 series).
• Alaska arctic sedge freshwater marshes occur in small patches throughout arctic Alaska, typically on the margins of ponds, lakes, and streams but also on floodplains in oxbows, wet depressions, low-lying areas, and abandoned channels. Patches are small to large. Soils are muck or mineral and water is nutrient-rich. On floodplains, permafrost is absent. These marshes typically have standing water. They are often dominated by water sedge or tall cottongrass, but other emergent species may occur (17050 series).
• Aleutian freshwater marshes typically occur with other wetland systems on the margins of ponds and lakes. They occur mostly in small patches and are semipermanently flooded. However, some may be seasonally flooded. Water is at or above the surface for most of the growing season. Soils are muck or mineral. These marshes have >10% cover of emergent herbaceous vegetation. Common species include water sedge, Northwest Territory sedge, buckbean, purple marshlocks, water horsetail, marsh horsetail, and mare's-tail (17220 series).

• Alaska arctic wet sedge-sphagnum peatlands occur on the Yukon-Kuskokwim Delta and the Kotzebue Sound lowlands ecoregions. They are common in wet depressions and old lake basins. Soils are poorly drained and acidic, typically with a well-developed peat layer. Permafrost may be present. Patches are small to large. Sphagnum cover is >25% and usually continuous, and herbaceous species (primarily sedges such as Eriophorum spp. and Northwest Territory sedge) cover is >25% (17020 series).
• The Aleutian wet meadow and herbaceous peatland - complex includes a variety of herbaceous wetlands. They occur in or along shallow depressions, seepage channels, pond margins, streams, lakes, wet slopes, terraces, and late-melting snow beds. Peat ranges from thin to thick (sometimes >16 inches (40 cm)) and may be floating or submerged. Vegetation has >25% herbaceous cover and <25% shrub cover (17230 series).
• Western North American boreal herbaceous fens occur in shallow depressions and basins, along pond margins, and in thermokarst pits. They are nutrient-rich and have a thick peat layer that may be floating or submerged. Standing water is usually present. Dominant species may include buckbean, water horsetail, purple marshlocks, water arum, tall cottongrass, and water sedge. Shrubs are occasionally present but contribute <25% cover (16181 series).

• Alaska arctic mesic herbaceous meadows occur throughout arctic Alaska on hill and mountain slopes, upper drainages, and lowlands, including drained lake basins. They typically occur in small patches on mesic sites, primarily in western arctic Alaska, and have >25% cover of herbaceous species. Common species include Bering Sea sedge on mountain slopes, icegrass and alpine meadow foxtail in collapsed acidic lowland snow beds, and bluejoint reedgrass in drained lake basins (16990 series).
• Alaska arctic wet sedge meadows occur on wet sites (typically with <10% visible surface water) with >25% sedge cover. These meadows also occur on small to large floodplains in oxbows, wet depressions, low-lying areas, and abandoned channels. Sites are flat to sloping in valley bottoms, basins, patterned ground, water tracks, and adjacent to streams. Soils are saturated during the summer and have an organic horizon over silt with permafrost, although permafrost is absent on floodplains. Sites are typically dominated by water sedge and tall cottongrass but other sedges may dominant or codominant. Dwarf shrubs may be common but make up <25% cover (16980 series).
• Alaska subboreal and maritime alpine mesic herbaceous meadows occur on mountain sideslopes in the subboreal and maritime regions of Alaska. They often occur in a continuous band above or mixed with subalpine and alpine shrublands on moderate to steep slopes. Species composition is diverse, with no single species dominant. Longawn sedge and woolly geranium are the most typical species (16450 series).
• Aleutian mesic herbaceous meadows occur on all slopes and aspects with mesic moisture regimes, including windswept coastal headlands, coastal bluffs, old beach ridges, hillside slopes, stabilized talus, alluvial fans, and ravine sideslopes. They commonly form a mosaic with alder patches. Four meadow types are common: lady fern, fireweed, bluejoint reedgrass, and mixed herbaceous (16510 series).
• Western North American boreal alpine mesic herbaceous meadows occur on gentle slopes in subalpine and alpine environments throughout Alaska. Bigelow sedge is dominant and dwarf shrubs are usually present but contribute <25% cover. This system often forms a mosaic with dwarf and low shrub systems (16330 series).
• Western North American subboreal mesic bluejoint reedgrass-forb meadows occur in boreal and subboreal regions of Alaska and extend south and east into western Canada. Soils are typically mineral and may be poorly drained (on flats) to well drained (on sideslopes). The vegetation is usually dense. Species composition ranges from nearly pure stands of bluejoint reedgrass to mixtures of bluejoint reedgrass with forbs, such as fireweed. Forb- or fern-dominated patches also occur. These meadows often occur in a mosaic with shrublands or deciduous forests. They also occur as seral stages in drained lakebeds or after disturbances such as fire or logging. However, these seral stages are considered part of the forested systems and are not included in this BpS series (16110 series).

