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Fire regimes of Alaskan quaking aspen and balsam poplar communities

Table of Contents:

 
  Figure 1. Mosaic of quaking aspen and spruce stands in Denali National Park. USDI, National Park Service photo by Tim Rains.
Citation for this synthesis:
Fryer, Janet L. 2014. Fire regimes of Alaskan quaking aspen and balsam poplar communities. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Missoula Fire Sciences Laboratory (Producer). Available: www.fs.usda.gov/database/feis/fire_regimes/AK_aspen_balsam_poplar/all.html [].

INTRODUCTION
This Fire Regime Synthesis brings together information from 2 sources: the scientific literature as of 2014, and the Biophysical Settings (BpS) models and associated Fire Regime Data Products developed by LANDFIRE, which are based on literature, local data, and/or expert estimates. This synthesis is intended to: It does not include information on fire regimes of quaking aspen and balsam poplar vegetation types in the conterminous United States or Canada. Reviews [16,21,94] and primary literature are cited in this synthesis.

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.

SUMMARY
This section summarizes fire regime information available in the scientific literature as of 2014. Details and documentation of source materials follow this summary. Where applicable, information on quaking aspen and balsam poplar communities is separated.

Quaking aspen:
Quaking aspen is a minor but widespread forest type throughout boreal interior Alaska. Quaking aspen stands generally occur within a deciduous-spruce forest mosaic, occupying warm slopes lacking permafrost. Quaking aspen forest is generally a seral type that succeeds to spruce forest.

Wildfires in quaking aspen forests are usually ignited by summer lightning and occur in drought years. Documented fire-return intervals range from about 40 to over 200 years in Alaskan quaking aspen stands; these are shorter than those documented for spruce forests. Fires typically stay on the surface and are of low severity, although stand-replacement fires occur infrequently. In deciduous-spruce forest mosaics, fires typically burn in mosaic patterns, with quaking aspen and paper birch-quaking aspen stands either burning at low severity or not burning at all, and spruce stands experiencing stand-replacing crown fire.

Compared to the 1940s and 1950s, fire frequency increased in interior Alaska during the 1960s-1990s. Climate warming may be partially responsible for this change and may favor quaking aspen and paper birch at the expense of spruces. Paleoecological studies show that previous periods of climate warming have favored quaking aspen and balsam poplar over white spruce and black spruce.

Balsam poplar:
Balsam poplar forest is a minor type confined to floodplains. It occurs throughout boreal interior and much of subboreal Alaska. Balsam poplar forest is generally a seral type that is succeeded by white spruce. Fire regimes of balsam poplar and balsam poplar-quaking aspen communities were not documented as of 2014. Experts suggest that balsam poplar forests have both infrequent low-severity surface and infrequent stand-replacement fires. Stand-replacement fire is most likely in late succession, when white spruce is replacing balsam poplar.

LANDFIRE models showed severe and mixed-severity fires in quaking aspen and mixed-deciduous communities, but not low-severity fire. 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 quaking aspen and balsam poplar communities is:

Table 1A. Modeled fire intervals and severities for Alaskan quaking aspen, quaking aspen-balsam poplar, and paper birch-quaking aspen communities [63]
Fire interval¹
Fire severity² (% of fires)
Number of Biophysical Settings (BpS) in each fire regime group
  Replacement Mixed Low I II III IV V NA³
73-159 years 22-84 16-78 0 0 0 3 1 0 0
Table 1B. Modeled fire intervals and severities for Alaskan balsam poplar and quaking aspen-balsam communities [63]
  Replacement Mixed Low I II III IV V NA
159-294 years 3-22 78-97 0 0 0 1 0 3 2
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% {Barrett, S.; Havlina, D.; Jones, J.; Hann, W.; (and others). 2010. [85876],LANDFIRE Rapid Assessment. 2005. [66741]}.
³NA (not applicable) refers to BpS models that did not include fire in simulations.
DISTRIBUTION AND PLANT COMMUNITY COMPOSITION

Figure 2. Distributions of Alaskan quaking aspen and balsam poplar-quaking aspen communities based on the LANDFIRE Biophysical Settings (BpS) data layer [63]. Numbers indicate LANDFIRE map zones. LANDFIRE did not map every BpS in this group. Click on the map for a larger image and zoom in to see details.
Quaking aspen:
Quaking aspen is a minor but widespread forest type of interior Alaska. The species occurs throughout boreal and subboreal Alaska, excluding the western half of the Alaska Peninsula and portions of the Alaska Panhandle [42,88]. Quaking aspen forests cover an estimated 2% [5,97] to 11% [43] of interior Alaska's taiga. At the landscape level, they form mosaics with paper birch and spruce forests. In these mosaics, quaking aspen is most common on uplands [13,29,95] and moraines [70], preferring warm, dry, south- and west-facing aspects [28,29,31,56,67,85,87,89,95,104]. In contrast, paper birch tends to dominate cool slopes [31,96]; white spruce typically occupies ridgetops, riparian zones, or forms stringer forests in depressions [28,75]; and black spruce occupies the coldest, northerly aspects [28,89]. On Esker Dome in interior Alaska, for example, quaking aspen dominates warm, south-facing slopes with thick (>3 feet (1 m)) active layers. Paper birch and/or white spruce dominate west- and east-facing slopes that are cooler than slopes dominated by quaking aspen. Black spruce dominates cold, wet sites underlain with permafrost [56]. Near the Kobuk River in northwest-central interior Alaska, quaking aspen-paper birch communities were positively associated with warm, dry topography (convexities, crests, shoulders, and east- west-, or south-facing midslopes) and shallow, sandy, or gravelly soil (P=0.05 for all variables) [85].

