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Fire regimes of Northern Rocky Mountain ponderosa pine communities


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Hundreds of articles are published about wildland fires in Northern Rocky Mountain ponderosa pine communities. For this Fire Regime Synthesis, FEIS staff reviewed over 300 publications on historical and contemporary fuel loads, stand structure, and fire regimes in ponderosa pine communities. Most studies found that prior to fire exclusion, low- to moderate-severity surface fires typically burned every 6 to 13 years, and stand-replacement surface or crown fires were less frequent. Since fire exclusion, many ponderosa pine communities are denser and multistoried, dominated by Douglas-fir, and have higher fuels loads than in presettlement times. Managing for resilience in most fire-excluded ponderosa pine communities requires reducing fuel loads, restoring historical stand structure, and returning frequent fire to the landscape.

SUMMARY

This section summarizes fire regime information available in the scientific literature as of 2016. Details and documentation of source materials follow this summary.

Ponderosa pine dominates low-elevation savannas, woodlands, and forests of the Northern Rocky Mountains. Bunchgrasses and shrubs dominate the ground layer and understory. Douglas-fir often codominates the overstory, especially at midelevations and in late succession. Historically, ponderosa pine communities were a mosaic of open stands, denser patches of young conifers, and areas without trees. Frequent fire maintained ponderosa pine as the seral dominant in many Douglas-fir habitat types.

Prior to fire exclusion in the early 1930s, fuels were mostly herbaceous species and litter. Fires were ignited by lightning and American Indians. Lightning fires occurred in summer, while Indians set fires in spring and fall.

The historical fire regime of ponderosa pine communities in the Northern Rocky Mountains was mostly frequent, low- to moderate-severity surface fires that burned in a mosaic pattern. Stand-replacement surface and crown fires were an infrequent but important part of the fire regime.

Results from fire history studies show mean fire-return intervals in ponderosa pine habitat types ranged from 6 years to 31 years, with ranges of 6 to 13 years reported most often. In Douglas-fir habitat types where ponderosa pine is the seral dominant, mean fire-return intervals historically ranged from 6 to 50 years, with 10- to 15-year fire-return intervals reported most often. Fire frequency and severity increased with slope and decreased with elevation, soil moisture, and northerly aspect. Therefore, low-severity fires were most common at low elevations and on southern aspects, while moderate-severity fires were most common at midelevations and on relatively moist, steep slopes. Two paleo studies found stand-replacement fires in the Northern Rocky Mountains were most frequent during periods of global warming in the early Holocene. Table A2 summarizes results of fire history studies reviewed for this Fire Regime Synthesis.

Fire sizes were variable in presettlement ponderosa pine communities. They ranged from 15 to ~7,000 acres (mean ~1,000 acres) in northeastern Washington and from 5 to ~129,000 acres (mean ~2,800 acres) in the Selway-Bitterroot Wilderness. Large, stand-replacement fires occurred when fire weather was severe. Over decades, large fires accounted for most area burned.

Fuel loads, particularly ladder fuels, have increased with fire exclusion and resultant advancing succession. Many ponderosa pine communities have become multistoried, with Douglas-firs and firs in the understory. Forest patches have become denser and young stands larger than in presettlement forests. Cheatgrass has invaded some ponderosa pine communities, increasing surface fuel continuity.

In many ponderosa pine communities of the Northern Rocky Mountains, fire exclusion has resulted in longer fire-return intervals and more severe fire than what occurred historically. Although mean area burned annually decreased in ponderosa pine-Douglas-fir communities in the late 20th century, the incidence of large fires that occur during severe fire weather has increased in the 21st century. Large, severe fires threaten the integrity of ponderosa pine ecosystems. Size and frequency of severe fires are likely to continue to increase with climate change.

Table A1 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 Northern Rocky Mountain ponderosa communities is [104]:

Table 1—Modeled fire intervals and severities in Northern Rocky Mountain ponderosa communities [104].
Fire intervala
(years)
Fire severityb (% of fires)
Number of Biophysical Settings (BpSs) in each fire regime group
  Replacement Mixed Low I II III IV V NAc
13-21 4-7 19-34 59-76 8 0 0 0 0 0
aAverage historical fire-return interval derived from LANDFIRE succession modeling (labeled "MFRI" in LANDFIRE).
bPercentage 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% [27,103].
cNA (not applicable) refers to BpS models that did not include fire in simulations.


Citation:
Fryer, Janet L. 2016. Fire regimes of Northern Rocky Mountain ponderosa pine 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.fed.us/database/feis/fire_regimes/Northern_RM_ponderosa_pine/all.html [].
INTRODUCTION

This Fire Regime Synthesis brings together information from two sources: the scientific literature as of 2016, and the Biophysical Settings (BpS) models and associated Fire Regime Data Products developed by LANDFIRE, which are based on literature, local data, and expert estimates. This synthesis:

It provides information on fire regimes of ponderosa pine habitat types and Douglas-fir habitat types where ponderosa pine was historically the seral dominant. The following reviews on fire regimes of ponderosa pine ecosystems are cited throughout this synthesis: [4,8,9,23,47,96].

In this synthesis, "presettlement" refers to the period before fire suppression became effective and natural fire regimes were still functioning. That date varies with study site, but most researchers target the early 1930s as the period when fire exclusion began in the Northern Rocky Mountains (e.g., [31,45,112,113]).

Common names are used throughout this Fire Regime Synthesis. Two varieties of ponderosa pine grow in the Northern Rocky Mountains: Columbia ponderosa pine and Rocky Mountain ponderosa pine occur on the western and eastern slopes of the Northern Rocky Mountains, respectively. Herein, both are referred to as "ponderosa pine". "Douglas-fir" refers to Rocky Mountain Douglas-fir, and "lodgepole pine" refers to Rocky Mountain lodgepole pine. See table A3 for a complete list of common and scientific names of plant species discussed in this synthesis and links to FEIS Species Reviews.

The following FEIS publications cover ponderosa pine ecosystems in regions adjacent to the Northern Rocky Mountains:

The Northern Rocky Mountain montane mixed-conifer report provides fire regime information on Douglas-fir-ponderosa pine-western larch-lodgepole pine habitat types, which occur at higher elevations than the habitat types discussed in this synthesis.
DISTRIBUTION AND PLANT COMMUNITIES

The Northern Rocky Mountain floristic zone extends south of Jasper National Park, Alberta, to Yellowstone National Park in southern Montana and northern Wyoming. Ponderosa pine cover types occupy an estimated 7 million acres (3 million ha) in northeastern Washington, Idaho, Montana, and western Wyoming [174]. They are sparse to absent in southwestern and south-central Montana [131,179]. Figure 1 shows the range of ponderosa pine ecosystems covered in this synthesis.

Figure 1—Land cover distribution of Northern Rocky Mountain ponderosa communities based on the LANDFIRE Biophysical Settings (BpS) data layer [104]. Numbers indicate LANDFIRE map zones. Click on the map for a larger image and zoom in to see details.

Vegetation in ponderosa pine habitat types is usually limited by water availability. Ponderosa pine communities are often subject to drought [87,153], although soil moisture regimes across ponderosa pine's distribution in the Northern Rocky Mountains range from moist to xeric [145,153]. In this region, mean annual precipitation increases from east to west and south to north [47,149,153]. Ponderosa pines tend to receive more precipitation and occur at higher elevations on the western slopes of the Northern Rocky Mountains than on the eastern slopes [47].

Ponderosa pine stands grow at low elevations (~2,600-5,400 feet (790-1,600 m)) [47,153] in all soil textures [162] and aspects. The soils are derived from a variety of parent materials, including granite, basalt, sedimentary rock, and glacial till [87]. Ponderosa pine series in the Northern Rocky Mountains are most common on dry soils and south- and west-facing slopes [52,173]. The ponderosa pine zone lies between the mountain big sagebrush-mountain grassland and Douglas-fir zones [47,153]. The Douglas-fir series replaces the ponderosa pine series at midelevations [131]. In 1917, Weaver [173] noted that prior to the fire of 1910, the town of St Maries, Idaho, was surrounded by ponderosa pine on south-facing slopes and exposed rocky sites and by Douglas-fir on more mesic sites.

Ponderosa pine has many characteristics that make it fire resistant, including deep roots; thick, relatively inflammable bark; thick scales that protect leaf and stem buds; high foliar moisture content; self-pruned or fire-killed lower branches; light to moderate lichen growth; and an open branch, crown, and stand habit [9,60,90,118,174]. In the Northern Rocky Mountains, only western larch is more fire-resistant [60,155]. Ponderosa pines in riparian settings may be more vulnerable to fire damage than trees on uplands. Their bark tends to be thinner, and dense riparian stands with ladder fuels are subject to severe fire [77,78].