• Aleutian floodplain wetlands occur within active and inactive portions of floodplains. They develop on poorly drained deposits, oxbows, and abandoned channels, and are often mosaicked with well-drained floodplain vegetation. Patches are small and often linear. River channel migration, flooding, and other fluvial processes are major disturbances. Wetland succession and species composition vary due to variation in water depth, substrate, and nutrient input. These wetlands are part of a complex that includes aquatic beds, marshes, wet meadows, herbaceous peatlands, and mesic-wet willow shrublands (17290 series).
• Aleutian shrub and herbaceous meadow floodplain systems occur on small active and inactive unforested floodplains and outwash plains. They are widespread on the Aleutian Islands and Alaska Peninsula and form a mosaic with Aleutian floodplain wetlands. The substrate is typically well-drained sand or cobble alluvium, and permafrost is absent. Patches are often linear. This floodplain system includes mesic herbaceous meadows, American dunegrass grasslands, and tall or low willow and/or Sitka alder shrublands. More information on alder and willow floodplain shrublands is available in the Fire Regime Synthesis Alaskan alder and willow shrublands (17270 series).
• Western North American boreal shrub and herbaceous floodplain wetlands occur within the active and inactive portions of floodplains. Wetlands develop on poorly drained deposits, oxbows, and abandoned channels, and are often mosaicked with well-drained floodplain vegetation. Patches are small to large. River channel migration, flooding, and other fluvial processes are major disturbances. These wetlands can also develop through paludification on poorly drained, fine-textured deposits. They are part of a complex that includes aquatic beds, marshes, wet meadows, herbaceous fens, low shrub peatlands, sphagnum peatlands, and sedge-dwarf shrub bogs (16170 series).

Some Native Alaskans used fire historically [78]. However, the frequency and locations of fires historically set by Native Alaskans were not well documented. Historical and contemporary accounts of native Gwich’in Athabaskan groups in eastern interior Alaska describe intentional burning of the landscape when conditions were not conducive to extensive fire spread, such as during wet periods. In central interior Alaska, however, Koyukon Athabaskan groups did not historically use fire to modify the landscape [83].

Human-caused fires have greatly outnumbered lightning-caused fires throughout Alaska in recent decades [1,6,21,52,100]. Most ignitions are near human populations [23]. However, lightning-caused fires tend to be larger and account for most of the area burned in interior boreal forests [21,24,100]. See Contemporary Changes in Fuels and Fire Regimes for more information.

Fire season
The typical fire season in Alaska starts in May after snowmelt and lasts until late July or August at the start of mid- to late-summer precipitation [34,93]. The peak fire season—from June to mid-July—coincides with high temperatures, intense lightning activity, low humidity, and sparse precipitation. Before June, soils are typically still wet from snowmelt. After mid- to late-summer rains, fuel moisture increases and lightning strike density declines, reducing the number of lightning-caused ignitions [21,108].

Herbaceous fens may be most likely to carry fire in April, May, and early June because grass and litter are dry during this period before new leaves emerge [41]. They may also carry fire after plants senesce in late summer [117]. Herbaceous bogs, like forested bogs, are most likely to carry fire during severe drought years when soils and vegetation are dry in July, August, and September [41].

Mesic meadows such as bluejoint reedgrass grasslands [11] and umbel meadows [16] carry fire readily in spring because grass and litter are dry before new leaves emerge [16,23,24,41]. They may also carry fire after plants senesce [16]. Bluejoint reedgrass grasslands, which senesce in August [45], burned in August during the 1977 Bear Creek Fire near Farewell [37]

Large fire years (i.e., years where area burned is >1.5 times the long-term average) are associated with extended warm, dry periods and burning late in the growing season. In a typical year, most ignitions (76%) and fire spread occur from the beginning of June to mid-July. In large fire years, most fires occur in July, and they may burn into August and September [53].