Quaking aspen forests can be "extremely dry" in the fire season [29]. The permafrost layer is either deep (>4 feet (1 m) below the soil surface [69]) or lacking [67,92]. Compared to conifer forests, soil moisture content is lower, soil nutrient levels are higher, and the litter layer is thinner [89].

Deciduous-conifer forests occupy about 36% of interior Alaska's landscape [5,97]; these mixed forests often succeed to conifer forests. On upland deciduous-conifer sites, quaking aspen typically dominates in early succession, although quaking aspen may "persist indefinitely" on some dry south- and west-facing slopes [34]. White spruce [52,67,92] or black spruce [56,67] generally replace quaking aspen successionally around postfire year 70 to 80 [29,34]. Early-seral quaking aspen stands are often dense, pure [67,94], and even-aged, with quaking aspen density decreasing with time since fire. Quaking aspen mortality in stands >60 years old promotes establishment and growth of spruces [67]. Conifer frequency usually increases with time since fire; late-successional quaking aspen communities may occur only where spruce seed sources are lacking (expert estimate cited in [61]). A chronosequence study near Delta Junction in east-central interior Alaska found northern rough fescue dominated a 3-year-old burn, quaking aspen and willows a 15-year-old burn, and black spruce an 80-year-old burn [65]. Near Delta Junction, a black spruce site had a crown fire around 1920 (severity unknown) and again in 1987 (severe; most of soil organic layer burned). Quaking aspen dominated the site by postfire year 12 (1999). Willows dominated the shrub layer; haircap mosses and fire moss the ground layer [103].

In midsuccession, quaking aspen often occurs in deciduous-conifer stands with paper birch, balsam poplar, and spruces [69]. It is frequently important or codominant in white spruce forests [29,34], and it may codominate with black spruce on warm sites [29]. White spruce usually replaces quaking aspen around postfire year 60. Fires at shorter intervals strongly favor quaking aspen [34].

As with upland forests, quaking aspen is seral to spruce in riparian zones. Mature quaking aspen stands are rare to infrequent on floodplains of interior Alaska [29,104]. On the Tanana River floodplain, Zasada [104] observed that although mature quaking aspens were scarce, quaking aspen seedlings were common.

Quaking aspen and other Populus forests are the most productive forests of interior Alaska [91,95]. The thick moss and organic layers of spruce forests insulate soil and have low bulk density and thermal conductivity, which tend to decrease site productivity and biodiversity [90]. Deciduous forests tend to retain more heat but use more water than conifer forests. Summer and winter albedo in boreal deciduous forests is about twice that of conifer forests; in summer, heat loss from boreal conifer forests to the atmosphere is about 41% to 70% more than heat loss from boreal deciduous forests. However, evapotranspiration rates are 50% to 80% higher in deciduous forests than in conifer forests [14].

NatureServe [74] described 3 quaking aspen ecological communities of Alaska:

Foote [29] recognized 2 quaking aspen community types of interior Alaska:

LANDFIRE's [63] list of quaking aspen and balsam poplar Biophysical Settings and links to their descriptions are provided in Appendix A. For quaking aspen, they include these BpS groups:

Balsam poplar:
Balsam poplar is a minor forest type: balsam poplar and black cottonwood forests occupy about 9% of interior Alaska's taiga [43]. The species has a scattered distribution throughout boreal and subboreal Alaska, excluding the western half of the Alaska Peninsula and portions of the Alaska Panhandle [42,88]. Balsam poplar stands are most common on the floodplains of interior, south-central, and southwestern Alaska, especially along major rivers of interior Alaska [31,87,94,96]. Balsam poplar-quaking aspen communities occupy warm upper slopes; they may extend up to elevational treeline in western boreal Alaska [61]. Small balsam poplar stands occur on timberline slopes throughout boreal Alaska [94]. A few isolated stands occur on northern foothills of the Brooks Range. These stands grow near warm springs and may be relicts from a warmer climate [94]. Balsam poplar is the only tree that grows north of the Brooks Range [16]; groves of balsam poplar grow west and southwest of white spruce's arctic treeline [98]. In western Alaska, balsam poplar occurs in isolated clumps as far as the Seward Peninsula [97,98]. On Kodiak Island and the Alaskan Peninsula, its distribution extends farther north than that of Sitka spruce [98].

Balsam poplar forests are usually seral, succeeding to white or black spruce forests. Viereck and others [94] reported that the oldest balsam poplar stands they found in Alaska were about 200 years old, but most stands were replaced successionally about 100 years after disturbance [94].