Diversity in ponderosa pine communities increases with latitude and soil moisture [49,77]. Moist Douglas-fir-ponderosa pine forests in extreme northern Idaho are generally more diverse than ponderosa pine forests in drier subregions of the Northern Rocky Mountains. Western hemlock and western redcedar are frequent associates in northern Idaho [145], but not in drier areas. Fire type and pattern also affect diversity. Malanson [115] suggests that compositional and structural diversity is greatest in ponderosa pine communities with a history of mixed-severity fire across large portions of the landscape.

At its lowest elevations, ponderosa pine sometimes establishes in Palouse prairie [173]. In eastern Washington and northern Idaho, the transition from Palouse prairie or mountain big sagebrush steppe to mixed-conifer forest is relatively abrupt [52]. A narrow belt of ponderosa pine savanna rims the steppe-forest ecotone [52,173]. Ponderosa pine habitat types are uncommon on the Colville National Forest of northeastern Washington; they are most prevalent at elevations below the national forest boundary. These low-elevation ponderosa pine habitats are generally open, and they are too dry to support most other conifers. At higher elevations in eastern Washington, ponderosa pine is a common seral species in warm, dry Douglas-fir/bluebunch wheatgrass habitat types [177]. In extreme northern Idaho, where soil moisture is relatively plentiful at low elevations, ponderosa pine communities occur only on the driest sites [49].

Habitat Types

Ponderosa pine habitat types predominate on dry, low-elevation sites. Historically, Douglas-fir dominated draws, stream bottoms, and some north- and east-facing slopes. Ponderosa pine became increasingly dominant with decreasing moisture and elevation [78,143,151].

Ponderosa pine is not limited to dry sites, however; it is a seral dominant in several moist habitat types. On the Lick Creek Demonstration Area on the Bitterroot National Forest, Montana, ponderosa pine historically dominated not only relatively dry Douglas-fir/pinegrass habitat types, but moist Douglas-fir/big huckleberry and Douglas-fir/dwarf bilberry habitat types as well [11]. In eastern Washington and northern Idaho, ponderosa pine is sometimes a seral dominant in moist western redcedar series [102]. In western Montana, it is a seral dominant in low-elevation quaking aspen [159] as well as Rocky Mountain Douglas-fir [11] habitat types.

Ponderosa pine is sometimes the successional climax species in balsam poplar [78], narrowleaf cottonwood [78], water birch [122], and other riparian communities.

Ponderosa pine formations in the Northern Rocky Mountains are:

Douglas-fir formations in which ponderosa pine is the seral dominant are:

Historical Stand Structure and Succession

Stand structure
Ponderosa pine communities were historically mostly open, but stand density varied across the landscape. In many locations, ponderosa pines grew in parklike, open-grown stands. Stand density increases with increasing soil moisture and elevation [13,151]. In the Boise Basin of central Idaho, habitat type and, to a lesser extent, topographic position, were the two most important factors determining stand density, composition, and tree size of low- to midelevation (4,400-6,800 feet (1,300-2,000 m)) forests [61].

Ponderosa pine forests in the Northern Rocky Mountains were historically patchy [90]. Small crown fires created openings in what were generally low- to moderate-density stands [139]. Ponderosa pine stands with short fire-return intervals typically form a mosaic of widely spaced trees, clumps of young conifers, and openings without trees. The relative abundance of these three elements varies widely across time and space [106]. Intermittently shallow soils help maintain open sites that have widely scattered or no trees [90].

Historically, surface fires maintained a variety of stand ages in ponderosa pine communities [9,17,18,31,32,131], from young to old-growth classes [9]. Stand reconstructions show sites on the Bitterroot and Lolo national forests had mostly uneven-aged structure from the 1600s through the 1800s, while sites on the Flathead National Forest were mostly even-aged. Old-growth ponderosa pines were up to 500 years old. Even on new, mostly stand-replacement burns with young trees, there were usually a few old ponderosa pines that survived past fires and bark beetle attacks [17].

Studies of presettlement forests in Montana and Idaho document dominance of ponderosa pine in Douglas-fir habitat types. On the eastern slope of the Bitterroot Mountains, basal area of low-elevation forests (4,500-5,800 feet (1,400-1,800 m)) was about 52% ponderosa pine and 19% Douglas-fir before 1900 [79]. On plots across western Montana, historical stand density in ponderosa pine stands and Douglas-fir stands where ponderosa pine is the seral dominant varied from 47 to 100 trees/acre (118-250 trees/ha) [17]. Before fire exclusion, ponderosa pine was the seral dominant in Douglas-fir habitat types in the Boise Basin of Idaho. Stands were generally most open in draw bottoms and in Douglas-fir/mallow ninebark habitat types (table 2). These forests usually had small clumps (<2.3 acres (1 ha)) of young trees, patches of dense trees, and open areas [61].

Table 2—Estimated stand density (feet²/acre) of presettlement (prior to the 1862 Gold Rush) Douglas-fir habitat types in the Boise Basin. Most trees were ponderosa pine, although some were Douglas-fir. Data are means (SE) [61].
Topographic position Douglas-fir/mallow ninebark Douglas-fir/white spirea Douglas-fir/common snowberry
Draw bottom 57.5 (9.1) 60.0 (13.7) 94.5 (10.1)
Midslope 66.7 (7.5) 124.4 (10.9) 93.8 (10.0)
Ridgetop 76.0 (9.0) 97.5 (11.6) 127.7 (11.4)

Ponderosa pine needs an open stand structure to reproduce and maintain dominance. Arno et al. [17] suggest that a basal area of no more than 40 to 60 feet²/acre is required for natural regeneration of ponderosa pine. Results from fire history studies show that stands on the Lick Creek Demonstration Area were historically dominated by old-growth ponderosa pines ranging from 200 to 400 years old. Distance to the next tree averaged >30 feet (9 m), and density ranged from 30 to 40 trees/acre (75-100 trees/ha). About 10% of old growth was Douglas-fir; it grew mostly on north-facing slopes and in swales. Old-growth ponderosa pines were interspersed with scattered mature individuals and patches of ponderosa pine and Douglas-fir seedlings and saplings [13]. Some studies [17,58] and General Land Office records [74] show pre-1900s basal areas of ponderosa pine stands in western Montana averaged <100 feet²/acre (23 m²/ha). On the Bannock Creek Research Natural Area in the Boise Basin of Idaho, large ponderosa pines dominated Douglas-fir habitat types before the 1900s. Estimated density in 1850 ranged from 6 large trees/acre (15/ha, basal area = 61 feet²/acre (14 m²/ha)) in plots with a pinegrass understory to 83 large trees/acre (205/ha, basal area = 19 feet²/acre (4 m²/ha)) in plots with an elk sedge understory [151].

In 1901, Pattee Canyon near Missoula, Montana, ponderosa pine/bluebunch wheatgrass savannas occupied low-elevation, south-facing slopes, while Douglas-fir/mallow ninebark forests occupied north-facing slopes. Based on General Land Office records, Habeck [74] estimated mean density of these stands in 1901 at 13.3 trees/acre (32.8/ha), and mean basal area at 36 feet²/acre (8.2 m²/ha). Stumps from old-growth ponderosa pines logged in the early 1990s averaged 22 inches (57 cm) in diameter [73,74].

Some of the largest ponderosa pines in the Northern Rocky Mountains grow in low-elevation, open savannas in Glacier National Park [15,40,71] and the Bob Marshall Wilderness [94,156]. Many of these giant trees range from 20 to 30 inches (51-76 cm) DBH, with some approaching 48 inches (122 cm) DBH [71,75]. In the absence of traditional American Indian fires, Douglas-firs and western white spruces have established in the understory of these ponderosa pine savannas in Glacier National Park [40,71,75], while western larches and lodgepole pines have established in the Bob Marshall Wilderness [105].

Succession
Wildfire, insects, disease, and wind are the principal natural disturbances in ponderosa pine communities [43,87,153]. Historically, these disturbances maintained a full range of successional stages, with seral stages predominating [87]. Hann et al. [76] estimated that most (65%) ponderosa pine and other dry forests of the Northern Rocky Mountains were historically in early to midseral stages of succession (table 3).

Table 3—Historical (1850-1900) and 1991 percent distributions of successional stages for dry forests of the Northern Rocky Mountains [76].
Successional stage Historical (%) 1991 (%) Change (%)
Grass/forb/shrub 18 1 -17
Early-seral (shade) intolerants 15 14 -1
Early-seral tolerant 3 3 0
Midseral intolerant 21 35 +14
Midseral tolerant 8 22 +14
Late-seral, intolerant single story 21 5 -16
Late-seral, tolerant single story 2 3 +1
Late-seral, intolerant multistory 9 8 -1
Late-seral, tolerant multistory 3 9 +6

Throughout its wide distribution in the West, ponderosa pine historically maintained its dominance in Douglas-fir habitat types because of frequent fires [9,49]. Postfire succession tends to advance more quickly on moist Douglas-fir habitat types than on drier sites [145].


