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Fire regimes of Alaskan black spruce communities

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.

Common names are used throughout this synthesis. For a complete list of common and scientific names of plant and lichen species discussed in this synthesis and links to FEIS species reviews, see Appendix B.

Black spruce is the most common forest type in Alaska and is particularly prevalent in interior regions. Fire history, aspect, topography, hydrology, depth or absence of permafrost, and latitude are important in determining the distributions and species compositions of Alaska's black spruce communities. Fire regimes of black spruce forest, woodland, bog, and black spruce-hardwood formations are discussed in this review.

Most wildfires in Alaskan black spruce communities are ignited by summer lightning. Documented mean fire-return intervals (MFRIs) since the 1700s range from about 40 to 200 years (see Table 2). Wet and stringer black spruce communities generally have longer fire-return intervals than mesic communities. Fires typically crown, with accompanying surface and ground fire. Most fires are small, but a few large fires account for most acreage burned. Except those near settlements, few Alaskan black spruce communities have been subject to fire exclusion.

Humans are increasingly the source of ignitions in black spruce taiga, but fires resulting from human ignitions tend to be small. Sizes of lightning-ignited fires have increased since the 1970s compared to the 40 years prior. Climate warming is likely responsible for this change. Climate change is predicted to result in type shifts, with some communities transitioning from black spruce forests to mixed hardwood-spruce forests and some treeless tundra communities shifting to black spruce woodlands.

Appendix A summarizes data generated by LANDFIRE succession modeling for the Biophysical Settings (BpSs) covered in this review. The range of values generated for fire regime characteristics in Alaskan black spruce communities is:

Figure 1. Land cover distribution of Alaskan black spruce communities based on the LANDFIRE Biophysical Settings (BpS) data layer [75]. Numbers indicate LANDFIRE map zones. Click on the map for a lager image and zoom in to see details.

Black spruce communities are widely distributed in boreal Alaska. Around 30% to 40% of Alaska's landscape is boreal forest [6], and black spruce is the most common boreal forest type [23,104]. Black spruce communities are especially common in interior Alaska, occupying 39% to 44% of that region [23]. Alaskan black spruce communities typically occur on cold, poorly drained, nutrient-poor sites with a shallow permafrost layer; hence, their productivity is usually low [104]. The most common black spruce types are broadly divided into upland and lowland types. Typically, upland types occupy low-gradient or north slopes, while lowland types occupy broad valleys or old river terraces. Less common types are productive black spruce forests on south slopes and black spruce-lichen woodlands. Black spruce-lichen woodlands are typically a transitional type between taiga and tundra, occurring on glacial deposits at high latitudes [23].

Black spruce communities are most common between the Brooks Range in the north and the Alaska Range in the south. Eastward, they extend to the US-Canada border and beyond; westward, they extend to the Bristol Bay and Yukon Delta regions. Black spruce communities are not common in southern Alaska [104], but there are some by Cook Inlet [23,38,69,102].

Black spruce dominates many mixed-conifer and conifer-hardwood formations of Alaska [38,104]. The black spruce overstory varies from <10% canopy cover in scrub to 10% to 25% in woodland, 25% to 60% in open forest, and >60% in closed forest formations. Groundlayer dominants vary with site but usually include feather mosses, sphagnum, and/or reindeer lichens. The shrub layer is absent to well developed; huckleberries, prickly rose, Labrador-teas, birches, and/or willows may dominate [38,104]. Black spruce communities are often adjacent to and may intergrade with white spruce or quaking aspen-birch (hardwood) communities. Fire history and development of thermokarsts are important in determining which plant communities dominate the landscape [67].

Appendix A lists the Biophysical Settings (BpSs) covered in this review and summarizes data generated by LANDFIRE's [73] successional modeling for those BpSs.

In their review of black spruce plant community classifications of Alaska, Viereck and others [104] identified 6 aggregations of black spruce communities based on community compositions, distributions, and site characteristics. Similar BpS types are provided in parentheses.

• Black spruce forest occurs on cold, poorly drained organic soils that are often underlain with permafrost. This type is widely distributed in interior and south-central Alaska and includes feather moss and shrub understory types. Varies from open to closed in late succession [104] (BpS 16220 and 16300 series).
• Black spruce-paper birch forest occurs on cool, wet sites. Found mostly in interior and south-central Alaska; a minor type in northwestern and southwestern Alaska. Usually closed in late succession [104] (BpS 16041, 16211, and 16220 series).
• Black spruce-white spruce forest occurs on terraces and the bases of south-facing slopes near the northern and western treeline limits; includes feather moss and shrub types. Usually closed in late succession but open near latitudinal treeline [104] (BpS 16041 and 16042 series).
• Black spruce-tamarack open forest occurs on wet lowlands underlain with permafrost in interior Alaska. Understory type is not described [104] (BpS 16211 and 16220 series).
• Black spruce dwarf scrub occurs in interior, south-central, and western Alaska. Soils are so cold or wet that that tree production is marginal. Includes shrub, feather moss, and cottongrass types [104] (BpS 16211 and 16212 series).
• Black spruce woodland bogs and black spruce-lichen woodlands are lumped in this classification. Black spruce bogs occur in wet lowlands of interior, south-central, southwestern, and northwestern Alaska; includes shrub, sphagnum, feather moss, and cottongrass types (BpS 16212 series). Distribution of black spruce-lichen woodlands is similar to that of black spruce bogs, but the lichen type occurs on drier, upland sites [104] (BpS 16220 series).