Fire frequency
Most communities in Alaska, including marshes, herbaceous peatlands, wet and mesic meadows, and herbaceous floodplain wetlands, support fire spread during severe fire weather [4,11,23,48,87,92,113,115]. In interior Alaska, wet and mesic herbaceous systems occur within a matrix of other boreal vegetation types (e.g., coniferous forest, deciduous forest, and shrublands) [84]. Wildfires are common in boreal forests, and they are usually the source of fires that occur in adjacent wet and mesic herbaceous systems [117].

Vegetation flammability may influence fire frequency in wet and mesic herbaceous systems. The systems covered in this review are comprised of numerous plant species varying from very high to low flammability. Sylvester and Wein [97] studied the fuel characteristics of common arctic tundra and forest-tundra species and genera near Inuvik and ranked them in order of decreasing flammability as: dead leaves of graminoids (tussock cottongrass and bluejoint reedgrass)>evergreen ericaceous shrubs (northern Labrador tea and black crowberry)>fruticose lichens>dwarf deciduous woody shrubs (dwarf birch, bog blueberry, and grayleaf willow)=sphagnum>fireweed [97]. DeWilde and Chapin [24] categorized 4 fuel types in interior Alaska and ranked them in order of decreasing flammability as: boreal spruce>mixed hardwood/spruce >>open tundra, shrub/grass=boreal lichen. The "open tundra, shrub/grass" fuel type consisted of open tundra, which was considered to have generally low flammability because of high moisture content and small fuel loads, and grass meadows, which were considered very flammable in spring because of dry leaf litter, but less flammable during the growing season because of high leaf moisture content. Boreal forest was considered the most flammable because of its fine twigs and needles, high resin content, low moisture content, and ladder-like structure that carries fire into the canopy [24]. Wein [111] noted that dwarf birch-ericaceous shrub tundra communities were "particularly susceptible to fires" due to the relatively high flammability of ericaceous shrubs, while sphagnum-dominated tundra communities appeared to be "much less susceptible". Sphagnum absorbs and retains water and tends to be one of the last ground-layer fuels to combust during boreal forest fires [14,91,104].

Fires do not spread in areas with standing water, such as aquatic beds, drainage areas, streams, rivers, lakes, and areas with bare mineral soil, such as mud boils [16,92,115].

Freshwater marshes: Klein (1971 cited in [109]) observed that productivity of some marshes in Alaska was apparently maintained by periodic fires but that data are needed to back up this generalization. As of this writing (2015), fire frequency had not been reported for Alaskan freshwater marshes. LANDFIRE models do not include fire in these communities (fire regime group="NA") [58,62,72].

Herbaceous peatlands: Limited information suggests that fire may occur in Alaskan herbaceous peatlands. Viereck [108] noted that fires are relatively common in treeless bogs and fens in interior Alaska, with a large portion (43%) of "forest fires" actually occurring in these areas. Morrissey and others [80] noted that fires often spread from upland forests into adjacent peatlands. Neiland and others [85] stated that evidence of past fire is common in Carex spp.-grass bogs, sphagnum bogs, and tussock cottongrass bogs. In contrast, Timoney [99] stated that boreal forest fires typically bypass boreal wetlands because of wet soils. LANDFIRE models do not include fire in these communities [61,71,73].

The importance and role of fire differ greatly with the type of peatland and the amount of soil drainage [80]. The water table in peatlands is usually at or above the soil surface immediately after snowmelt but falls during the summer. Peatlands in which the water table falls well below the surface during summer (e.g., raised bogs underlain by permafrost) are the most susceptible to burning. Fens, which have water tables at or above the surface most of the summer, are the least susceptible to burning [80].