HISTORICAL FUELS AND FIRE REGIMES

Historical Fuels

Prior to fire exclusion, fuels in low-elevation ponderosa pine ecosystems were mostly herbaceous species and forest floor litter [9]. Grasses in intermountain valleys were dry in summer and fall, so fires sometimes spread between mountain ranges [10]. Estimates of historical fuel loads in ponderosa pine forest of the Northern Rocky Mountains range from 1 to 4 tons/acre (2.2-9.0 t/ha) [9].

Historical Fire Regimes

Ponderosa pine communities historically had a mixed-severity fire regime of frequent, low- to moderate-severity surface fires and less frequent, stand-replacement surface or crown fires [2,62]. Surface fires would often crown in or burn severely enough to kill trees in dense thickets, creating a mosaic of burn severities on the landscape. Fire histories based on fire scars document that prior to European-American settlement (1890s to early 1900s), surface fires in ponderosa pine habitat types returned every 6 to 13 years on average [2,55,139]. In Douglas-fir habitat types where ponderosa pine was the seral dominant, low- and moderate-severity surface fires returned every 20 to 30 years on average [5,6,17,31,33,157,161]. Under these fire regimes, mature ponderosa pines survived low-severity surface and many moderate-severity surface fires. Most conifer seedlings and saplings were killed [9], although in mosaic fires some young conifer patches and scattered young conifers either survived fire or were missed. Crown fires were usually passive or limited to dense thickets [139]. Stand-replacement fires occurred when fire weather was extreme [4,85,86]. Table A2 summarizes fire-return intervals for ponderosa pine communities in the Northern Rocky Mountains.

Figure 2—Although of low severity in this stand, the 2016 Roaring Lion Wildfire killed most saplings in the thicket in the foreground. U.S. Forest Service photo by Janet Fryer.

Smith [152] placed ponderosa pine habitat types of northern Idaho in Fire Groups One or Two. These Fire Groups have mostly low-severity surface fires, with some moderate-severity surface and stand-replacement fires [152].

Table 4—Fire Groups of ponderosa pine habitat types in northern Idaho [152].
Fire Group One (warm, dry)
ponderosa pine/bluebunch wheatgrass
ponderosa pine/Idaho fescue
ponderosa pine/common snowberry
Fire Group Two (warm, mesic to dry)
ponderosa pine/mallow ninebark

American Indian Use of Fire
American Indians regularly burned ponderosa pine sites in and near areas where they lived [28,37,48]. They intentionally burned in fall or early spring, when cured grasses ignited easily and fire severity was likely to remain low. According to American Indian elder and European-American settler accounts, Indians set fires for multiple objectives: to improve forage for game and horses, increase hunting access in forests, promote production of berries and other food plants, for communication, and to reduce the chances of crown fire near villages [28,38,109]. Old-growth ponderosa pine stands in the Bob Marshall Wilderness were likely maintained by frequent surface fires [156] set by the Salish, Kootenai, and Blackfeet [94,128]. West of the Continental Divide, the Salish and Kootenai set fires in spring and fall, when weather and fuel conditions were conducive to low-severity fires [28,29]. Elders report that the Salish set fires in the Ninemile Valley west of Missoula every fall, before the tribe moved to their winter village in Missoula Valley [38]. The Kootenai set fall fires in the Swan Valley. Settlers reported summer fires near Indian settlements, although the summer fires might have been unintentional [28].

American Indians apparently set fires near their villages and hunting grounds more often than in remote locations. A western Montana study compared presettlement (1612-1861) fire frequency of ponderosa pine stands in Salish villages to fire frequency of stands in remote areas. The study found fires were significantly more frequent in 10 "heavy use" (village) ponderosa pine stands on the edge of the Bitterroot Valley, Montana, than in 10 "remote" ponderosa pine stands in canyons several miles upland (<7- vs. 15- to 20-year intervals, P = 0.05). By 1910—the end of the settlement period (1861-1910)—the differences between heavy-use and remote areas were no longer significant, with fire-return intervals averaging 35 years for both areas. The authors concluded "Euro-Americans reversed untold centuries of frequent Indian and lightning-caused fires in grasslands and dry ponderosa pine forests" [38]. Based on fire scars from 120 individual trees, Barrett [28] reported similar differences in heavy-use vs. remote ponderosa pine-Douglas-fir stands in sites across western Montana (table 5) [28].

Table 5—Mean fire-return intervals (range (SD)) of heavily used and remote ponderosa pine-Douglas-fir stands in western Montana, for 3 times [28].
Time period   
1695-1860
(presettlement)
1861-1910
(settlement)
1911-1980
(postsettlement)
Heavy use (n = 10) 9.1 years
(2-35 years (2))
10.8 years
(1-35 years (6))
25.9 years
(4-69 years (7))
Remote (n = 10) 18.9 years
(3-64 years (9))
15.0 years
(4-31 years (5))
26.9 years
(2-69 years (11))

Because records of Indian-set fires date to a time when tribes were decimated by disease and/or moved from their native territories [4], the presettlement impact of American Indians on fire regimes of ponderosa pine communities will likely remain uncertain.

The sections below provide discussions and documentation of historical fire regimes in ponderosa pine communities of the Northern Rocky Mountains. Table A2 summarizes results of these studies.

Fire Ignition
Lightning and people ignite fires in ponderosa pine communities [65]. In remote locations, lightning might have been the only historical source of ignition [1,8]. Ignitions from American Indians were probably common near villages and camps [8] (see American Indian Use of Fire). Gruell [68] compiled historical narratives of fires in the Inland West from 1776 (the Lewis and Clark Expedition) to 1900. Of 145 fire reports, 60 (41%) were attributed to American Indians, 78 (54%) made no mention of the ignition source, and 7 (5%) were attributed to European-Americans [68].

Although acreage burned from lightning ignitions has changed since presettlement times, rates of lightning ignition probably have not. The Rocky Mountains receive an average of about 0.5 to 3.0 strikes/km²/year, a low rate compared to the Midwest. In the Northern Rocky Mountains, strike rates generally increase with elevation and to the south [20,21,125]. However, higher strike rates to the south do not always result in higher ignition rates. From 1970 to 2002, the number of lightning-ignited fires averaged 8 lightning fires/million acres (4000,000 ha) on the Gallatin and Beaverhead national forests and 65 lightning fires/million acres on the more northerly Bitterroot and Nez Perce national forests (data from the National Interagency Fire Center cited in [91]).

Thunderstorms occur throughout spring and summer in the Northern Rocky Mountains, peaking in early summer [174]. Dry thunderstorms are most common in late summer and early fall [140], with strikes that result in ignitions most frequent in summer [174]. Many lightning fires in the Northern Rocky Mountains burn down to ponderosa pine communities from higher-elevation forests [117].

Even though lightning strikes are more common at higher elevations, dry fuels in ponderosa pine stands support higher ignition rates than in higher-elevation, moister forest types. On the Lolo National Forest, ponderosa pine cover types had the highest density of lightning-ignited fires among all forest cover types, averaging 476.5 fires/million acres/year from 1960 to 1974. Mean elevation for these wildfires was 4,000 feet (1,200 m). Western larch cover types had the second highest density of lightning-ignited fire, averaging 201.3 fires/million acres/year [44].

Fire Season
Summer is the main fire season in the Northern Rocky Mountains [174]. Most wildfires occur in July and August [41], when fuels are dry and lightning strikes most frequent [174]. Although total annual precipitation is higher on the western than the eastern slope of the Continental Divide, the western slope receives less precipitation during the fire season [41], and lightning strikes tend to occur later in the fire season on the western slope. From 1931 to 1945 on the western slope of the Divide, most lightning strikes resulting in ignition occurred August. On the eastern slope, most strikes resulting in ignitions occurred in July [41].

Fire scars document that past fires occurred mostly in the growing season. Near Sheafman Creek on the Bitterroot National Forest, 51% of fire scars occurred in ponderosa pine latewood and 23% in earlywood. The remaining scars (26%) occurred when the cambium was dormant (late fall-early spring). Fire scars dated from 1571 to 1910 [83]. On the Sawmill Creek Research Natural Area of the Bitterroot National Forest, season of fire scarring for seven mature ponderosa pines was nearly evenly divided between the growing season (earlywood and latewood, 12 scars) and the dormant season (earlywood and latewood boundaries, 9 scars) [63]. American Indians set fires early and late in the fire season in many low-elevation locations [29,38].