Balsam poplar and other Populus types are the most productive forests of interior Alaska [92,94]. Balsam poplar litter is abundant and decays rapidly. Soils are relatively warm in balsam poplar forests, soil nutrients are readily available, and soil water is available during most—if not all—of the growing season [97]. Balsam poplar stands grow on thin, nonpermafrost soils or on soils with a deep active layer [16,94]. Balsam poplar is common on fresh alluvium [68,97,98] and other river-deposited sediments [16]. Stands are most productive before white spruce replaces balsam poplar as the canopy dominant [101].

Balsam poplar is restricted to floodplains; it favors banks of large streams [68,73] and rivers, floodplains, and low terraces [16,68]. Stands are generally confined to narrow bands along rivers, although some balsam poplar stands along the Tanana, Yukon, and Kuskokwin rivers are several kilometers wide [24]. Floodplain forests along major rivers are typically mixes of balsam poplar, quaking aspen, paper birch, and white spruce [87], with the deciduous trees dominating in early postdisturbance succession [87,92]. Typically, balsam poplar forests have >60% canopy closure at maturity [94]. Along major rivers such as the Tanana, balsam poplar dominates the overstory about 20 to 40 years after flood creates a seedbed. Willows, alders, and prickly rose typically dominate the shrub layer; meadow horsetail and bluejoint reedgrass dominate the ground layer [97,101]. Balsam poplar and white spruce codominate the overstory about 100 years after a scouring flood or top-killing fire; at that stage, splendid feather moss and other feather mosses dominate the ground layer. White spruce dominates the overstory around postdisturbance year 120 to 150 [97].

NatureServe [74] described 1 balsam poplar ecological community of Alaska:

Viereck and others [94] recognized 10 balsam poplar community types. Except for the woodland, these communities are scattered throughout northwestern, interior, south-central, southwestern Alaska [94].

Forests: Woodland:

LANDFIRE's [63] list of quaking aspen and balsam poplar Biophysical Settings and links to their descriptions are provided in Appendix A. For balsam poplar, they include these BpS groups:

HISTORICAL FIRE REGIMES
Wildfire is the dominant disturbance in Alaskan taiga [44]. Three to 5 million acres (1.2-2 million ha) burn in interior Alaska annually [31]. Both quaking aspen and balsam poplar often survive low-severity surface fire, and both may sprout after top-kill by more severe fire. Quaking aspen generally survives low-severity surface fire when >6 inches (15 cm) DBH [12,54]. It sprouts from the roots [18,96] and establishes from seed [53] after stand-replacement fire. Balsam poplar's thick bark allows it to survive low-severity surface fire [16,32,35,66], and it sprouts from the roots and establishes from seed after stand-replacement fire [16,83]. For balsam poplar, these are likely adaptations to flooding and ice scouring rather than fire [16].

The sections below provide discussions and documentation of historical fire regimes in Alaskan quaking aspen and balsam poplar communities. See FEIS's Fire Regime Table for links to Fire Regime Summaries of other Alaskan vegetation communities.

Fire ignition
Most of interior Alaska has a natural fire regime: lightning-ignited fires with few human ignitions and negligible fire suppression [19]. From 1940 to 1969, 78% of ignitions in interior Alaska were due to lightning [97]. In taiga, >90% of forest fires from 1956 to 1999 were lightning-ignited. Most fires occurred between the Alaska and Brooks ranges [36].

Native Alaskans and Euro-Americans ignited fires historically. A review of fire history studies in the Tanana River Basin reported that Native Alaskans started fires for many reasons, particularly for insect control and to reduce undergrowth for better hunting. Euro-Americans documented ignition and use of fire by Native Alaskans during the settlement period (1867-1914) [77]. During the Gold Rush era (1886-1925), miners probably increased ignition rates over that of presettlement times [28,51].

Human-caused fires are more prevalent than lightning-caused fires in some upland quaking aspen communities [16]. Between the Brooks and Alaska ranges, humans caused 66% of ignitions in upland deciduous communities (1992-2001). These human-caused fires tended to be small (<1,000 acres (400 ha)), and they extended the fire season about 1 month earlier in spring and 1 month later in fall [16,19]. On a site near the Wrangell Mountains of southeastern Alaska, humans have been the main source of ignitions since 1951. Lightning strikes are rare in the region [71].

Quaking aspen and balsam poplar communities are less flammable than conifer communities. Deciduous trees provide more shade for fine surface fuels during the fire season—resulting in higher fuel moisture—they have less continuous vertical fuels, and communities dominated by deciduous trees have a lower proportion of resinous fuels [16,73]. In the Northwest Territories, a 17 June prescribed fire conducted under extreme fire weather crowned in black spruce-jack pine stands but extinguished in quaking aspen stands [1].

Fire season
Summer is the peak fire season in interior Alaska [104]. Large fires tend to occur late in the fire season [52]. Leaf litter dries out earlier in deciduous than in conifer forests, so the fire season may be longer in quaking aspen and balsam poplar than in spruce forests [93]. Quaking aspen stands are most flammable in spring and from late summer to early fall, when surface vegetation is dormant and dry [16,21].

In Alaska, the Canadian Forest Fire Weather Index (FWI) system is used to track effects of weather on fuel moisture conditions. Based on the Fine Fuel Moisture Code of the FWI, areas near Fairbanks were within prescription for burning during 7 of 25 years (1963-1995) . Duff moisture fell within the acceptable range on 11 May or later. After 11 August, the Duff Moisture Code fell and the Drought Code was above prescription more frequently than earlier in the summer. Generally, prescription conditions were met from late June through late July [79].