As of 2013, there was generally more scientific literature on fire regimes of black spruce forest types than on black spruce-lichen, black spruce scrub (dwarf-tree), and black spruce bog types (for example, see Table 2). No information was available on fire regimes of the black spruce-tamarack forest type. Therefore, information on fire season, frequency, and other components of fire regimes discussed below is more complete and probably more accurate for black spruce forest than for other black spruce vegetation types.

The sections below provide discussions and documentation of historical fire regimes in Alaskan black spruce communities.

Ignition
Lightning is the main source of ignitions in spruce ecosystems of Alaska [6,94,105]. Data from 1956 to 1999 show >90% of fires in Alaskan taiga (all forest types) were started by lightning [6,44]. Black spruce forests have a higher incidence of lightning strikes than other common boreal forest types [27]. Lightning-burned areas tend to vary more in size than burns resulting from human ignitions (see Contemporary Changes in Fire Regimes). From 1956 through 2000, sizes of lightning-caused burns ranged from 0.8 to 4,432 miles² (2-11,476 km²), while human-caused burns ranged from 0.2 to 2,163 miles² (0.5-5,601 km²). Mean burn size ranged from 0.02 to 37 miles² (0.07-95 km²) for lightning-ignited fires and from 0.004 to 14 miles² (0.01-36 km²) for human-ignited fires [16]. A fire history study of boreal forests between the Brooks and Alaska ranges found most of the area had a natural fire regime: many large, lightning-caused fires, few human ignitions, and "negligible" fire suppression [26].

Figure 2. The lightning-ignited Bear Creek Wildfire burned several thousand acres of spruce forest along the Parks Highway, roughly 80 miles southwest of Fairbanks. Photo courtesy of the Alaskan Fire Service.

Burning by Native Alaskans is likely a long-time cultural practice [82,105]. A review noted that historical records of Indian-set fires along the Copper River and northward to the Alaska Range date back to 1889 (Glenn 1900 in [80]). European settlers observed Indian-set fires across Alaska in the 1890s and early 1900s [80]. However, the frequencies and locations of fires historically set by Native Alaskans were not well documented.

Igntions probably increased during the early 1900s due to mining [62].

Black spruce forests are the most flammable vegetation types in interior Alaska [23]. Because black spruce needles and branches are highly resinous and usually distributed continuously from ground to treetop, they ignite and burn easily [23,86,109]. Black spruce/feather moss and mixed conifer-hardwood stands are usually moister than black spruce-lichen stands, so they are less likely to burn in most years [13]. Since lichens dry more quickly than mosses, they are more likely than mosses to promote fire spread. During extreme drought, all strata (ground to canopy) of a black spruce community become highly flammable and burn readily. Dyrness and others [32,106] reported that in open-canopy black spruce communities with feather moss, lichens, and low shrubs, the shrub layer is not critical to continuity of ladder fuels; fire easily travels from the moss-lichen ground layer to the black spruce canopy.

Season
Most wildfires in Alaskan taiga burn in summer, but annual variability in fire season is high [23]. The fire season may extend from 1 April to 30 September [105]. May through late August are typically the most active fire months [24,105] because they have the highest average temperatures and lowest average humidity and precipitation [105]. In "average" fire years, the fire season ends in July, when low-pressure systems replace the high-pressure systems that block incoming precipitation, and relative humidity and precipitation increase [23].

Most fires in Alaska taiga occur in dry years [20]; severe fire years are correlated with warm, dry air flows [54]. Fire seasons extending into late summer or early fall are associated with moderate to strong El Niño phases. Increased lightning strikes are also associated with El Niño phases [23]. During the summer of 2004, interior Alaska had the largest area burned (6.7 million acres (2.7 million ha)) since fire records were started in the 1940s [50] (see Contemporary Changes in Fire Regimes). Sites that burned in late July and August had a larger proportion of high-severity fires than sites that burned earlier in the summer (n=90 sites) [50].

In spruce-birch and tundra ecosystems of interior Alaska, DeWilde [25] found the fire season lasted about twice as long in areas near relatively large human population centers, such as Fairbanks, as in areas away from large human settlements. About half of all ignitions occurred in the most flammable vegetation types of interior Alaska: boreal black spruce/feather moss and boreal black spruce heathland forests [25].