Fire Frequency
Fire scars show that fires historically occurred at frequent intervals in ponderosa pine communities of the Northern Rocky Mountains. Mean fire-return intervals reported from fire history studies in ponderosa pine habitat types ranged from 6 years in a ponderosa pine/Idaho fescue habitat in the Bitterroot Valley [7,16] to 31 years at Sawmill Creek on the Lolo National Forest [17]. Means of 6 to 13 years were reported most often [7,8,16,17,35,83]. Fire frequency tends to increase with slope and decrease with elevation, soil moisture, and northerly aspect [30,47,142]. Fires were historically least frequent on north-facing slopes and in moist Douglas-fir habitat types such as Douglas-fir/mallow ninebark [30]. On the Flathead National Forest, fire-return intervals averaged 21 years for ponderosa pine habitat types and 30 years for Douglas-fir habitat types [33]. In the Boise Basin, they ranged from 10 to 13 years for ponderosa pine habitat types and 16 to 22 years for Douglas-fir habitat types [157].

Historical frequencies of moderate and severe fires are largely unknown. Because fire history in ponderosa pine ecosystems is largely based on fire scars, it difficult to determine the frequency of moderate- and high-severity fires: Such fires burn up some or all of the overstory, leaving incomplete or no fire-scar evidence of their passing [166]. Table A2 summarizes fire-return interval means and ranges of study sites in the Northern Rocky Mountains where ponderosa pine was historically dominant. These studies reported that fire-return intervals of 10 to 15 years were historically most common in those communities (figure 3).

Figure 3—Frequency distribution of composite fire intervals of fire study sites listed in Table A2.1. This chart includes only those sites for which composite fire intervals were determined.

Trends in Fire Frequency: Fire-return intervals tend to lengthen as warm ponderosa pine habitat types transition to cool Douglas-fir habitat types [131]. Studies across the Northern Rocky Mountain illustrate this trend. Arno [8] conducted fire history studies in ponderosa pine and Douglas-fir habitat types in the Bitterroot Valley and on the Lolo National Forest. He found that fire-return intervals in ponderosa pine habitat types averaged 6 years on dry, south-facing slopes and 12 years on mesic, north-facing slopes. In Douglas-fir habitat types where ponderosa pine is the seral dominant, fire-return intervals averaged 13 years on the north end of the valley and 26 years on the south end (Arno unpublished data cited in [8]). He emphasized that variability in fire-return intervals is, at least in part, due to patchiness of the vegetation on the landscape, local terrain, and surrounding habitats [8]. In northern Idaho and western Montana, mean fire-return intervals ranged from 6 years in dry ponderosa pine types to 40 years in moist ponderosa pine-Douglas-fir types [16,49]. On some sites, frequent surface fires maintained open ponderosa pine forests for several centuries; on others, fires crowned where longer fire-return intervals had led to heavy fuel loads [8,153].

Figure 4—Locations of fire history studies from Table A2.1. Click on the map for a larger image and zoom in to see details.

A fire history study in the Boise Basin found low-elevation, dry ponderosa pine sites historically burned about every 10 to 13 years, while moist, midelevation ponderosa pine-Douglas-fir sites burned about every 16 to 22 years (table 6, Table A2.1). Indian-set fires were probably historically common in the area; the gentle terrain of the basin makes for good campsites. Low-elevation fires likely spread to steeper terrain upslope. Fire-return intervals increased with elevation and across dry to moist habitat types, with a couple of exceptions. The authors suggested that fire spread less often than expected into Stands 4 and 5 because they were adjacent to moister sites. Although Stand 4 was at relatively high elevation, it was directly upslope from a stream terrace. Stand 5 was the driest of the habitat types, but it was surrounded by "much wetter sites" [157].

Table 6—Mean fire-return intervals of stands on a habitat-type moisture gradient in in Boise Basin of west-central Idaho [157].
Stand number Elevation (feet) Habitat type (from driest to moistest) Mean fire-return interval (years) during 1700-1895
5 5,625 ponderosa pine/mountain snowberry 11.4
3 5,000 Douglas-fir/elk sedge 9.8
1 4,600 Douglas-fir/white spirea 10.3
4 4,975 Douglas-fir/white spirea 18.1
6 4,850 Douglas-fir/mallow ninebark (dry) 12.8
2 5,820 Douglas-fir/mallow ninebark (mesic) 15.9
7 5,600 Douglas-fir/mallow ninebark (moist) 21.7

A study in the Quartzite Planning Area of northeastern Washington found ponderosa pine-Douglas-fir stands historically burned very frequently on dry sites. The study separated fire histories into the presettlement (1671-1885) and settlement (1860-1920) period. Fire frequency and size "changed abruptly" after 1920 (table A2.1). On average, a fire burned every 2 to 3 years somewhere in the 7,669-acre (3,104-ha) study area; fire-return intervals averaged 8 years for the presettlement period and 6 years for the settlement period. For six of seven study sites, presettlement fire-return intervals averaged about 7 years. The seventh study site was in a mesic area and had a longer mean fire-return interval (11.3 years). Historically, the mesic study site may have had more mixed-severity fires, while the other study sites had mostly low-severity fires [146]. Simulation models suggest that fire-return intervals of <20 years are needed to retain ponderosa pine dominance in the area [93].

Stands on cold sites probably had long fire-return intervals relative to other ponderosa pine stands. A ponderosa pine-Douglas-fir community near Fales Flat Campground southwest of Darby, Montana, is near the cold limits of ponderosa pine. From the 1500s to 1900, mean fire-return interval for the site was 50 years [17], which is longer than typical for ponderosa pine stands in the Bitterroot Valley [9]. Fire history literature [17] and modeling [93] suggest that 50 years is near the maximum limit for ponderosa pine to maintain dominance in areas where it cooccurs with Douglas-fir [17]. However, Steele and Geier-Hayes [158] suggest that in central Idaho, wildfire every 50 to 100 years historically maintained ponderosa pine dominance in moist to mesic Douglas-fir/mallow ninebark habitat types [158].

Based on fire history studies he conducted across the Northern Rocky Mountains, Barrett [36] estimated that warm, dry ponderosa pine forests had mean fire-return intervals of 10 to 25 years (n = 137 plots) from about 1900 to 1935. These stands had mostly frequent, nonlethal surface fires. Dry ponderosa pine-Douglas-fir forests stands had both frequent, nonlethal surface fires and moderate-interval, moderate-severity fires, with mean fire-return intervals ranging from 20 to 40 years (n = 117 plots). Overstory mortality was generally low: <20% in dry ponderosa pine stands and ≤30% in mesic ponderosa pine-Douglas-fir stands [36].

Before fire exclusion began in the area in the early 1930s, fires in ponderosa pine communities were frequent both inside and outside of the River of No Return Wilderness. Barrett's [30,31] fire history studies in east-central Idaho compared fire histories on two study sites inside the wilderness (Salmon River Breaks corridor; Chamberlain and Disappointment creeks) and on two sites outside the wilderness (Colson Creek, Camp Creek). Before fire exclusion began in the area in the early 1930s, fire-return intervals were similar on all slopes in the Salmon River Breaks corridor of the wilderness (mean = 20 years) and outside the wilderness at Colson Creek (mean = 14 years). Stands at both sites were uneven-aged. Sample trees inside the wilderness had fewer fire scars than trees outside; Barrett attributed this to the higher elevations sampled in the wilderness [30,31]. Pre-1930s fire-return intervals in the Chamberlain and Disappointment creek drainages of the wilderness were slightly longer (mean = 22 years) than those on other sites [31] (Table A2.1).

Inside the wilderness in the Salmon River Breaks corridor, fires during 1702 to 1934 were less frequent on north-facing than south-facing slopes (means of 30.9 vs. 13.9 years, respectively), even in canyon bottoms. Fires were most frequent on open, dry sites; "virtually all" of those sites had surface fires [31]. Ponderosa pine dominated both north- and south-facing slopes before 1935. On north- and east-facing slopes, stand-replacement fire sometimes occurred after one or more surface fires [32]. Some Douglas-firs were >150 years old, leading Barrett to suggest that fire-rotation intervals for stand-replacement fire within the corridor may exceed 100 years [30].

Outside the wilderness at Camp Creek, fire-return intervals from 1623 to 1948 averaged 15 years and were shortest in dry sites. Thirteen stands in the driest, low-elevation ponderosa pine stands had historical mean fire-return intervals of 12 years (range: 1-20 years; n = 13 stands), and the fires were of low severity. In mesic, higher-elevation ponderosa pine-Douglas-fir stands, mean fire-return interval was 22 years (range: 20-30 years; n = 4 stands), and the fires were of both low and moderate severity. Ponderosa pine-Douglas-fir stands were mostly in riparian areas and on north-facing slopes [35].