Fire frequency Quaking aspen:
Quaking aspen communities have relatively frequent low-severity surface fires. Mean fire-return intervals are shorter than those of white or black spruce communities; in fact, quaking aspen and paper birch have the highest fire frequency of all forest types of interior Alaska [16]. Frequent fires help maintain quaking aspen stands [67]. On most sites, fire must return within about 100 years for quaking aspen to retain dominance. With longer fire-return intervals, the site succeeds to white spruce or, less often, to black spruce [92,96]. In the Yukon Flats Wildlife Refuge, northeastern Alaska, time since fire (or age of quaking aspen stands) averaged 60 years (range: 47-81 years) compared to 77 years for white spruce and 81 years for black spruce stands [20]. The oldest quaking aspen stands Zasada [104] observed in interior Alaska were 150 to 200 years old.

Within stands, fires become more frequent as the proportion of spruce or age of quaking aspen increases [31]. From 1956 to 1965, 21% of wildfires statewide were in deciduous-conifer forests, 36% percent were in conifer forests, and 29% were in tundra [4]. On lowlands of the Kenai Peninsula, scattered quaking aspen stands within a black spruce forest were <50 years old. Four large fires, ranging from 3,700 to 270,000 acres (1,500-110,000 ha), burned in the area between 1940 and 1999 [17].

A few studies provided information on fire-return intervals in Alaskan quaking aspen and balsam poplar communities or stands. Table 2 provides historical and paleological fire-return intervals for those few studies.

Paleobotanical studies on the Kenai Peninsula found quaking aspen dominated during the late Pleistocene and early Holocene. Climate was drier and fires more frequent during that time (10,700-8,500 BP; MFRI*=77 years) than during the early Pleistocene (13,000 BP; MFRI=138 years), when the site was a tundra grassland [2].
*MRFI=mean fire-return interval.

Table 2. Fire frequency information for quaking and balsam poplar types of boreal and subboreal Alaska
A. Historical fire frequencies
Area Fire-return interval
(years)		 MFRIs predicted by LANDFIRE models* (BpS) Scope of study;
period studied		 Site type
Caribou Poker Creeks Research Watershed, interior Alaska

125-200+

138 (16050) across 2 subbasins, ages of 4-47 trees determined on 21 sites;
1749-1924		 mosaic of black spruce, white spruce, & quaking aspen-paper birch boreal forests; paper birch-quaking aspen on south-facing slopes [28]
Riley Creek near the Nenana River, south-interior Alaska ~40-60 294 (16141) ages of 56 quaking aspen & ~35 white spruces determed on a ~150-ha riparian sites w/ 8 terraces, 2 terraces were white spruce-balsam poplar;
1740-1920s		 boreal white spruce-balsam poplar-quaking aspen/russet buffaloberry/lichen riparian forests on low terraces [73]
Ester Dome near Fairbanks 80-100
 138 (16050) ages of 4 quaking aspen stands across ~40 ha on northwest side of dome (15-60 quaking aspen/stand sampled);
1800-2000		 mosaic of black spruce, quaking aspen, & paper birch boreal forest stands; quaking aspen on warm sites [56]
Yukon Flats Wildlife Refuge, eastern interior measured w/ fire scars:
45-148;
x̄=82		 138 (16050) determined stand ages on 27 sites (1-5 ha) w/ 40 total fires, quaking aspen on 3 sites & white spruce-quaking aspen on 6;
1843-1998 for fire scar method
 mosaic of black spruce, white spruce, quaking aspen, & mixed conifer-hardwood boreal forests w/ quaking aspen & white spruce-quaking aspen well-drained, warm uplands [20]
measured w/ cohort age:
37-166;
x̄=24
1751-1999 for cohort method
*MFRI=mean fire-return interval. See Appendix A for further information on similar BpS groups.
B. Paleological fire frequencies
Area Mean fire-return interval (years) MFRIs predicted by LANDFIRE models* Scope of study;
period studied		 Current site type
south-central Brooks Range 258 not applicable 4 lakes, 13-32 sediment cores;
10,300-8,250 BP		 riparian balsam poplar & upland quaking aspen [38]
Alaska Range 170 not applicable sediment cores;
6,600-2,000 BP		 Populus spp./willow (abstract [72])
near Moose Lake, southeastern Alaska >500 prior to 3,800 BP;
~200 from 3,150 BP to present		 not applicable 7 sediment cores;
5,800 BP to present		 upland white spruce-quaking aspen-paper birch/mountain alder [71]
Kenai Peninsula 77 (SE 49) not applicable 4 sediment cores;
10,700-8,500 BP		 lowland quaking aspen-Kenai birch/mountain alder [2]

Analyses of fire frequencies across boreal Alaska from the 1940s to the 1990s found mean fire cycles (time required to burn an area equal in size to the study area) were shortest in central and east-central interior Alaska and longest in the Davidson Mountains of northeastern Alaska. Infrequent fire in the Davidson Mountains—despite high total tree cover and high proportion of conifers to deciduous trees (spruces:quaking aspen and paper birch)—was attributed to infrequent lightning activity. Yukon-Old Crow Basin and the North Ogilvie Mountains had relatively low proportions of conifer:deciduous trees (30% and 43%, respectively) and relatively frequent fire cycles (97 and 12.2 years, respectively) [52].