Frequency
In interior Alaska, most black spruce stands burn before reaching 100 years old [38]. Fire frequency generally increases with black spruce cover [48]. Todd and Jewkes [99] summarized fire frequency in Alaskan black spruce communities: "the question is not if a boreal spruce forest will burn, but when. A given stand of spruce in the boreal forest will burn every 50 to 150 years, and some areas burn even more frequently. This is inevitable in a fire-driven ecosystem." Fire history studies of Alaskan black spruce wildlands show a range of MFRIs varying from 25 [24] to 146 years [35] from the 1700s to the 2000s (see Table 2). Fire-return intervals of <30 years may result in black spruce recruitment failure. On the Kenai Peninsula, there was at least 35 years between fires in black spruce stands [24]. Seedling establishment is sparse before then because cone crops of small trees are so sparse [108]. In their review, Cronan and others [23] reported MFRIs that varied from 57 years in black spruce-lichen woodlands of the central Brooks Range to 146 years in a watershed covered by black spruce forest near Fairbanks. In eastern-interior Alaska and adjacent Yukon, 6 of 30 black spruce forests had not burned for >150 years [111]. Heinselman [45] calculated mean fire-rotation intervals of 100 years for closed black spruce forests and 130 years for open black spruce-lichen woodlands across Alaska. Lutz [80] provides a history of individual forest fires documented in Alaska from 1893 to 1950.

Fire studies reporting fire-return intervals or fire-rotation intervals are shown in Table 2.

LANDFIRE's modeled MFRIs range from 73 to 188 years (see Appendix A), which is similar to MFRIs reported in the literature. Fire-return intervals in LANDFIRE BpS descriptions varied as follows. LANDFIRE designates the Alaska Range as separating boreal from subboreal zones.

Generally, bodies of water on a landscape and wet soils reduce fire frequency. Across interior Alaska, plant communities near streams, rivers, and lakes were positively correlated with low fire frequencies [23,63]. Dry upland black spruce types are thought to experience fire more frequently than wetter lowland types [23] such as black spruce bogs and stringer forests [108].

Forests: The research summarized above is consistent with earlier studies on fire regimes in Alaskan black spruce. According to Heinselman [45] and Dyrness and others [33], Alaskan black spruce forests burn about every 50 to 100 years. The coldest, wettest, and/or most open communities tend to have the longest fire-return intervals. Heinselman [45] estimated the fire cycle at about 100 years for closed black spruce forest and 130 years for open black spruce-lichen woodlands. Fire-return intervals are generally shortest in interior Alaska, while southern and southwestern Alaska have long fire-return intervals [110].

Fire frequencies vary locally as well as on broad geographic scales. How often black spruce communities burn is determined by a site's stand composition (mixed or pure black spruce), solar insulation, altitude [65,105], slope, drainage, presence and thickness of permafrost, and fire history [105,117].

Landscape-level studies found that compared to other Alaskan vegetation types, black spruce forests have short to intermediate MFRIs. A spatial analysis of the fire histories of taiga and tundra communities between the Brooks and Alaska ranges found fire was most common in mixed hardwood-white spruce forests (70 fires/million ha/decade), followed by tundra (58 fires), black spruce forests (47 fires), and black spruce-lichen woodlands (20 fires) [23,26]. A review reports that fire frequencies decrease from south to north in the progression from black spruce forests in the interior to black-spruce-lichen woodlands that transition to tundra in arctic zones [23].

Since fires in Alaskan taiga usually kill spruces and top-kill hardwoods, stand age is the most convenient method of dating the last fire [33]. A large-scale analysis of stand ages in the Porcupine and Upper Yukon river drainages of eastern Alaska found hardwood stands had the shortest mean fire cycle (60 years), followed by black spruce (100 years), then white spruce (113 years). Researchers determined the ages of 371 stands across 8.9 million acres (3.6 million ha) [23,116]. However, another fire history study of 27 sites in the same area found no significant difference in fire frequency between sites dominated by white spruce/quaking aspen (82 years) and black spruce (67 years) [23,28].

A few studies were able to use dendrochronology to determine fire history of a site. A dendochronology study on the Caribou-Poker Creeks Research Watershed found the site experienced 3 stand-replacement fires in 103 years (1896-1999): in 1896, 1909, and 1924. North-facing slopes were underlain with permafrost and occupied by black spruce forest stands. South-facing slopes were free of permafrost and occupied by quaking aspen and paper birch stands. Ericaceous species dominated the shrub layer of these stands; Schreber's moss and splendid feather moss carpeted the forest floor [97].

Bogs and stringers: Wet black spruce communities usually burn only in July, August, and/or September of severe drought years [13]. Black spruce bogs experience longer fire-return intervals than nearby upland black spruce stands; these bogs usually become uneven-aged over time [108]. Studies documenting fire-return intervals in Alaskan black spruce bogs were few as of 2013. On burns south of the Yukon River in interior Alaska, Foote [38] aged black spruce/feather moss-lichen stands clumped within lichen tundra at 40 to 70 years, but the dates of previous fires were not determined. Heinselman [45] estimated a fire-rotation interval of 100 to 150 years for large black spruce forest bogs of Minnesota, which was longer than that of adjacent upland forests [45].