Little published information is available on the length of fire cycles and fire-return intervals in herbaceous peatlands of Alaska, although a review of North American boreal peatlands may be relevant to the herbaceous peatlands of Alaska. The review divided peatlands into 3 regions: subarctic, boreal continental, and boreal humid wetland regions (Figure 4). Based on observations in the literature, air photos, and fire maps, fire-return intervals for surface fires in North American boreal peatlands ranged from 75 to 1,000 years and varied regionally and by peatland type. Charcoal layers in peat profiles in western Canada indicated that fires are common in peatlands in the subarctic region and frequent in peatlands in the continental boreal region, while fires are infrequent in peatlands of the humid boreal region. Continental areas, such as those in boreal interior Alaska, are subject to more frequent fire than subarctic or coastal regions, and bogs burn more frequently than fens (Table 2). Regional differences in fire frequency, severity, and extent were attributed to climatic and hydrological controls on the depth of the water table relative to the surface [117].

According to Zoltai and others [117], peat (ground) fires are less common than surface fires in northern peatlands. Although specific information on Alaskan herbaceous bogs was not provided, these researchers estimated that in subarctic and boreal continental bogs, return intervals were 150 years for surface fires and 400 years for peat fires. The return interval for surface fires in fens was also 150 years, while peat fires rarely occurred in fens (Table 2) [117].

If fire is absent from boreal forests for long periods, treeless bogs may form through the process of paludification [109].

Wet and mesic meadows: Researchers documented occasional fires in Aleutian mesic bluejoint reedgrass [38] and Arctic wet sedge-grass meadows in Alaska [87,88]. LANDFIRE models included fire in boreal alpine mesic herbaceous meadows [65] and subboreal mesic bluejoint reedgrass meadows [64] but not in other wet or mesic meadow types [59,60,66,70]. They indicated that subboreal mesic bluejoint reedgrass-forb meadows burn with replacement (85%) or mixed (15%) severity at intervals averaging 769 years, but mesic bluejoint reedgrass and other meadows on the Aleutian Islands and in the Arctic do not burn (Appendix A). Hanson [38] recorded fire in several bluejoint reedgrass communities in Alaska. Near Homer, in the Aleutian mesic herbaceous meadow system [66], bluejoint reedgrass grassland fires were apparently "frequent", as indicated by 2 charred layers in the 0 to 1.5-inch (3.8 cm) soil horizon of one community. In this community, bluejoint reedgrass constituted about 90% cover and there was a 3- to 10-inch (8-25 cm) layer of litter on the surface [38]. On the Seward Peninsula in northwestern Alaska, wet sedge-grass meadows [59] burned occasionally in the 1970s [87]. Authors suggested that fire, as well as wind and edaphic conditions, may maintain some mesic grasslands in Alaska [38,106].

Wind-driven fires beginning in surrounding communities may spread into wet and mesic meadows and influence their fire regimes. For example, LANDFIRE modelers note that the mean fire-return interval of 769 years for mesic bluejoint reedgrass meadows [64] is similar to that of adjacent mesic subalpine alder shrublands [67]. LANDFIRE models include a mean fire-return interval of 400 years for boreal alpine mesic herbaceous meadows [65]. Modelers suggested that because adjacent dwarf shrublands [68] and ericaceous-dwarf shrublands [69] burn occasionally, some fires may spread into alpine mesic herbaceous meadows during dry periods [65].

Herbaceous floodplain wetlands: Fire is probably less important than other disturbances in herbaceous floodplain wetlands. Flooding, shifting channels, and sediment deposition are the most common disturbances [84]. As of this writing (2015), fire frequency had not been reported in Alaskan herbaceous floodplain wetlands. LANDFIRE models do not include fire in these communities. However, the modelers suggested that fire may occasionally move into boreal shrub and herbaceous floodplain wetlands from adjacent upland systems [63].

Fire type and severity
Surface and ground fires occur in Alaskan wet and mesic herbaceous systems [23,113,117], and severity is determined mainly by hydrology [16,52]. Some authors noted that fire is generally more severe on well-drained than poorly drained sites [37,87,113]. For example, on well-drained sites on the Seward Peninsula, complete burning of the vegetation and organic soil layer was common, while on poorly drained sites, unburned patches of vegetation, particularly of sphagnum, were common within burns [87].

Freshwater marshes: Anecdotal information suggests that surface fires may occasionally occur in marshes in northern circumpolar ecosystems [113].