Table 3. Fire cycles across boreal Alaskan taiga [52]
Region Fire cycles
(years)		 Mean growing-season temperature
(°C)		 Mean growing-season precipitation
(cm)		 Mean lightning strikes
(per 10 × 10 km/year)		 Tree cover
(%)		 Total conifer cover
(%)
Yukon-Old Crow Basin (east-central) 97 8.8 18.8 4.9 90 27
Ray Mountains (central) 136 14.7 17.4 4.7 73 45
North Ogilvie Mountains (east-central) 138 12.2 17.6 3.7 92 40
Yukon River lowlands (central) 146 12.2 13.8 0.3 78 55
Tanana-Kuskokwim lowlands (south-central) 214 14.9 18.5 3.7 83 64
Kobuk ridges and valleys (west-central) 215 13.0 16.6 3.0 47 43
Kuskokwim Mountains (south-central) 253 15.8 17.1 3.4 70 43
Seward Peninsula (west) 340 11.1 17.6 0.3 1 1
Nulato Hills (west) 356 16.5 11.3 1.2 19 17
Davidson Mountains (northeast) 464 9.7 14.8 1.6 92 70
Yukon-Tanana uplands (southeast) 540 16.3 17.1 5.4 71 54

Yarie [102] found a fire cycle of 26 years for deciduous stands in the Porcupine River watershed (3.6 million ha). Quaking aspen, paper birch, and balsam poplar were present, but the specific deciduous mix was not described in the study.

Balsam poplar:
Floods, not fires, are the primary disturbances in balsam poplar stands [66]. Experts state that fire is rare in balsam poplar stands [16,67,68]. Lutz [66] wrote that fires "are not nearly as common in the tacamahac (balsam) poplar type as in other forest communities but they are not unknown". Cronan and others [16] reported that "balsam poplar likely represents a fire refugia in the boreal forests of Alaska". Riverbottoms of interior Alaska seldom burn because soils are moist [97], the vegetation is relatively nonflammable [25,55,97], and such sites are often adjacent to sloughs and oxbow lakes [97] that break up fire continuity. However, balsam polar stands occur near flammable spruce types, so fires likely burn into balsam poplar stands in extreme fire years [16].

Balsam poplar requires a scouring flood or stand-replacing fire about every 100 years, or spruce replaces it successionally [92]. Unfortunately, as of 2014 there were few fire history studies documenting fire-return intervals of Alaskan balsam poplar communities. The Riley Creek study (see Table 2A) found a fire-return interval of 40 to 60 years [73]. Stand ages of 1 and 2 centuries [25,55] suggest stand-replacement fires are uncommon in balsam poplar stands [16]. On the North Slope, for example, balsam poplars were 100 to 250 years old [25]. Paleological studies found fire was relatively infrequent when Populus spp. dominated the landscape [41,72] (see Table 2B).

Fire type

Quaking aspen:
Alaskan quaking aspen stands have frequent surface [21,67,81,97] and infrequent stand-replacing crown [21,87] fires. Surface fires are typical in pure or mixed-deciduous stands [81,97]; crowning usually occurs only in extreme fire years [10,81]. Todd and others [87] suggested that fires in deciduous stands are not as severe as fires in conifer forests due to lack of low branches and resinous needles in deciduous forests. Lutz [67] reported that in interior Alaska, fires in pure quaking aspen stands often either burn at low severity or fail to burn. Quaking aspen stands are firebreaks in many years [1,61,97]. Many quaking aspen stands survived the 1947 Kenai Wildfire, while adjacent black spruce stands experienced stand-replacement crown fire [67].

In deciduous-conifer stands, the probability of crowning increases with the proportion of conifers (usually spruces) [81]. Mixed deciduous-conifer stands often have mixed-severity fires. On the Kenai National Moose Range of south-central Alaska, the Russian River and Swanson River wildfires of 1969 burned about 86,000 acres (35,000 ha) of quaking aspen-paper birch-black spruce-white spruce mosaics. In quaking aspen-paper birch stands, wildfires were mixes of low- and moderate-severity surface fires and stand-replacing fires. In some places, the fires burned into the roots of the deciduous trees, but in other places, the deciduous trees were top-killed or only slightly damaged [33].

Ground fires are rare in quaking aspen and mixed-deciduous forests but not in deciduous-conifer forests. Fires do not linger on quaking aspen forest floors because quaking aspen grows on relatively warm, dry sites lacking permafrost [49,50,104], and the litter and soil organic layers are often thin [67]. On 6 sites in the Tanana River valley, wildfires removed most of the litter, moss, and humic layers in quaking aspen and white spruce stands but not in black spruce stands. However, in the overstory, more biomass burned in black spruce than in white spruce stands, and more burned in white spruce than in quaking aspen stands [50]. Ground fires may linger for weeks, or even months, in deciduous-spruce forests (review by [87]).

In pure quaking aspen communities, fires are usually of low severity in stands of all ages [21]. Old quaking aspen stands are most likely to burn severely— particularly stands that are breaking up—because they have greatest fuel accumulations [21]. When low-severity fires do not kill conifers in the understory, conifers may replace quaking aspen successionally [11,81]. Likelihood of stand-replacement fire increases as conifers increase [21].