Studies near Fairbanks found black spruce-white spruce stringer forests typically had longer MFRIs than surrounding, contiguous conifer forests. Stringer forests were often located in concave depressions or swales below springs [23]. Quirk and Sykes [93] concluded that these sites were less conducive to fire spread than surrounding forests due high soil moisture, low soil temperature, and sheltered topography. Estimates of fire-return intervals for stringer spruce forests ranged from 100 to 150 years [93], while estimates for surrounding spruce forests ranged from 40 to 60 years [35]. Fastie and others [35] note that actual fire-return intervals in spruce stringers may be shorter than estimated. Because low fire severity may result in low spruce mortality, some fires may be undetected [35].

Ancient times: Paleobotanical studies of ancient fire regimes show several apparently climate-driven fluctuations in fire frequencies in Alaskan boreal forests. At the end of the Ice Age (~13,000 BP), areas on the Kenai Peninsula that are now black spruce taiga were herbaceous tundra. As the climate warmed during the early Holocene (~10,700 BP), these areas succeeded to willow-alder shrub tundra. Fire-return intervals averaged 138 years (SD 65) during this type shift. As climate warming continued during the mid-Holocene (~5,000 years BP), Populus spp. and white spruce gained dominance, and fire-return intervals shortened (x̄=77 years (SD 49)). Black spruce became dominant during the Little Ice Age (~1,000 years BP), and fire-return intervals lengthened (x̄=130 years (SD 66)) during this period of climate cooling [2,52].

Other analyses of mid-Holocene sediment charcoal and fossils—on the Kenai Peninsula [83], northwestern Alaska Range [51,85], and central Brooks Range [15,47]—found fire-return intervals increased when black spruce dominance increased with climate cooling. On the central Brooks Range, birch shrubs dominated during the early Holocene, and fire-return intervals ranged from 119 to 170 years [47]. Black spruce increased during the mid-Holocene, and fire-return intervals increased significantly (P>0.1) [15,47], ranging from 131 to 238 years in forest-tundra transition zones [47].

Type and severity
Black spruce types have mostly stand-replacement fires [13,29,110]; crowning [110] is concurrent with burning of surface and ground layers [21,58,81,103,113]. Lethal surface fire occurs alone on some sites [39,42,45,103] but is less common than a mix of surface and crown fire. Fires spread easily and uniformly through crowns in closed black spruce forests [3,81,86,103]. Intermittent or passive crown fires occur in lichen woodlands and other open black spruce communities [55,89]; these may cooccur [1,38] with lethal surface fires [39]. Active crown fires usually occur in closed black spruce forests [55,89]. Typically, mosses and/or lichens carry the surface fire, with crowning occurring behind the fire front [89] (see Figure 3). Consumption of the organic soil layer (ground fire) typically accompanies both crown and lethal surface fires [58,81,103,113]. Fires in late summer—especially those occurring in dry years or with hot winds—may burn down to mineral soil. However, most ground fires are patchy, leaving some organic material [29,103]. In black spruce bogs, which generally have deep layers of soil organic material, ground fires may burn for weeks, months, or even years [113]. While leading an expedition to Denali in the early 1900s, Dunn (1907 cited in [99]) wrote "We struggled among the ponds, crossed a river, and toiled through burned forest, where smoldering fire gnawed the moss, and black bark scaled from the spruces as if by disease." In black spruce forests near Palmer, a firefighter reported that "fires sometimes smoldered for weeks or even months in the deep moss beneath the surface before reemerging during dry weather" (Demming 1947cited in [99]).

Figure 3. Advancing surface and crown fire in a black spruce-hardwood stand. Photo used with permission of Wildlandfire.com.

Fires are typically intense enough to kill the black spruce overstory and top-kill or kill understory shrubs [12,29]. The tendency of black spruce to layer increases continuity of vertical fuels. Black spruce branches—which are typically draped with flammable lichens [80,81]—carry fire to crowns. An open, flammable understory of ericaceous shrubs can also carry fire to black spruce crowns [110]. Dyrness and others [32,106] reported that in open-canopy black spruce communities with feather mosses and lichens, a shrub layer is not critical to continuity of ladder fuels; fire easily travels from the moss-lichen ground layer to black spruce canopies.

Generally, lowland forests occupy wet to mesic, broad valleys or river terraces, while upland forests occupy north-facing or low-gradient slopes [23]. Depth of the soil organic layer helps regulate ground temperature, permafrost formation, and in part, site drainage [61]; in turn, this helps regulate severity of surface and ground fires. Many fires in wet or permafrost black spruce types may have little effect on the forest floor and may not substantially reduce the soil organic layer [61,105]. After the fires of 2004 and 2005 in interior Alaska, postfire organic layers were deeper on poorly drained plots and those that burned early in the fire season (late June) than on moderately drained plots (P<0.001) and plots that burned late in the fire season (late August, P=0.01) [61].