Herbaceous peatlands: Boreal peatlands typically burn with surface fire but can burn with ground fire during very dry years [80,113,117]. Boreal peat fires are typically shallow because of wet soils, generally consuming the upper 4 to 6 inches (10-15 cm) of litter and peat. In extremely dry years, fires can burn deep into peat and smolder for long periods [80,92,117]. Specific information on Alaskan herbaceous peatlands, particularly on arctic and subarctic peatlands, was lacking.

The position of the water table relative to the surface influences the moisture content of the surface peat and thus fire type and severity [39,103,117]. Peatlands having water tables well below the surface throughout much of the growing season are the most susceptible to peat fire, while peatlands that have water tables at or above the surface most of the summer are least susceptible. The position of the water table is deepest in bogs (all types), followed by conifer swamps, treed fens, and shrubby fens, and is shallowest or above the surface in open, graminoid fens [80,117]. Thus, susceptibility to peat fire is greatest in bogs and least in fens. Permafrost may impede the downward movement of water flow, thus either waterlogging the active thaw layer or forming steep moisture gradients, with low moisture content at the surface and saturated peat near the permafrost table. Peatlands underlain by permafrost are often elevated ≥3 feet (1 m) above adjacent unfrozen peatlands and are thus better drained and able to carry fire [117].

Water table position in boreal peatlands changes seasonally in response to weather conditions. Flooding is common during snowmelt in many northern peatlands. Drought may cause the water table to drop. The susceptibility of any particular peatland to burning, therefore, varies both between years and within any given growing season [117].

Because sphagnum absorbs and retains water [12], fuel consumption in boreal bogs with a high proportion of sphagnum tends to be highly variable [52,91].

Fire effects: Short-term fire effects on boreal peatlands include reduced primary productivity, increased soil respiration in and decomposition of remaining soil organic matter, water table fluctuations, and—in permafrost areas—degradation of permafrost and increased development of thermokarsts. Long-term effects include increased productivity and changes in plant community composition, including paludification and terrestrialization. In addition, removal of the insulating peat layer during peatland fires often increases soil temperatures and active layer depth in permafrost regions [109,115,116,117].

Fuels: There is usually sufficient fuel to carry fire in northern ecosystems because primary production exceeds decomposition [113]. For estimates of available fuels and annual biomass loss from northern peatlands due to surface and ground fires, see Zoltai and others [117].

The moisture content of peat fuels determines whether they can ignite and sustain combustion [117]. Zoltai and others [117] summarized moisture content of surface peat measured in 151 peatlands across boreal Canada and the northern United States during 4 summers. Their analysis revealed that bogs underlain by permafrost were most likely to have peat with moisture contents within combustible levels because these bogs are elevated above the local water table. Sixty-six percent to 75% of the peats underlying raised permafrost peatlands and 36% of those underlying nonpermafrost bogs were susceptible to combustion in an average year. Less than 1% of fens had peat within the combustible moisture range, suggesting peat fires in fens are rare. Typically only the upper 4 to 6 inches (10-15 cm) of peat in bogs are within ignitable moisture ranges. Once ignited, the heat of the fire may dry the underlying peat, allowing deeper burning [117].

Wet and mesic meadows: LANDFIRE models estimated 85% replacement-severity fires and 15% mixed-severity fires for mesic bluejoint reedgrass meadows. Modelers suggested that fires occurring prior to green-up are likely to be surface fires because of wet soils [64]. Limited published information indicates that post-senescence fires in bluejoint reedgrass meadows may be stand-replacing ground fires. The 1977 Bear Creek Fire near Farewell burned bluejoint reedgrass meadows as stand-replacing surface and ground fire. It started in early August. It burned the meadows down to grass root crowns and burned 2 to 10 inches (5-25 cm) into the 14-inch (35 cm) organic mat, exposing mineral soil over 5% of the area. Charred organic soil comprised 85% of the ground cover and unburned litter comprised the remaining 11% [37].

Information on fire in other wet and mesic meadow types was sparse to lacking as of this writing (2015). Researchers documented patchy, low- to moderate-severity surface fires in Arctic sedge-grass wet meadows on the Seward Peninsula [88]. LANDFIRE models estimated 100% replacement severity fires for boreal alpine mesic herbaceous meadows [65], but as of this writing, no published studies were available.

Frequent severe fires in boreal white spruce and black spruce forests may result in these forests being replaced by herbaceous or shrub systems, such as bluejoint reedgrass-Carex spp.-herb meadows, fireweed-grass communities, or arctic dwarf birch-willow shrublands [55,78,106].