Paleo studies: Pierce et al. [133] suggest that fluctuations in the frequency of stand-replacement fires in ponderosa pine forests of Idaho are within the range of variability over long time scales (100s to 1,000s of years). Charcoal accumulations in alluvial fan deposits [132] and lake sediment cores [135] may span thousands of years, providing information on changes in fire activity during shifts in climate and vegetation [135]. These proxies of long-term fire records indicate that summer-dry regions of the Northern Rocky Mountains exhibited highest fire activity during the very early Holocene (11,000-7,000 BP) and the Medieval Warm Period (~1,000 years BP). Drought conditions during those times were more severe than at present. However, determining Paleolithic fire regimes of ponderosa pine and other dry-site forests is difficult because dry forests with frequent fires tend to produce indiscreet charcoal peaks [176]. Pierce et al. [133] estimated a range of 154 to 286 years for fire-return intervals of ponderosa pine communities in central Idaho (Table A2.2).

Charcoal deposits in alluvial fans indicate that there were frequent, low- to moderate-severity fires during relatively cold periods of the early to mid-Holocene (6,800-7,400 and ~5,000 BP) in ponderosa pine forests in central Idaho. There were stand-replacement fires and attendant large debris flows during warm periods with severe drought. The most recent periods of frequent, low- and moderate-severity fires occurred from approximately 1,200 to 1,500 years BP, and during the Little Ice Age from about 1450 to 1800 AD. Severe fires that resulted in large debris flows were apparently infrequent in ponderosa pine stands and were associated with a period of mutidecadal drought interspersed with unusually wet intervals. From 24% to 27% of total charcoal deposits occurred from nine debris flows that occurred approximately 1,000 to 800 years BP [132,133].

Lake charcoal sediment studies in Idaho, Montana, and Wyoming show that fire regimes in the Northern Rocky Mountains have been strongly governed by regional differences in summer moisture and by centennial- and millennial-scale variability in climate (see Climate Change). During the early Holocene, extreme summer drought in sites that are presently summer-dry (e.g., Hoodoo Lake, Idaho) corresponded with a period of "protracted" fires. During the same time span, fires were infrequent in sites that are now summer-wet (e.g., Baker Lake, Montana) [176]. The study did not address differences in fire regimes by plant community. Sediment records from Foy Lake in the Flathead Valley of western Montana show open ponderosa pine and Douglas-fir forests were prevalent during a period of regional warming (11,500-9,000 BP), with Douglas-fir and firs increasing during a period of regional cooling (7,500-2,200 BP). Fire activity was high during the warm period and lower than at present during the cold period [135].

Fire Type, Severity, and Intensity
Ponderosa pine ecosystems of the Northern Rocky Mountains historically had mosaic surface fires of mostly low to moderate severity, with some crowning in dense, young conifer patches [4,10,36]. High-severity fires were less common [4,10,23,85,86,137]. Based on fire history studies he conducted in ponderosa pine stands across the Northern Rocky Mountains, Barrett [36] estimated overstory mortality from wildfire was <20% before fire exclusion began (1900-1935, depending on location). Low- to moderate-severity fires (Fire Regime Group I) were characteristic of ponderosa pine communities on relatively dry sites [153]. Low-severity surface fires were most common in low-elevation areas with relatively flat topography and an overstory of large, widely spaced ponderosa pines [90]. Fires tended to transition to mixed and moderate severity with increasing elevation, although local variation was common [90].

Moderate-severity fires were historically common in ponderosa pine stands on relatively moist, steep slopes [9]. Fire severity generally increases with slope [140] due to a "chimney effect" based on landform [165]; convective heat transfer to upslope fuels is most efficient on steep slopes [3]. Across the Northern Rocky Mountains, moderate-severity fires were most common in the northern and eastern portions of ponderosa pine's distribution [90], especially east of the Continental Divide [9].

Moderate- and high-severity fires were a historically important component of low-elevation conifer systems of the Northern Rocky Mountains, although large, severe fires were infrequent [9,86]. Moderate-severity fires were apparently common on relatively moist Douglas-fir-ponderosa pine forests of northeastern Washington [145]. On the Flathead Lake Eastshore Analysis Area near Bigfork, Montana, Douglas-fir/mallow ninebark habitat types on warm, moist slopes had both low-severity surface and mixed-severity, mosaic fires, while ponderosa pine-Douglas-fir habitat types on dry, south-facing slopes had mostly frequent surface fires (Table A2.1) [33]. LANDFIRE data (cited in Hutto et al. [86]) show that about 85% of forested land in the West, including ponderosa pine communities of the Northern Rocky Mountains, had a mixed-severity fire regime. Hutto et al. [86] define mixed-severity fire as including "proportions of low-, moderate, and high-severity (lethal to more than 70% of all trees) fire that vary widely across vegetation types and biophysical settings".

Infrequent, stand-replacement fires occur in drought years, and they play a key role in the long-term fire regime of ponderosa pine communities [4,85,86]. Since the relative proportions of low-, moderate-, and high-severity fire vary spatiotemporally, some suggest that fire regimes of ponderosa pine in the Northern Rocky Mountains are best viewed as a continuum, with moderate and severe fire integral parts of the fire regime [2,86]. Stephens et al. report "all fires have high-severity effects, where most of the trees are killed, at some spatial scale and patch size" [163].

Large fire size does not mean 100% stand replacement within the fire perimeter [145]. Typically, large burns have unburned patches and patches burned by low-, moderate, and high-severity fire [27,86,103,145]. In the Upper Swan Valley of Montana, fires large enough to burn at least 30% of a 6,000-acre (2,000-ha) study area historically (1600-1919) occurred an average of every 46 years in ponderosa pine and mixed-conifer forests. However, most of these fires were of low or mixed severity [34]. On the Colville National Forest, a Douglas-fir-ponderosa pine stand near the confluence of two streams had been buffered from several fires, including a severe, extensive fire in 1831. For 3 centuries (1600s-1800s), most fires were not stand-replacement across a 50,192-acre (20,312-ha) watershed, which included ponderosa pine and higher-elevation, mixed-conifer communities [145].

In a 1900 publication describing forests in what are now the Idaho Panhandle and Bitterroot national forests, Leiberg [110] noted that ponderosa pine forests near Sandpoint, Idaho, showed evidence of past mixed-severity fires. He also alluded to stand-replacement fires: "Where they are burned over, red fir (Douglas-fir) and western yellow (ponderosa) pine are the first trees in the reforesting process". He noted "the destruction has involved a great deal of timber" [110].

There is no evidence that a "true" stand-replacement, crown fire regime, such as that associated with lodgepole pine, historically occurred in Northern Rocky Mountain ponderosa pine ecosystems [90,96]. Patches of severe fire typically occurred within a mixed-severity fire mosaic, with patches of less severe fire and unburned patches. Uneven age structure suggests dominance by low-severity fire [24] or long periods without fire.

Figure 5—The 2016 Roaring Lion Wildfire on the Bitterroot National Forest was a mixed-severity, mosaic fire.
Figure 6—A patch of the Roaring Lion Fire that burned at low and moderate severity.
Figure 7—A patch of the Roaring Lion Fire that burned at high severity.
U.S. Forest Service photos by Janet Fryer.

The few publications that discuss fire intensity in ponderosa pine ecosystems describe intensity as mostly low [2,4], and none provided quantitative measures. Fire intensity varies spatiotemporally and by vegetation type in ponderosa pine communities. Dry, low-elevation ponderosa pine woodlands tend to have shorter flame lengths than moist, midelevation Douglas-fir forests where ponderosa pine is seral [2,59]. When historical fire regimes were still functioning in ponderosa pine communities, the range of energy that accumulated as undecomposed litter before the next fire averaged about 20,000 to 29,000 kJ/m² [1,172], an insufficient amount of energy to support intense surface fires [1].

Fire Pattern and Size
Climate, local weather, topography, and vegetation determine sizes and spatial patterns of fires at landscape scales [81,140]. Historically, mosaic fires in ponderosa pine communities produced patterns of unburned to severely burned areas across the landscape (see Fire Type, Severity, and Intensity). Analysis of fire patterns in the Forest Service's Northern Region from 1931 to 1945 found that most fires (>35%) occurred on gentle slopes of <20%. However, large fires (≥100 acres (40 ha)) were most common on steep slopes [41]. Barrett et al. [39] estimated that across 200 million acres (80 million ha) spanning all terrestrial habitat types in the interior Northwest, an average of 5.9 million acres (2.4 million ha) burned annually before 1900.

Small fires were apparently most common in ponderosa pine communities of the Northern Rocky Mountains, although documentation of historical fire sizes is sparse [62]. Although less common, large fires accounted for most acreage burned. In and near the Frank Church-River of No Return Wilderness, 59 fires burned from 1707 to 1919. Eleven were >1,000 acres (400 ha) [30,31]. Historical fire sizes apparently varied widely in the 7,669-acre (3,104-ha) Quartzite Mountain area of northeastern Washington. Estimated mean fire size in presettlement ponderosa pine forests was 1,076 acres (435 ha), but estimated sizes ranged from a 15-acre (6-ha) fire in 1879 to a 7,271-acre (2,942-ha) fire that burned about 95% of the area in 1882. The authors stated that actual fire sizes were "probably somewhat larger" than their estimates [146], but some small fires that did not cause scarring may have gone undetected [23,56]. A study in the Deep South Watershed of northeastern Washington found fire sizes ranged from 28 to 15,000 acres (3-6,100 ha) in the presettlement period (1683-1860), averaging 520 acres (210 ha) [145].