Stand-replacing surface fires or passive crown fires may occur if ladder fuels are sparse [103] or discontinuous. Hanson [42] found that all black spruce trees were killed following a "low-intensity" fire in an open black spruce-tamarack community in interior Alaska. (The top 2 to 4 inches (5-10 cm) of the 6- to 14-inch (15-35 cm) soil organic layer was consumed.) The Taylor Complex Wildfire, which burned in the summer and fall of 2004, produced a mosaic of unburned patches and patches that burned from low to high severity. Satellite imagery of the 1,181,337-acre (478,274 ha) burn—taken in the first few postfire weeks—showed that more than half the mosaic was composed of severely burned patches. The prefire plant community was a mixed forest of white spruce, black spruce, quaking aspen, and paper birch on a moist site. Black spruce died even where fire severity was low [76].

These FEIS Research Project Summaries describe behavior of crown, surface, and ground fires on Washington Creek Fire Study and Training Area north of Fairbanks. These prescribed fires produced mosaics [32,107] that were similar to patterns observed in nearby areas burned by wildfires [107]:

• Research Project Summary: The effects of experimental fires in an Alaskan black spruce/feather moss community
• Research Project Summary: Forest floor and plant responses to experimental fires in an Alaskan black spruce/feather moss community

In a FROSTFIRE experiment on the Caribou-Poker Creeks Research Watershed near Fairbanks, a prescribed fire burned at mixed severity over 3 days (8-10 July 1999), with extreme fire behavior in the canopy. The fire burned all strata (ground, surface, and crown). The initial surface and crown fire settled to a smoldering, patchy ground fire on the night of 9 July. Northeasterly winds increased on the morning of 10 July, fanning smoldering fuels in the lowest part of the valley. From there, the fire spread up a ridge, torching black spruces. Repeated images of flames bursting from the fire's core to upslope black spruce crowns were captured on video. Upslope winds created by these bursts were 10 times stronger than ambient environmental winds, and played a "powerful role" in propagating crown fire and increasing rate of fire spread. After this run, a ground fire smoldered until 15 July, when rains extinguished it [21]. About one-third of a 2,600-acre (1,050 ha) area burned; most was black spruce forest. Wet areas and areas dominated by quaking aspen and paper birch did not burn [21,49]. Conservative estimates of fire conditions in the black spruce stands were [21]:

maximum updraft speed: 96 to 198 feet (32-60 m)/s
maximum downdraft speed: 60 to 100 feet (18-30 m)/s
maximum horizontal wind speed: 40 to 92 feet (12-28 m)/s
maximum windspeed parallel to ridgeslope: 92 to 132 feet (28-40 m)/s
mean spread rate: 2.0 feet (0.6 m)/s
mean spread rate on fire flank: 2.48 to 3.66 feet (0.75-1.11 m)/s
mean spread rate over any 10-hour period: 4.16 feet (1.26 m)/s

Fire spread was greater than 0.6 m/s and still accelerating when the fire reached the end of a 150-m² plot. Winds created by the fire moved upslope parallel to the ground. Fingers of flame in the canopy accelerated to 165 to 200 feet (50-60 m)/s. This fire behavior was attributed to internal forces produced by the fire (pressure gradients, advection, and buoyancy) [21].

Pattern and size
Fires tend to burn irregularly across Alaskan taiga landscapes, creating patches typical of mosaic fires. Site characteristics, plant community composition, and weather and climate affect burn patterns. Fires that are only partially lethal often occur across landscapes where black spruce stands intermix with less flammable, mesic hardwood stands and/or wet tundra. These landscapes often experience mixes of surface and stand-replacement fires that leave unburned to severely burned patches (for example, [26,76]). Generally, upland black spruce communities burn more evenly than lowland black spruce communities. Fires often burn unevenly in lowland black spruce bogs, leaving unburned stringers of black spruce [80]. The FROSTFIRE prescribed fire on the Caribou-Poker Creeks Research Watershed left a mosaic of stand-replacement burns in black spruce, unburned hardwood stands [49], and probably less severe burns in between.

Fire sizes in black spruce communities range from small to very large [23]. Average size of burns in interior Alaskan taiga is estimated at 2.3 to 3.9 miles² (6.1-10.1 km²) [6], but most fires are smaller. Barney and Stocks [6] reported that from the 1940s to the 1970s, 60% to 80% of fires in Alaska were <12 acres (5 ha), with a few large fires responsible for most of the total area burned [6,94] (range of large fires: 10,000-19,000 miles² (26,000-50,000 km²)) [6]. In interior Alaska, lightning ignitions generally burned ≤10 acres (4 ha), even though about half of all ignitions occurred in boreal black spruce/feather moss or boreal black spruce/heathland, the most flammable vegetation types in the region [25]. "Ecologically important" fires generally burn ≥100 miles² (250 km²) [110]. Fire records for interior Alaska show a pattern of increasing frequency of large fires (≥1% of total land surface burned) from the 1960s to the 1990s [60]. This trend is discussed further in Contemporary Changes in Fire Regimes.

Summer weather [33] and climate patterns largely determine fire sizes in Alaskan taiga. Alaskan fire records for 1950 through the 1990s show that most large fires occur late in the season during extreme fire years. In a handful of such years (1940, 1941, 1957, 1969, 1977), most fires were >125,000 acres (50,000 ha), and 33% were >250,000 acres (100,000 ha) [63]. Between-year variation in fire size correlates closely with the strength and phase of the Pacific Decadal Oscillation, suggesting a connection with global-scale climatic patterns [30,36,37].