Severe fire that results in permafrost melt can result in a type shift from black spruce to quaking aspen [48].

Figure 3. The 2007 McKinley River Wildfire, in Denali National Park, burned mostly on the surface but with some torching of spruces. Balsam poplar is visible in the top right. USDI, National Park Service photo.

Balsam poplar:
To date (2014), fire history studies of balsam poplar forests were few. Experts suggest that balsam poplar stands historically experienced a mix of infrequent low-severity surface fire [16] and infrequent, stand-replacement fire [66,97]. The latter experts did not speculate on whether stand-replacement surface or crown fire was typical for balsam poplar communities.

Stand-replacement fire is most likely late in riparian succession, when white spruce is replacing or has replaced balsam poplar [97]. This is generally after about postdisturbance year 100 [30]. Stand-replacement fire sets back succession, favoring balsam poplar over spruces. Severe fires that burn into uplands can favor expansion of balsam poplar into upland spruce types. Lutz [66] reported that severe fire tends to maintain balsam poplar stands, while frequent low-severity fire tends to maintain balsam poplar-white spruce stands. Geographically isolated balsam poplar sites may not burn for centuries. Viereck [97] reported that some balsam poplar stands on islands of major rivers were >200 years old.

Fire intensity

Quaking aspen:
Fire intensity in Alaskan quaking aspen types is usually low. The under- and overstory fuels are relatively nonflammable and shade the ground, so fine surface fuels are moist during most of the fire season. Fireline intensity in a quaking aspen stand in central Alberta, burned before leaf-out, ranged from 15 to 581 kW/m. Fireline intensity in a quaking stand in the same location, burned in late summer under severe fire weather, was estimated at 4,200 kW/m [16].

Quaking aspen stands are most flammable before spring leaf-out and after fall leaf drop, but fire intensity is always lower in quaking aspen than in coniferous stands [34].

Hardy and Franks [34] rated relative rates-of-spread in quaking aspen fuel types of Alaska as medium, except for on ridgetops (medium-high) and southerly slopes (high).

Balsam poplar:
No information was available on this topic as of 2014.

Fire severity Quaking aspen:
Quaking aspen forests experience low-, mixed-, and high-severity fires. In quaking aspen-spruce mosaic forests, surface fire tends to be more severe and ground fire more common in spruce than in quaking aspen stands [22,26,80]. On the Bonanza Creek Experimental Forest, for example, the 1983 Rosie Creek Wildfire burned quaking aspen, paper birch, white spruce, and black spruce stands. Fire severity was low in the quaking aspen and paper birch stands, while it ranged from low to high in white spruce and black spruce stands. Most acreage burned (3,902 acres (1,579 ha) total) was spruce [22]. In many years, fires that crown in white spruce or black spruce stands burn around quaking aspen stands [34].

Severe fire sometimes favors quaking aspen by decreasing the relative abundance of black spruce in early postfire communities [47]. Postfire sprouting and seedling establishment of quaking aspen is highest on severely burned sites, although rate of quaking aspen seedling establishment is generally less than that of associated conifers [44]. Analyses of fire severity and patterns in Alaskan boreal forests from 1950 to the 1990s showed severe fire years occurred about once every 4 years, although landscape patterns (deciduous:conifer) within these burns were not noted [52].

Very moist black spruce sites where fire has exposed mineral soil are most likely to succeed to deciduous species in early postfire landscapes. Sites with moderately moist (5%-30% soil moisture content) mineral soil will likely transition to mixed deciduous-black spruce forest [45]. Studies on 4 burns near Delta Junction and Tok Junction in interior Alaska found that from postfire years 10 to 20, quaking aspen dominance was positively correlated with sites that burned severely and therefore had shallow soil organic layers. Black spruce dominance was positively correlated with sites that burned at low severity and had deep residual organic layers (P<0.001) [84]. On 90 burned black spruce sites near Fairbanks, seedling establishment and early postfire dominance of quaking aspen and paper birch were positively associated with severe fire that removed much of the soil organic layer (P<0.001) [46]. Johnstone and others [45] provide a key for predicting postfire successional trajectories in taiga of interior Alaska.

Figure 4. A patchy surface burn on FROSTFIRE's experimental site (see Fire pattern below). USDA, Forest Service photo.
Figure 5. Severe fires may favor quaking aspen over conifers. Here, a firefighter sprays a hot spot during the 2006 Parks Highway Wildfire near Nenana. USDI, National Park Service photo.

Balsam poplar:
Experts suggest that balsam polar stands probably experience low-severity surface [16] and severe, stand-replacement [66] fire. Documentation of this—and relative frequency of low-severity and stand-replacement fire—was not was available as of 2014.