Fire sizes generally decreased during European-American settlement. Livestock grazing during the mid- to late 1800s removed many of the fine fuels, irrigation and land development broke up fuel continuity, and American Indian-set fires were lessened or curtailed [10]. In the Deep South Watershed, mean fire size dropped from 520 acres (210 ha) in the presettlement period (1683-1860) to 337 acres (140 ha) in the settlement period (1861-1910) [145]. However, areas near mining towns sometimes had large fires. In 1904, Leiberg [111] reported that 47% of the Little Belt Mountains Forest Reserve, Montana, burned over during 1864 to 1874. The reserve was 238,170 acres (96,380 ha), 111,600 acres (45,160 ha) of which burned. He concluded "more ground has been burned over by the occupancy of the region by the white man than by the last three generations of Indians" [111]. The policy of fire exclusion was initiated in the early 1930s in the Northern Rocky Mountains [10] (see Contemporary Changes in Stand Structure, Fuels, and Fire Regimes).

A landscape-level study across the Selway-Bitterroot Wilderness found that a few large fires accounted for most area burned over a century. All but one of the fires occurred before fire exclusion began in the wilderness. The landscape was composed of ponderosa pine, Douglas-fir, lodgepole pine, and subalpine fir-Engelmann spruce forests. Six fires (1889, 1910, 1919, 1929, 1934, 1988) accounted for 72% of the total area that burned from 1880 to 1994 in a 1.94 million-acre (785,090-ha) study area. Mean fire size was 2,849 acres (1,153 ha), ranging from 5 acres (2 ha) to 129,050 acres (52,223 ha, the 1910 Moose Creek Fire). Median fire size was 334 acres (135 ha) [142]. Twenty percent of the 42,078 acres (103,973 ha) that burned during 1880 to 1935 reburned at least once [141] (table 7). In the 1900s, fires were most frequent in the ponderosa pine-Douglas-fir zone (1,970-5,770 feet (600-1,760 m)). Douglas-fir habitat types—which included ponderosa pine forests, Douglas-fir forests, and shrubfields—burned more frequently than other habitat types [142]. Data were based on fire atlases (archived fire reports and operational fire perimeter maps) [141,142].

Table 7—Fire-rotation intervals, area burned, and area reburned in the Selway-Bitterroot Wilderness during 3 times. Ponderosa pine, Douglas-fir, lodgepole pine, and subalpine fir-Engelmann spruce forests were included in the analyses [141,142].
Time period    
1880-1935
(presettlement)
1936-1974
(fire exclusion)
1975-1996
(wildland fire use)
Fire-rotation intervala (years) 92-110 3,206-3,888 218-271
Total area burned (ha) 482,030 9,622 58,427
Total area reburnedb (ha) 75,502 29 3,546
Percent of total study area burned 60.9 1.2 7.4
Percent of total study area reburned 20 0.5 4
aFire-rotation intervals were calculated based on extent of fires across 785,090 ha.
bReburned area = total area that burned more than once.

Large, mixed-severity fires occurred near the South Fork of the Salmon River from the 1700s to the 1900s. On the Salmon National Forest at Colson Creek, there were 11 fires >1,000 acres (440 ha) from 1707 to 1919. Three of the fires—in 1863, 1880, and 1919—spread over nearly all of the 3,000-acre (1,000-ha) study area, scarring 70% to 85% of sample trees. Fires scarred more trees in 1919 than in other years at both Colson Creek and in the Salmon River Breaks corridor, suggesting that in 1919, fires were widespread within and beyond the two study areas. The 1919 fires were stand replacing on north slopes, although earlier fires on north slopes were of mixed severity [30,31].

Ponderosa pine communities may experience large fires during periods of severe fire weather. Fire atlases for the Selway-Bitterroot Wilderness show that years with large fires are drier than other years (P < 0.01) [140]. Extended drought and high winds can spread fire across a broad spectrum of fuel loads and stand structures, including open ponderosa pine stands [24,64]. Rapid increases and changes in wind speed often result from jet streams or cold fronts [21]. Cold fronts in spring and fall, often accompanied with downsloping Chinook winds, may lead to high-severity fires that spread rapidly in ponderosa pine stands [24].

Large fires typically occur under extreme fire weather conditions. For example, the 1910 wildfires in northern Idaho and western Montana burned across all forest types, consuming over 3.5 million acres (1.4 million ha). Most of that area (2.65 million acres (1.07 million ha)) burned on 20 and 21 August. Fires spread 30 to 35 miles (50-55 km) in 36 hours, driven by gale-force winds from the southwest [53,169,178]. The "Big Burn" of 1910 likely started, crowned, and spread from ponderosa pine into higher-elevation communities. Egan [54] reports that "wind took the hot floor of the simmering forest and threw it into the air, where it lit the boughs of bigger ponderosas and white pines, which snapped off and also rode the force of upward acceleration" [54]. Although there were 1,736 fires that summer, most area burned was due to 50 fires. Nine of those burns probably exceeded 200,000 acres (81,000 ha), with 10 fires accounting for 66% (2.3 million acres (0.93 million ha) of total area burned [53,169,178]. The Big Burn is thought to be the largest fire in recorded U.S. history, and it shaped fire policy and fire management from that time until today [53,136]. Wildfires were widespread across the West in 1910. The spring of 1910 was anomalously warm, and it was followed by a dry summer. The Yellowstone wildfires of 1988—which involved mostly lodgepole pine communities—spread under similar, anomalous severe fire weather [53].

In extreme fire years, large wildfires such as those in 1910 often burn across elevation gradients, into all terrestrial plant communities. From 1908 to 1969, average annual area burned in Washington, northern Idaho, and Montana ranged from 315 acres (127 ha) in 1950 to 2,573,172 acres (1,041,326 ha) in 1910 [174]. Based on field work and map data, Leiberg [109] estimated in 1900 that portions of the 4 million-acre (1.7 million-ha) Bitterroot Forest Reserve (now part of the Bitterroot National Forest) burned four times between 1719 and 1898 [72,109], with total acreage burned in all fires ranging from approximately 75 to 1,600 acres (30-650 ha) [109]. Among conifers, ponderosa pines showed least fire damage from these fires. Leiberg [108] estimated a 1% loss of timber in pure ponderosa pine stands, compared to 4% in ponderosa pine-Douglas-fir stands and 50% in pure Douglas-fir stands.

In wet years, ponderosa pine forests may build up fine fuels [101] and understory woody vegetation [133] that fuel large fires in succeeding dry years. In the Selway-Bitterroot Wilderness, northern Idaho, the 15 largest fires between 1880 and 1995 occurred in summers that were drier than average (P < 0.001), with a wet year occurring 4 years prior to a large fire (P < 0.05) [101].

Burned areas can reduce severity of the next fire or act as fuelbreaks on a landscape. Analyses of geospatial data (Monitoring Trends in Burn Severity) from the Frank Church-River of No Return Wilderness, the Selway-Bitterroot Wilderness, and the Crown of the Continent ecosystem revealed that old burns in ponderosa pine and other forest types serve as fuelbreaks for about 6 to 18 postfire years; however, that function weakens with increasingly severe fire weather. For example, in the Crown of the Continent ecosystem, the probability of a 10-year-old burn limiting spread of a subsequent fire is high under moderate fire weather (P = 0.97) but low under extreme fire weather (P = 0.30) [129].

Large-scale climatic patterns influence fire patterns over time [82,84,120]. Heyerdahl et al. [84,120] developed fire chronologies for 21 ponderosa pine communities in the Northern Rocky Mountains of Idaho and Montana and related them to regional climate patterns (temperature, Palmer Drought Severity Index (PDSI), El Niño-Southern Oscillation (ENSO), and the longer-term Pacific Decadal Oscillation (PDO)) from 1650 to 1900 and from 1988 to 2003. They found that widespread fires in the Northern Rocky Mountains occurred in years when dry summers followed warm springs. PDO was in a negative (weak) phase in the 1600s and 1700s, so it was not a significant driver of fire at that time. However, it is a strong driver of spring climate and regional fire years in modern times. In the 20th century, widespread fires occurred during the positive (strong) phase of the PDO. Eleven of the most extensive fire years occurred from 1900 to 1934 and from 1988 to 2003. [84].