Fire exclusion: Fire suppression in Alaskan taiga began in 1940 [6,33]. Estimates of area burned annually in Alaska before 1940 range from about 1 to 6 million acres (0.4-2 million ha) [33]. In 1983, Gabriel and Tande [41] concluded that there were insufficient data to estimate either the "natural" fire regime of Alaskan taiga or the effect of fire exclusion on the natural fire regime. However, they suggested that most of Alaskan taiga was still in a "wilderness fire" regime, with fire exclusion having little effect [41]. A 1992 to 2000 study across interior Alaska found that in the 17% of the landscape designated for fire suppression, there was a 50% reduction in area burned relative to areas not designated for fire suppression. However, there were 50 times more fires in the suppression area, and the fire season began 2 months earlier. Most fires were human-ignited and small (<1 acre (0.4 ha)). Human ignitions were "of negligible importance" for fires >100 acres (40 ha). Most of the landscape studied was classified as closed-canopy black spruce forest within the boreal spruce fuel model [26].

Although the number of wildfires has increased with human population increases, human-caused wildfires tend to be small. For example, fire occurrences increased during the Klondike Gold Rush at the turn of the 19th century and during construction of the Alaskan Pipeline in the 1970s. These increases were mostly from human-ignited wildfires, and fire sizes were small compared to lightning-ignited fires. From 1970 to 1978, human-caused fires averaged about 106,000 acres (43,000 ha)/year across Alaska, while lightning-caused fires averaged 6.2 million acres (2.5 million ha)/year [6]. Across interior Alaska, human-ignited fires accounted for <7% of total area burned from 1980 to 1999 [60]. Most fires near settlements were suppressed [16].

Fire records show a general increase in fire frequency in Alaska from 1940 to the 2000s [40]. In 1990 and 1991, 1,562 wildfires burned a total of 5 million acres (2 million ha) of forest: about 3.4% of the total forest area of 140 million acres (56 million ha). Ninety percent of these wildfires were <1,000 acres (400 ha), but these small fires represented only 2.5% of total area burned. The 12 largest fires accounted for >50% of total area burned, with the largest fire covering nearly 500,000 acres (200,000 ha). Most fires were located in interior, boreal regions of Alaska [40]. In summer 2004, 6.7 million acres (2.7 million ha) burned between the Alaska and Brooks ranges [50]. This was the largest area burned in interior Alaska since fire records were started in the 1940s [50,99].

Figure 4. Crowning fire in a black spruce stand during the lightning-ignited, 2004 Taylor Complex Wildfire in southeastern interior Alaska. Photo used with permission of Wildlandfire.com.

Climate change: Climate warming has led to an increase in fire sizes in Alaska taiga [19,20,43]. In the early 2000s, there was a shift towards larger, more frequent fires compared to the last half of the 20th century [8]. Large fires in 2004, 2005, and 2009 burned 14.1 million acres (5.7 million ha), or 5%, of interior Alaska [11,31].

In the 2000s, increases in mean annual air temperatures and fire sizes continued in Alaska despite a return of the Pacific Decadal Oscillation to its negative phase [20]. Westerling and others [114] suggest these increases were largely due to climate variation, not fuel buildup as a result of fire exclusion. With changes in the Pacific Decadal and other global oscillations—particularly the El Niño-Southern Oscillation—large fire years may become more common in Alaskan taiga, with increased fire sizes and frequencies [36,46], changes in species composition, and decreased forest ages [36]. Since 1940, 15 out of 17 of the largest fire years in Alaska occurred during El Niño episodes. Those 15 large-fire years accounted for almost 63% of total area burned over 58 years [46].

Researchers on the FROSTFIRE project predict regional warming in Alaskan black spruce communities will result in higher-severity fires, with increased consumption of biomass, especially in soil organic layers. However, this effect may be only temporary in southern boreal regions. Increased fire severity is predicted to favor hardwood regeneration at the expense of black spruce regeneration, which could switch the forest type from black spruce to less flammable hardwood-black spruce forest or hardwood forest [97]. Using data and modeling, many researchers report that severe fires in interior Alaska promote hardwood trees over black spruce and other conifers [10,11,56,65]. Beck and others [10,11] predict that because hardwood forests are less flammable than black spruce forests, the combination of climate warming and a shift to less frequent, less severe fires may lead to a biome shift in some areas of interior Alaska, with black spruce dominance shifting to that of quaking aspen, alders, birches, and white spruce. Their predicted effect of climate warming is consistent with vegetation changes during climate cooling during the Little Ice Age (1500-1800 AD). In the Copper River Basin, black spruce die-back coincided with Ice-Age climate cooling. The cooler climate favored alder and birch shrubs at the expense of black spruce, white spruce, and tree-sized birches [98].