Herbaceous floodplain wetlands: As of this writing (2015), no published information was available regarding fire type and severity in herbaceous floodplain wetlands.

Fire pattern
Because of variation in topography, plant community composition, soil and fuel moisture, and weather, fires in Alaska typically produce a mosaic of unburned, lightly burned, and severely burned areas (e.g., [3,29,86,87,94,105,111,112,113]). Monitoring Trends in Burn Severity data from 2004 (large fire year) and 2006 (small fire year) indicate that 20% and 66% of the area within fire perimeters in Alaska did not burn, respectively, indicating the prevalence of mosaic fires in both large and small fire years. An analysis of the burned and unburned fuel types within fire perimeters from 2004 showed that deciduous and nonforested fuel types, such as grasslands and shrublands, burned less often than mature spruce fuel types within the boreal forest ecoregion of interior Alaska. The occurrence of deciduous and nonforested fuel types was lower within fire perimeters (40%) than within the ecoregion (53%). The occurrence of mature spruce fuel types was higher within fire perimeters (47% of the area) than within the ecoregion (39%). Within the fire perimeters, however, there were only small differences in the fraction of fuel types that burned and those that did not, indicating that during large fire years, fire burns evenly across all these fuel types [52]. In peatlands, fires tend to be patchy because of discontinuous fuels and the presence of surface water [117], which inhibits fire spread [2,8].

Fire size
The published literature reports little information on fire size in Alaskan wet and mesic herbaceous communities. However, it is likely that general patterns regarding fire size in Alaska apply to these systems. In general, fire sizes in Alaska range from very small to very large [21,24,35,109] and vary regionally [35]. Approximately 60% to 80% of all fires in Alaska are <12 acres (5 ha) [5], although large fires account for most of the total area burned [5,24,32,52,53]. From 1957 to 1979, average fire size ranged from as little as 9 acres (4 ha) in the Pacific Border Ranges province of southeastern Alaska to as much as 25,831 acres (10,453 ha) in the Alaska-Aleutian province of south-central Alaska, which encompasses the Alaska and Aleutian ranges (Table 3) [35].

Figure 5. Percent of each physiographic province in Alaska burned from 1957 to 1979. USDI, BLM image from Gabriel and Tande [35].

When and if fire occurs in Alaskan wet and mesic herbaceous systems, it is likely to occur during large fire years, when dry and warm weather occurs for extended periods. In Alaska, large fires typically occur episodically. Since the 1860s, large fire years occurred 17% of the time and accounted for 68% of the area burned throughout Alaska [52]. From 1950 to 1999, the average fire size during large fire years was 50,200 acres (20,300 ha), while the average fire size was 19,300 acres (7,800 ha) in all other years [53]. Large fire years occurred every 4 years on average [53], although the frequency of large fire years and extreme fire events has varied since the 1940s [52].

In general, fire size in Alaska is primarily driven by summer weather and climate patterns [7,21,28,33,52]. Large fires occur when extended periods of warm and dry weather (≥10 days) occur across the landscape [33]. This weather pattern—often caused by surface-blocking high-pressure systems—dries out the fuels across the landscape and increases the fire danger [28,33]. Positive relationships between the area of peatland burned in western Canada during 20 years and weather variables calculated for each fire event, including maximum air temperatures and duff moisture, suggest that dry and/or warm conditions result in increased area of peatland burned [102].

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 [21,28,33,42]. From 1940 to 1998, 15 of the 17 largest fire years occurred during a moderate to strong El Niño episode. These 15 years account for nearly 63% of the total area burned during that period [42].

Fire regimes and human activity
Fire regimes in Arctic and interior Alaska may not differ much from historical fire regimes because these regions are generally sparsely populated, have little road access, and have had minimal suppression activity [17,23,24,26,41,108]. However, fire regimes in localized areas, such as near human populations or along roads, may be influenced by human activity. In populated areas in interior Alaska designated for fire suppression, less area burns and more ignitions are human-caused than in remote areas [23]. 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 (i.e., boreal spruce, mixed hardwood-spruce, grassland and shrubland tundra, and boreal lichen), it was somewhat more effective in nonforest vegetation [24]. In interior Alaska, total ignition rates were higher near rivers than in uplands. Human-caused ignitions were mostly within 0.6 mile (1 km) of rivers, even though lightning was the major cause of fires near rivers [15].