Fire pattern and size
A few large fires account for most acreage burned in interior Alaska [52]. From 1940 to 1994, 3 large fires (in 1940, 1957, and 1969) burned about 4.5 million acres (1.8 million ha) in interior Alaska (USDI, BLM data cited in [31]). During that period, individual fires ranged from 0.25 to 1.16 million acres (0.10-4.69 million ha). Most fires (87%) were <300 acres (100 ha); 12.5% were 300 to 50,000 acres (120-20,200 ha); and 0.5% were >50,000 acres (Alaska Interagency Fire Management Plan 1984 cited in [31]). Large fires occurred in drought years, when several small fires would usually merge into a large fire [31]. Another study of fire records of Alaskan taiga from 1940 to the 1990s found most large fires (73%) occurred during 10 severe fire seasons. During these severe fire seasons, fires averaged 50,200 acres (20,300 ha). In less severe fire years, average fire size was 19,000 acres (7,800 ha), and no fires were >500,000 acres (200,000 ha) [52].

Alaskan taiga usually burns in mosaics, with low- to high-severity and unburned patches [31,39,96] such as that shown in Figure 6 below. Quaking aspen and balsam poplar communities usually do not burn except in drought years [26].

Quaking aspen:
In most years, quaking aspen and quaking aspen-paper birch stands are too moist to burn, even when adjacent, highly flammable black spruce stands do burn [39]. Quaking aspen self-prunes [34] and the shrub layer is often sparse [34], so fires that crown in adjacent spruce stands usually burn around quaking aspen stands [34]. Except in extreme fire years, quaking aspen or other deciduous stands may be firebreaks [26]. If they do burn, they typically comprise low- and mixed-severity patches of the mosaic [16].

Figure 6. The 2009 Bear Creek Wildfire in Denali National Park left a mosaic of different burn severities and unburned patches. USDI, National Park Service photo.

The FROSTFIRE experimental fire during 8 to 15 July 1999 burned about one-third of a subbasin in the Caribou Poker Creeks Research Watershed. The landscape was a black spruce/feather moss and quaking aspen-paper birch mosaic. Quaking aspen-paper birch occurred on south-facing slopes lacking permafrost. The understory of deciduous stands was sparse, with alders and willows in the tall shrub layer and ericaceous shrubs in the low shrub layer [39]. Within the fire's 2,590-acre (1,050 ha)) perimeter, black spruce dominated most of the landscape that burned: About 1,100 acres (430 ha) of black spruce/feather moss on north-facing slopes underlain with permafrost experienced stand-replacement crown fire [15]. Most quaking aspen stands, paper birch stands, and sphagnum bogs did not burn [15,39,82]. Although the fire was prescribed, researchers concluded that as "with a natural wildfire, a range of burn severities developed from irregular burning patterns" [39].

High-elevation quaking aspen-steppe bluff types usually have fewer understory and surface fuels than similar low-elevation types. Therefore, fires in high-elevation quaking aspen-shrubland are likely patchier and fewer in number than fires in low-elevation quaking aspen-steppe bluff types (expert estimate cited in [59]).

Balsam poplar:
Balsam poplar communities are relatively nonflammable, and do not burn in most years [16]. No further information was available on this topic as of 2014.
CONTEMPORARY CHANGES IN FIRE REGIMES
The number of large forest fires increased in Alaska from the 1960s to the 1990s compared to the previous 20 years. Large-fire years were drier than small-fire years [100].

The combination of more frequent fires and a warming climate may favor quaking aspen, balsam poplar, paper birch, and other birches, allowing these deciduous trees to establish on boreal sites that were formerly dominated by spruces [8,46,56,64,99]. In particular, high-severity fires will likely favor quaking aspen and birches over spruces [8]. Barrett and others [6] predicted that in interior Alaska, sites with <1 inch (3 cm) of organic soil remaining will succeed to quaking aspen-paper birch stands and lack permafrost. Sites with 1 to 4 inches (3-10 cm) of organic soil remaining will succeed to mixed quaking aspen-paper birch-black spruce stands, with either deep, active layers of warmer soil or permafrost taking longer to recover previous thickness. Sites with >4 inches (10 cm) of organic soil remaining after fire will succeed to black spruce stands, with permafrost eventually re-forming [6]. A model predicted a 450% increase in area of deciduous forest in interior Alaska, mostly from encroachment into areas that are currently black spruce forest [13].

Previous periods of global warming have favored quaking aspen expansion in Alaska. Quaking aspen and poplars apparently dominated larger portions of interior Alaska during the early Holocene than at present [37,38,40]. For example, in the south-central Brooks Range, Populus spp. forests dominated the landscape from about 10,300 to 8,000 BP, while white spruce was dominant from around 8,000 to 5,500 BP [38].

However, it is not certain that climate warming will favor quaking aspen expansion on Alaskan landscapes. Landscapes dominated by quaking aspen or poplars tend to have higher surface albedo [3,27] and rates of evapotranspiration [14] compared to landscapes dominated by conifers. This reduces sensible heat flux, which can result in a net cooling effect on climate [9,76,84].

LIMITATIONS OF INFORMATION
Fire histories of Alaskan taiga communities in general are incomplete. Alaska's fire records only date back to the 1940s [24], and records may be missing or incomplete [20]. The fire histories of communities dominated by Populus spp. are particularly difficult to reconstruct due to heart-rot decay, which destroys fire scars or renders them hard to interpret [86]. As of 2014, there were few fire studies of quaking aspen and balsam poplar communities in Alaska. Most of those were conducted in the northeastern part of the state. That region has lower mean annual precipitation, higher mean lightning strike density, and a shorter fire-rotation interval relative to the rest of Alaska [52]. Only tenuous inferences can be made based on such limited data, and further research is needed on fire regimes of these communities.