Models predict that with climate warming, black spruce-lichen woodlands near arctic treeline will expand northward at the expense of high-latitude hardwood stands [95] and tundra (e.g., [5,7,19]). Such a shift may increase forest flammability and shorten fire-return intervals. A model of treeline dynamics in interior Alaska predicted that fire sizes will increase and fires become more frequent with increasing abundance of black spruce. The fire-return interval for hardwood forests "decreased considerably" when black spruce was added to the model [95].

Permafrost is vulnerable to climate warming and attendant increased fire frequency, decreased organic soil thickness [90], and decreased soil albedo [34,77,100]. In boreal regions, increases in depth of the active layer above permafrost favors establishment of hardwood species at the expense of black spruce [19,50,59]. As fire severity and/or frequency increase in areas currently underlain with permafrost, models predict a shift from spruce to hardwood-spruce or hardwood forests due to permafrost melt [11,17,56,90]. Barrett and others [8] predict that sites with >4 inches (10 cm) of organic soil remaining after fire will succeed to black spruce stands, with permafrost eventually re-forming. Sites with 1 to 4 inches (3-10 cm) of organic soil remaining will succeed to mixed hardwood-black spruce stands, with either deep, active layers of warmer soil or permafrost taking longer to recover previous thickness. They predict that sites with <1 inch of organic soil remaining will succeed to hardwood stands and lack permafrost [8].

With warming temperatures across Alaskan landscapes, taiga forests may become a carbon source rather than the carbon sink they were historically [5,16,18,91,118]. The Yukon River Basin experienced both climate warming and increased wildfire frequencies between 1960 and 2006. Modeling of fire regime-climate change interactions suggests that the area has become a carbon source rather than the carbon sink of prior decades, with most carbon losses from mineral soil horizons. The authors concluded that climate warming and wildfires contributed approximately equally to carbon loss, with climate change-wildfire interactions responsible for about 28 million tons (25 million tonnes) of carbon reduction [118]. However, some boreal forest sites will likely remain carbon sinks due to regional variation in climate and substrate moisture [18]. This may occur when open wetlands encroach into black spruce woodlands. On the Tanana River floodplain, a paleobotanical study indicated that around 600 years ago, there was a shift from black spruce woodland to a sedge-dominated wetland. Around 1970, the sedge wetland succeeded to a sphagnum peatland. This 2nd shift coincided with increases in the growing-season temperatures recorded in nearby Fairbanks. In 2001, a wildfire occurred during a period of permafrost collapse, resulting in further warming of the substrate and expansion of 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 [87].

Sites where ground fire has removed much of or the entire soil organic layer have shown poor postfire development of insulating moss layers, so permafrost re-formation is compromised. In turn, this reduces the water-holding capacity of the forest floor [14,59]. As such sites become warmer and drier, quaking aspen and birches may replace black spruce successionally [14,50,59]. Johnstone (2007 cited in [14]) noted that "In a warmer climate we're going to get more fires and they'll be more severe". Lowland sites underlain with gravel are most vulnerable to loss of permafrost [19]. Following the 2004 fires in interior Alaska, sites that experienced high-severity fire (burned to mineral soil) showed significant losses in moss abundance and diversity, particularly sphagnum mosses. Reduction of moss layers may result in long-term loss of ground insulation, and could increase flammability of such sites as the substrate dries [50]. Permafrost on some poorly drained lowlands shows resilience to melting because the moss layer tends to thicken as postfire succession proceeds. Thus thermal insulation is restored, the soil becomes increasingly cold, and permafrost regains thickness [19].

In the current climate, black spruces on many upland and permafrost sites are already drought-stressed during the growing season. Todd and Jewkes [99] predict a "warmer climate would exacerbate this problem, resulting in drier needles and branches, leading to more frequent and intense fires. It could become too warm for spruce and other species, such as pine, or—if it is very dry—grasses could replace some of the forested stands" [99]. On the Caribou-Poker Creeks Research Watershed, wildfires in 1890 and 1924 may have initiated degradation of permafrost, from which the site had not yet recovered in 1999. Researchers speculated that if forest fires in black spruce and other sites with permafrost initiate widespread degradation of permafrost, substantial changes to boreal forest ecosystems will follow [49]. On a 21-year-old burn in the Bonanza Creek Long Term Ecological Research Site near Fairbanks, white spruce established in higher numbers than black spruce. The site was in a white spruce-black spruce transitional zone. Before the fire, black spruce dominated lowland, permafrost sites and white spruce dominated warmer, upland sites. The active layer increased on permafrost sites after the fire. The authors surmised that black spruce will regain dominance on its former sites if the permafrost layer regains its prefire thickness. However, they suggested that white spruce will dominate the entire landscape if permafrost continues to degrade with climate warming [115].

Variations in postfire successional pathways are discussed in detail in the FEIS species review of black spruce. The Workbook of potential successional trajectories in burned stands of black spruce in interior Alaska helps predict patterns of postfire plant community composition based on prefire vegetation, soil moisture, and fire weather. It includes a section on predicting succession in unburned black spruce forests and a key for identifying categories of soil moisture [59].