Humans ignite most wildfires in Alaska; however, human-caused fires tend to burn less area than lightning-caused fires [1,6,23,24,36,52,100]. Over the past 2 decades, nearly twice as many fires in Alaska were human-caused than lightning-caused [52]. However, within the interior, human-caused fires were rarely >100 acres (40 ha) and therefore accounted for only 4.6% of the total area burned from 1992 to 2001. Most of the human-caused fires that were >100 acres occurred in forests; few occurred in non-forest types such as grasslands [24]. Human-caused fires tend to be small because they typically occur outside of the fire season in populated areas where they are easily suppressed, and in only moderately flammable vegetation such as mixed hardwood-spruce and grassland and shrubland tundra [15,24].

Lightning-caused fires occur mostly in June and July [24,34,93] (see Fire season). Human-caused fires lengthen the fire season by 2 months [23,24,57]. In the Fairbanks region, human-caused fires in grasslands were most frequent in May after snowmelt and before green-up, when grasses and litter are dry [23].

Fire regimes and climate change
Warmer temperatures at northern latitudes are expected to reduce snow and ice cover and permafrost; decrease surface albedo; lengthen the snow-free season; and alter vegetation communities. These changes are likely to contribute to longer fire seasons and increased fire frequency, severity, and area burned (e.g., [1,8,10,18,19,21,31,47,51,52,95]). Over the last decade, fire size and the frequency of large fire years have increased in Alaskan boreal forest due to climate warming. Four of the 11 largest fire years on record since 1940 occurred between 2002 and 2009 [52]. The largest Arctic fire on record, the Anaktuvuk River Fire, occurred in 2007 [46]. Paleorecords from 14 lakes in the Yukon Flats ecoregion indicate that fire frequency has been higher in recent decades than at any other time during the past 3,000 years [54].

Although the response of fire regimes to climate change is complex, the area burned across Arctic and boreal regions will likely increase with lengthening fire season, increasing moisture stress, and increasing human ignition rates [43,52]. In Arctic tundra, where lowlands and poorly drained sites are abundant and permafrost is continuous [109], climate change has led to increased shrub height and cover and allowed boreal forest to spread into tundra at treeline. Predicted increases in the size and frequency of tundra fires and anthropogenic disturbances are likely to drive rapid shrub proliferation in the Arctic. Shrub-dominated sites have decreased albedo, increased net solar radiation, increased snow pack, and elevated near-surface ground temperatures relative to graminoid tundra [17,18,20,77,82,90,96,98,101,110].

Climate change effects on lowlands and poorly drained sites in interior and south-central Alaska differ between discontinuous permafrost and permafrost-free zones [18]. In some poorly drained lowlands in the discontinuous permafrost zone, spruce forests and woodlands may be converted to ponds or wetlands due to thawing of permafrost and subsequent development of thermokarsts. Ponds and wetlands absorb more radiation and thus are likely to accelerate rates of thaw and landscape change [18]. Forty-two percent of Alaska's boreal region is comprised of lowlands, of which 13% is susceptible to thermokarst development (Jorgenson and others 2007 cited in [49]). Researchers on the Tanana Flats in central Alaska estimated that >42% of their 652,269-acre (263,964 ha) study area had thermokarst development. Because of thermokarst development, fen meadows in the study area increased 9% from 1949 to 1995, while lowland paper birch forests decreased 8% [50].

In some boreal lowlands in the discontinuous permafrost zone of boreal Alaska, permafrost has been relatively resilient to climate warming because thermal insulation and low litter quality of mosses lead to cold, nutrient-poor soils [18]. However, predicted increases in fire frequency and severity may combine with climate warming to increase the rate of permafrost degradation and thermokarst development on these sites because fire consumes the insulating moss cover [18,49]. In these lowland areas, permafrost degradation may result in a shift from black spruce ecosystems to sphagnum bogs and herbaceous fens [18]. On the Tanana River floodplain, a paleobotanical study indicated that around 1970 a sedge-dominated wetland succeeded to a sphagnum-Carex spp.-tall cottongrass peatland. This shift coincided with increased growing-season temperatures. In 2001, a wildfire occurred during a period of permafrost collapse, further warming the substrate and expanding the sphagnum peatland. The authors concluded that with sufficient precipitation, future warming and/or fires could further melt permafrost, expand open peatlands, and increase carbon storage. However, warming during drought may decrease long-term carbon storage and reduce black spruce recruitment in the peatland [81].