Quaking aspen:
Fire regimes of quaking aspen communities of Alaska were not well documented as of 2014. A handful of studies show fire-return intervals ranging from 40 to over 200 years (see Table 2). Although both low and severe fires occur in quaking aspen [22,47], the relative proportion each was not documented. Moderate-severity fire was documented in only 1 study [33].

Balsam poplar:
To date, information on fire regimes in balsam poplar communities is limited almost entirely to expert opinion. One study at Riley Creek (see Table 2) documented fire-returned intervals of 40 to 60 years, suggesting frequent, low-severity fire is important on that site. Even studies of how balsam poplar regenerates after fire in Alaska [23], and its postfire successional role, are few [16].

Considerations for LANDFIRE: LANDFIRE's [61] placement of boreal mesic paper birch-quaking aspen forest in Fire Regime Group IV (35-200 year frequency, stand-replacement) is based on expert opinion. This placement may need to be reconsidered if studies show low-severity fire is important in paper birch-quaking aspen stands.

The fire regimes of quaking aspen-steppe bluff communities were undocumented [16] as of 2014, and these communities are not well described in vegetation classifications. LANDFIRE assigns low- [60] and high-elevation [59] boreal dry aspen-steppe bluff communities to Fire Regime Group III (35-200 years, mixed severity) based on expert opinion; the literature can provide no refinement of this estimate.

Alaskan balsam poplar communities cannot be assigned a Fire Regime Group based on research as of 2014: Except for the Riley Creek study [73], documentation of fire regimes of balsam poplar communities in Alaska was lacking. Cronan and others [16] noted that low-severity fires in riparian zones may leave little evidence of their passing, and low-severity fires may burn in balsam poplar communities more often than realized. A study in Alberta found fire-return intervals of 20 to 30 years for balsam poplar stands [78], suggesting placement in Fire Regime Group I (frequent, low-severity fire). A few researchers have published widely divergent expert opinions on fire regimes of Alaskan balsam poplar. Cronan and others [16] assigned a regime of infrequent, low-severity fire to pure balsam poplar communities, suggesting placement in Fire Regime Group V (long-interval; may include fires of any severity [7]). Lutz [66] stated that balsam poplar communities had infrequent stand-replacement fire, which would place them in Fire Regime Group IV or V. Further research is needed on the apparently variable fire regimes of Alaskan balsam poplar community types before Fire Regime Groups can be more confidently assigned to Alaskan balsam poplar communities.

Third-order streamside communities are missing from the Biophysical descriptions and models of white spruce-balsam poplar floodplain types. The Boreal riparian stringer forest and shrubland BpS applies to forests near small streams, with streamsides that generally lack gravelbars [58], while the Boreal lowland large river floodplain forest BpS applies to large river floodplains [57]. Since balsam poplar and white spruce are common along large streams [74,96], this omission fails to capture many white spruce-balsam poplar communities.

APPENDIX A: Summary of fire regime information for Biophysical Settings covered in this synthesis


APPENDIX B: Common and scientific names of plant and lichen species.
Follow the links to FEIS Species Reviews.
Common name Scientific name
Trees
balsam poplar Populus balsamifera subsp. balsamifera
black cottonwood Populus balsamifera subsp. trichocarpa
black spruce Picea mariana
jack pine Pinus banksiana
Kenai birch Betula papyrifera var. kenaica
paper birch Betula papyrifera
poplar Populus spp.
quaking aspen Populus tremuloides
Sitka spruce Picea sitchensis
spruces Picea spp.
white spruce Picea glauca
Shrubs
Alaska wormwood Artemisia alaskana
alders Alnus spp.
birches Betula spp.
Barclay's willow Salix barclayi
common juniper Juniperus communis
ericaceous shrubs Ericaceae
fringed sagebrush Artemisia frigida
halberd willow Salix hastata
highbush cranberry Viburnum edule
huckleberries Vaccinium spp.
kinnikinnick Arctostaphylos uva-ursi
prickly rose Rosa acicularis
mountain alder Alnus viridis subsp. crispa
russet buffaloberry Shepherdia canadensis
shrubby cinquefoil Dasiphora floribunda
silverberry Elaeagnus commutata
Sitka alder Alnus viridis subsp. sinuata
thinleaf alder Alnus incana subsp. tenuifolia
willows Salix spp.
Forbs
bunchberry Cornus canadensis
cow parsnip Heracleum lanatum
fireweed Chamerion angustifolium
false toadflax Geocaulon lividum
twinflower Linnaea borealis
wintergreens Pyrola spp.
Graminoids
arctic brome Bromus pumpellianus
bluejoint reedgrass Calamagrostis canadensis
northern rough fescue Festuca altaica
reedgrasses Calamagrostis spp. (includes bluejoint reedgrass
& circumpolar reedgrass (Calamagrostis deschampsioides))
Fern allies
horsetails Equisetum spp.
Bryophytes
feather mosses Hylocomiaceae
Schreber's moss Pleurozium schreberi
splendid feather moss Hylocomium splendens
Lichens
felt lichens Peltigera spp.

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