Heyerdahl et al. [84] classified temporal fire patterns in the Northern Rocky Mountains as: 1) no-fire years (0 sites with fires); 2) local fire years (1-4 sites with fire); and 3) regional fire years (≥5 sites with fire). From 1650 to 1900, there were 99 no-fire years and 120 local fire years, when fires burned only a few sites. Regional fire years occurred 32 times during that time, with fires recorded on as many as 10 sites per year. Regional fire years had significantly higher summer temperatures and more drought (based on PDSI), while no-fire years were significantly cooler and wetter. No-fire years tended to coincide with La Niña years when the Northern Rocky Mountains had high snowpacks, late snowmelt, and short fire seasons. ENSO was not significantly related to regional fire years. The authors found no timelag between wet years and fire occurrence (P = 0.01 for all variables) [84], although such timelags have been reported for forests in the Selway-Bitterroot Wilderness [101] and the Southwest [153]. They concluded that spring–summer temperature and moisture are the primary drivers of fire in the Northern Rocky Mountains and that even though ENSO and PDO have some effect on spring climate, climatic conditions conductive to regionally synchronous fires can occur regardless of ENSO and PDO [84].


CONTEMPORARY CHANGES IN STAND STRUCTURE, FUELS, AND FIRE REGIMES

Changes in Stand Structure and Succession

Generally, fire-excluded forests in the ponderosa pine zone have changed from dominance by ponderosa pine and other early-seral, shade-intolerant species (see Stand Structure) to mature, shade-tolerant Douglas-fir and firs. Fire exclusion, coupled with selective harvesting of large ponderosa pines, has led to development of thickets of shrubs and small ponderosa pines, Douglas-firs, and/or firs [10,12,14,15,17,33,57,74,97] (table 3). Increased basal areas in ponderosa pine's drought-susceptible forests probably contribute to epidemics of insects, dwarf-mistletoes, and root diseases, which are becoming widespread in seral ponderosa pine forests [12,17,57,123,164].

Douglas-fir has in particular become increasingly dominant in ponderosa pine-Douglas-fir habitat types since fire suppression became effective in the 1930s. Sometimes succession is so advanced that the crowns of understory conifers have nearly reached the canopy. Growth of old ponderosa pines has slowed, and many are dying [17].

Successional replacement by Douglas-fir is most advanced on warm, moist sites [33]. Ponderosa pine stands on very dry slopes are least affected by advancing succession because drought stress tends to inhibit establishment of other conifers [31,131]. However, dry sites may have scattered conifer thickets, and dry sites are subject to duff and litter buildup in the absence of frequent fire [33]. In the Salmon River Breaks corridor, succession from 1934 to 1984 was to Douglas-fir on north slopes, but ponderosa pine still dominated south-facing slopes [31]. On the South Fork of the Salmon River, successional changes in stand structure and species composition were greatest on mesic, productive sites, while dry-site ponderosa pine stands were still relatively open. Even on open, southerly exposures, however, there were scattered thickets of young Douglas-fir and duff and litter buildup around large trees [35]. In the early 1980s, ponderosa pine still dominated dry, low-elevation slopes and southwest-facing slopes from 5,000 to 5,600 feet (1,500-1,700 m) elevation. However, dense Douglas-fir-ponderosa pine stands occupied most north-facing slopes in 1984, with ladder fuels of young Douglas-fir in the understory. Duff and litter accumulation was greatest on dry, low-elevation slopes; sometimes, it was up to 1 foot (0.3 m) deep [30,31].

The Northern Rocky Mountains are losing old-growth ponderosa pine to shade-tolerant conifers. On the Colville National Forest, the number of large ponderosa pines present in the 1980s had decreased greatly compared to the number present in aerial photographs taken in the 1930s to the 1950s. In the 1930s, a least a few large ponderosa pines were present in all successional stages, including burns in early-seral stages after fires that were largely stand replacement. By the 1980s, most large ponderosa pines had been logged [145]. On the eastern slope of the Bitterroot Mountains, percent basal area of low-elevation forests (4,500-5,800 feet (1,400-1,800 m)) changed from about 52% ponderosa pine and 19% Douglas-fir before 1900 to about 26% ponderosa pine and 55% Douglas-fir in 1995. Data were based on tree cores from four sites (20-40 plots) and from standing or fallen dead trees [79]. On study plots in the Bannock Creek Research Natural Area, ponderosa pine was the only large tree present in the 1850s, while Douglas-fir dominated in 1993. Mean stand density was estimated at 36 trees/acre (90 trees/ha) in 1850 and measured at 175 trees/acre (438 trees/ha) in 1993 [151].

On many sites in the Northern Rocky Mountains, loggers removed most large ponderosa pines in the early 1900s [58]. In 1906, 37 million board feet of timber—mostly ponderosa pine—was harvested on the Bitterroot National Forest. Less than 1% of ponderosa pine forests in western Montana has no history of logging [17]. Arno et al. [15] estimate that <3% of old-growth ponderosa pine forests remain in the West.

Fire-excluded sites that have been logged may be more susceptible to severe wildfire and insect outbreaks than unlogged sites. On 10 sites across central and western Montana and northern Idaho, fire-excluded ponderosa pine/Douglas-fir forests that were logged prior to 1960 had higher stand densities, more homogeneous stand structure, more snags, and more late-seral tree species than paired forests in fire-excluded, unlogged ponderosa pine-Douglas-fir forests (P ≤ 0.10). The authors concluded that "to the extent that modern wildfires are driven by vegetation and fuel characteristics, historically logged stands are more prone to severe-stand-replacing wildfires than unlogged, fire-excluded stands" [124].

Gruell et al. [67] provide photo-point comparisons of succession in ponderosa pine/Douglas-fir stands in Montana during the settlement period (1870s) and in 1982; these photos document increasing density of ponderosa pine stands over time. Photo points include the mouth of the Thompson River, Knowles Creek near Perman [67], the Bitterroot National Forest [69], and a stand near Philipsburg [67]. A similar publication is available for the Boise National Forest [171]. Photo sequences from the 1950s to the 1980s document ponderosa pine invasion into meadows and a mountain big sagebrush community in Adams County, Idaho [168].

Changes in Fuels

Since the early 1900s, changes in stand structure and composition of ponderosa pine communities have resulted in increased fuel loads in general and ladder fuel loads in particular [8]. Photoseries illustrate this fuel buildup (see these publications: [67,69,168,171]). Accumulated fuels support fires of higher severity than what likely occurred historically, with more torching, crowning, and longer periods of burnout [157] (figure 7). Dense, late-seral forests have attendant epidemics of bark beetles, defoliating insects, western and Douglas-fir dwarf-mistletoe, and fungal root diseases [12,33,57,80,96,164,177]. In turn, this results in stand deterioration and accumulations of woody and fine fuels [12,57,177].

Ponderosa pine communities across the Northern Rocky Mountains have become multistoried under fire exclusion [65,95,96,99,114,141,157]. Forest patches have become larger, denser, and more homogeneous [65,95] as shrubs and advanced conifer regeneration establish in ponderosa pine understories [9,35,65,95]. Multiple canopy layers and dense seedling and sapling thickets can become ladder fuels [9]. Mean stand density in ponderosa pine communities of Pattee Canyon, Montana, increased fourfold from 1901 to 1992, from 13.3 to 53.6 trees/acre (32.8-132.5 trees/ha) [74]. Fuel loads that were an estimated 1 to 4 tons/acre (2.2-9.0 t/ha) in presettlement times have increased to 12 to 40 tons/acre (27-90 t/ha) in some ponderosa pine communities [9].

Live and dead fuel biomass increase with advancing succession under fire exclusion [65,95]. Longer fire-return intervals mean live fuels have longer periods to grow and dead fuels longer times to accumulate. Crown fuels increase because late-seral, shade-tolerant tree species such as Douglas-fir and grand fir tend to have more branches and higher leaf areas than ponderosa pine. This biomass tends to be more evenly distributed over the height of the tree than for ponderosa pine, and there is more leaf mass carried in the crown. More evenly-distributed branches, greater crown biomass, and high seedling and sapling densities in the understory allow flames from the surface climb into the canopy and become crown fires. Duff and litter depths generally increase with increasing crown closure and leaf area because of increased needlefall and decreased decomposition on the forest floor [96].

When the historical fire regime was still functioning, frequent fires burned off duff and litter [9,49], but duff and litter have accumulated since fire suppression became effective [9,35]. Duff mounds 6 to 24 inches (15-61 cm) deep can be found under some old-growth ponderosa pines [8]. When they burn, such mounds often girdle and kill trees [8,35]. Arno [9] cautioned that accumulated duff and litter can fuel ground fires in ponderosa pine communities, and that managers "can easily overlook the significance of forest floor fuels; the upper litter (O) and parts of the middle (fermentation) layer provide the highly combustible surface fuels for flaming combustion and extreme fire behavior during severe fire weather. The lower part of the fermentation layer and the humus layer make up the ground fuel that generally burns as glowing combustion" [9].