In lowlands underlain by gravel, permafrost is less common, and climate warming may result in drying of lakes, ponds, and wetlands. Shrubs and trees from upland areas establish in the newly dried areas, resulting in further drying [18,44]. Decreased size and number of ponds and wetlands have been reported in both interior [81,89] and south-central Alaska [11,56] since the 1950s. Researchers examined >10,000 closed-basin ponds throughout Alaska. They found reductions in the area and number of shallow, closed-basin ponds since the 1950s for all boreal regions but not the Arctic Coastal Plain region [89]. In the permafrost-free Kenai Peninsula of south-central Alaska, they found a reduction in wetland area (including muskegs, kettle ponds, closed- and open-basin lakes) by 67% for 3 subregions based on 1950 to 1996 aerial photography [56]. Berg and others [11] concluded that wet sphagnum-sedge fens initiating since the end of the Wisconsin glaciation (18,000 years ago) began to dry in the 1850s and that this drying has greatly accelerated in recent decades. Another study on the Kenai Peninsula showed that on 11 herbaceous wetland sites, herbaceous area shrank 6.2%/decade from 1951 to 1968 and 11.1%/decade from 1968 to 1996, while area covered by shrubs and trees increased. The authors commented that the conversion of herbaceous wetlands to shrublands or forests is "not a minor issue given that 41% of the Kenai Lowlands is classified as wetlands" [11]. Establishment of shrubs in herbaceous wetlands may reduce albedo and increase atmospheric heating in a positive feedback loop (e.g., [17,19]), also changing their fire regimes. Wetlands that may have served as firebreaks in the past may become "fuel bridges" as they convert to shrublands and forests [8,11], facilitating increased fire spread and extent [11]. The greatest increase in fire susceptibility is expected to occur in subarctic and boreal continental regions. Only small changes in water table levels are expected in the most northern and maritime climates [117]

In peatlands, any climate change that results in a drying of surface peats or lowering water tables is likely to increase fire frequency, intensity, and extent, and the probability of ground fires [80,117]. These increases would increase decomposition rates and carbon emissions. However, peat-forming plants are expected to rapidly colonize wet depressions, sequestering carbon [80,117].

LIMITATIONS OF INFORMATION
Fire history studies in Alaskan wet and mesic herbaceous systems are scarce, and our knowledge is incomplete. Surface and ground fires in these systems may be more common than the literature suggests. Fire history in Alaska is incomplete because fire records only date back to the 1940s [30], and some records may be missing or incomplete [26]. Studies based on macroscopic charcoal layers buried in peat may offer the best fire history information for wet and mesic herbaceous systems with peat, but such studies lack annual precision and have small sample sizes. In addition, they may not capture records of all fire types equally, thus biasing paleoecological reconstructions of fire history [11,38,80,102,117].

Considerations for LANDFIRE
LANDFIRE models for Alaskan herbaceous peatlands do not include fire as a disturbance, but there is evidence of fire in these systems. Berg and others [11] observed occasional burned trees in 24 minerotrophic fens (Aleutian wet meadow and herbaceous peatland - complex [73]) within the perimeter of a severe, mineral-soil exposing fire that occurred in the second summer of drought on the Kenai Peninsula. They found no evidence of fire in 27 peat cores taken from the same area, which suggests that peat cores may underestimate fire frequency [11]. Zoltai and others [117] estimated that for moss- and sedge-dominated subarctic and continental boreal fens in North America, surface fires occur approximately every 300 years (Table 2).

According to LANDFIRE modeling, boreal alpine mesic herbaceous meadows have a mean fire-return interval of 400 years [65]. This is based on the assertion that adjacent dwarf shrublands [68] and ericaceous-dwarf shrublands [69] burn, but this is inconsistent with the LANDFIRE models for these shrublands, which do not include fire. More information is needed on the frequency and behavior of fire in Alaskan wet and mesic herbaceous systems and adjacent communities.

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