Since Alaskan fire records only date back to the 1940s, information on Alaska's fire history is incomplete. The lack of long-term fire records makes it difficult to compare current and past fire regimes [110]. Since most fires in black spruce communities are stand-replacing, dendrochronological records (fire- scarred trees) are few. Paleobotanical studies are constrained by small sample sizes (see Paleological fire frequencies, Table 2).

Information on fire frequencies, types, severities, and other components of fire regimes is most easily applied to on-the-ground management when the communities being researched are well-described. Many past studies described vegetation types and physical characteristics of study sites incompletely. Incomplete information is difficult for managers to apply and difficult to use in models. It is important for research reports to describe stand structure (forest, woodland, dwarf scrub, or stringer); overstory codominants; shrub and groundlayer dominants; soil moisture, permafrost presence/absence; slope; relative position (lowland or upland); and elevation. Information on fire and other disturbance history is also helpful. Fortunately, there is substantial interest in studying the fire ecology of black spruce communities (for example, [49,97]). Studies that fully describe plant communities and study sites will contribute to refinement of information on the range fire-return intervals, patterns of burning, and other aspects of fire regimes in black spruce communities.

Too often, descriptions of fire behavior and type below the crown have been lacking. For black spruce communities, the fire type is sometimes reported as "crown fire" (for example, [101]), without reference to surface and ground components that typically accompany crown fires [32,38,107] (see Figure 3). Lethal surface fires also occur on sites with sparse understories (see Type and severity), but frequencies and sizes of surface fires are not well documented. More information is needed on the importance of surface fires in Alaskan black spruce ecosystems, and better documentation is needed of the complex fire behavior typical of Alaskan black spruce communities.

Patterns of postfire succession in black spruce communities are strongly driven by how deeply fires burn into organic soil layers. Therefore, fire severity in black spruce ecosystems is best measured by how much of the soil organic layer was removed (Hollingsworth 2008 cited in [14]). The term "severe" has not been applied consistently to fires in black spruce communities and thus, can be misleading [14]. Black spruce mortality is not a reliable indicator of fire severity [112] because the black spruce overstory is usually killed (for example, [88,92]) even when fire severity is low at ground and surface levels [88]. Ryan and Noste [96] stated that in black spruce communities, "the crown fire phase of a wildfire involves primarily the combustion of fine fuels. It devastates the overstory" but may have only slight effects on understory vegetation and organic soil layers [96]. If fire severity is low to moderate below the crown, black spruce communities may show no postfire changes in species composition [14].

Considerations for LANDFIRE: LANDFIRE's use of "mixed-severity" fire has been inconsistent [9]. According to their current definition, mixed-severity fires causes 26% to 75% kill or top-kill of the upper canopy layer [75]. Since even low-severity fires result in nearly 100% mortality of black spruce canopy [62,79,81], that definition cannot correctly be applied to black spruce communities.

Based on effects to the canopy, LANDFIRE models predicted a blend of mixed-severity (some canopy trees survive) and stand-replacement fires in boreal mesic forest and subboreal wet-mesic slope woodland black spruce BpS groups, which places those groups in Fire Group III (see Table 1). Although mixed-severity fire occurs in Alaskan black spruce communities [35], it is rare [62,79,81] and usually occurs only on forest ectones or near unburned patches [57,103]. Model results predicting both mixed and stand-replacement fires were not well-supported in the literature, and reassignment to Fire Group IV seems warranted for these 2 groups. Although some authors suggested that mixed-severity fires might occur in Alaskan black spruce communities [35,103], to date (2013), only one mixed-severity fire in Alaskan black spruce communities was documented in scientific literature [62]. Another study documented mixed-severity fires in jack pine-black spruce communities of west-central Québec, although most of the landscape (88%) experienced stand-replacement fires [53]. Studies in Alaska and northwestern Canada have consistently found nearly 100% mortality of the black spruce overstory, even when 100% crowning did not occur [4,38,79,80,81,99]. For example, in north-central Alberta, Kiil [64] reported that although 100% of black spruces died as a result of an 18 July prescribed fire, only about 54% of black spruce trees torched. Therefore, LANDFIRE's modeled results of 19% to 54% mixed-severity fire in black spruce communities (Table 1) may be inaccurate.

LANDFIRE predicted the portions of crown, surface, and mixed-severity fires likely in black spruce BpSs but did not include ground fire. Since ground fires are an integral fire type in black spruce ecosystems, incorporating their effects would model succession in black spruce communities and assign fire regime groups more accurately.

For fire managers, fire-return interval ranges are at least as useful as MFRIs; however, the ranges predicted by LANDFIRE are higher than those predicted in the literature to date (2013). According to LANDFIRE models for Alaskan black spruce, fire-return intervals range from a minimum of 112 years for stand-replacement fires in boreal mesic black spruce forests [70] to a maximum of 1,430 years for "mixed-severity" fires in subboreal mesic black spruce forests [69]. LANDFIRE's modeled ranges can be adjusted, based on on fire-return interval ranges (43-210 years) and other data presented in Table 2.

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