The forest floor of ponderosa pine stands becomes deeper and less fertile with fire exclusion. On the Lolo and Bitterroot national forests of western Montana, forest floor thickness—as well as Douglas-fir basal area and total shrub cover—increased in ponderosa pine stands with time since fire. Total carbon and phenolic compounds increased and nitrate content decreased in the forest floor with time since fire. The authors suggested that high levels of phenolics in the litter and soil of the late-successional forests might have decreased nitrogen availability [114].

Smoke emissions might have been greater before than after fire exclusion. Based on aerial photographs and expert opinion, Brown et al. [46] estimated that in the Selway-Bitterroot Wilderness, smoke emissions were 1.3 times greater before 1935 than after. About 12% of the wilderness is low-elevation ponderosa pine/Douglas-fir stands [46].

Nonnative invasives
Changes in historical stand structure, increased fire severity, and land development may lead to increased postfire establishment of nonnative invasive plant species. In turn, this may alter fuel characteristics of invaded stands. Cheatgrass has invaded some ponderosa pine/bunchgrass habitats, increasing fine fuel continuity and decreasing forage value for wildlife and livestock [52,70,130]. In the late 1960s, Daubenmire and Daubenmire [52] wrote that the ponderosa pine/Idaho fescue rangelands of eastern Washington and northern Idaho are "seldom managed prudently", with cheatgrass, Dalmatian toadflax, and common St. Johnswort invading and persisting with heavy grazing.

Nonnative invasive plants may increase with level of disturbance. On the Colville National Forest in eastern Washington, nonnative herbaceous species showed small but significant increases in richness and cover on thinned plots (0.5% cover) and plots burned under prescription (0.4% cover) compared to untreated plots (0.3% cover). On plots where thinning and burning treatments were combined, cover of nonnatives averaged 2% and did not exceed 7%. Cover of nonnatives increased with severity of disturbance and time since treatment (P ≤ 0.05 for all variables). Cheatgrass, Kentucky bluegrass, bull thistle, common mullein, and yellow salsify were among the most common nonnatives on study plots [127].

Changes in Fire Regimes

Anthropogenic causes for shifts in fire regimes in ponderosa pine ecosystems include fire exclusion, logging of old growth; climate change, and development in the wildland-urban interface [9,52,89]. Disruption of traditional burning practices of American Indians has also interrupted historical fire regimes [9].

The historical fire regime is probably the most important ecosystem process altered by fire exclusion. Analyses of fire atlases in northern Idaho and western Montana found that from 1974 to 2008, low-elevation, dry forest types such as ponderosa pine burned at the same frequency as colder, higher-elevation forests, with both forests types burning in proportion to their occurrence on the landscape. This was a shift from past fire patterns, when ponderosa pine forests burned more often. Fire-scar reconstructions of fire history on 14 low-elevation dry sites ranged from 4 to 14 fires/century from 1650 to 1900 and 0 to 2 fires/century from 1901 to 2008 [121].

Fire exclusion, insects, and disease interact and degrade ponderosa pine ecosystems [96,116,177]. Increases in insect attacks and disease increase stress and reduce growth of early-seral, fire-dependent species such as ponderosa pine. Increased stress is a direct result of increased competition from late-seral species for light, space, and other resources [92].

Fire frequency has decreased and severity increased in ponderosa pine ecosystems of the Northern Rocky Mountains under fire exclusion [9]. Across the Northern Rocky Mountains, Barrett [36] found mean fire-return intervals in dry ponderosa pine stands had increased from 17 years before fire exclusion to 76 years in the 2000s. In dry ponderosa pine-Douglas-fir stands, mean fire-return intervals had increased from 30 to 100 years [36]. On the South Fork of the Salmon River, fire-return intervals had increased to five times that of historical levels after 50 to 60 years of fire exclusion [35]. On the Flathead Lake Eastshore Analysis Area, fire intervals in ponderosa pine-Douglas-fir stands increased 5-fold, from a mean of 21 years in 1920 to 98 years in 1994. A site near Gunderson Creek had a historical mean fire-return interval of 16 years, but as of 1994, the site had not burned for 115 years. Barrett [33] calls this "the strongest evidence of fire exclusion in the study area today". Fire frequencies in Douglas-fir/mallow ninebark stands decreased 4-fold from 1920 to 1994. As of 1994, an estimated 10 fires had been precluded from ponderosa pine-Douglas-fir stands in the study area, and an estimated 5 or 6 fires precluded from Douglas-fir/mallow ninebark stands [33]. Fires are now so infrequent in many ponderosa pine sites that it is infeasible to calculate contemporary mean fire-return intervals [31,35] (Table A2.1 ).

Other studies confirm increases in fire-return intervals under fire exclusion. Anderson et al. [5,6] found fire-return intervals for Douglas-fir-ponderosa pine habitats in western Montana averaged 15.1 years before 1910, and 49.3 years from 1911 to 1983. In the Quartzite Mountain area of northeastern Washington, contemporary fires were less frequent and fuel continuity much greater than in presettlement times: "In many instances, the Quartzite Planning Area is out of synchrony with historic MFFIs (mean fire-frequency intervals) by a factor of 10. Vegetation is connected horizontally and vertically across the landscape, predisposing this area for fires that are of greater severity than those that occurred during the past several centuries" [146] (Table A2.1 ).

Several fire history studies in wildernesses and national forests illustrate the trend of decreasing fire frequency due to fire exclusion. In and near the Frank Church-River of No Return Wilderness, fire-return intervals in ponderosa pine-Douglas-fir communities were much longer after 1936 (combined mean for wilderness and nonwilderness = 61.9 years, range: 49-94) than before (combined mean = 17.7 years, range: 5-39) [30,31]. Similarly, fire-return intervals on the Bitterroot, Lolo, and Flathead national forests were about twice as long in the early 1990s as those before fire exclusion began [17]. In the Selway-Bitterroot Wilderness, presettlement fire-rotation intervals were shorter than those during either the fire exclusion or the wildland fire use period, but fire-rotation intervals during the wildland fire use period were shorter than those of the fire exclusion period [141] (table 7).

In another Selway-Bitterroot Wilderness study, fire-return intervals in ponderosa pine-Douglas-fir stands in the presettlement period (1528-1934) averaged 22 years compared to 48 years in more recent years (1979-1990). The authors stated that past frequent fires maintained ponderosa pine dominance. In presettlement times, all fires on study plots were nonlethal surface fires, while about 25% of fires from 1979 to 1990 were stand-replacement surface or crown fires [45,46].

Except in extreme fire years (see Fire Pattern and Size and Climate Change), annual mean area burned in many ponderosa pine communities was less in most of the 20th century than in presettlement times [35,41,46]. Across the Selway-Bitterroot Wilderness, there were almost no mapped fires from 1935 to 1979 [142] (table 7). Brown et al. [46] estimated that in the Selway-Bitterroot Wilderness, mean annual area burned in ponderosa pine-Douglas fir and other communities with a nonlethal understory fire regime was 3.7 times greater from the 1500s to 1935 (7,818 acres (3,164 ha)) than from 1979 to 1990 (2,130 acres (862 ha)) [45,46]. In the Quartzite Mountain area of Washington, average number of acres burned was less during the presettlement period (300 acres (120 ha)) than the settlement period (1,076 acres (435 ha)). There was an abrupt lengthening of fire-return intervals and lessening of acreage burned after 1920 [146] (figure 8).

Figure 8—Estimated acreage burned within sampled portions of the Quartzite Mountain area by decade [146].

Woodlands and forests with short fire-return intervals, such as ponderosa pine, are most sensitive to successional changes and fuel buildup due to fire exclusion [12]. Using digitized maps of the Interior Columbia Basin, Morgan et al. [119] compared land cover before 1900 to contemporary land cover. Ponderosa pine cover types had decreased by 23% in the region [119]. A landscape-scale successional model indicates that montane forests that historically had frequent, nonlethal surface fires have shifted from historical reference conditions more than other forest types [180] (table 8).

Table 8—Summary of simulated historical and current mean fire-return intervals (MFRI) and percent of high burn severity for three Biophysical Settings [180].
Dominant species (BpS series) Historical fire regime (frequency/severity) Current fire regime (frequency/severity) Simulated historical MFRI (years) Simulated current MFRI (years) Simulated average historical percent of severe fire (%) Simulated average current percent of severe fire (%)
ponderosa pine-Douglas-fir (10452) frequent/low infrequent/moderate 49 622 26 46
Douglas-fir (11660) frequent/low infrequent/moderate 61 308 22 48
Engelmann spruce-subalpine fir (10560) infrequent/high infrequent/high 167 335 43 67

With fire exclusion and climate change, there has been a trend toward large fires that are more severe than fires in presettlement times. This trend is particularly threatening to ecosystems that evolved under a fire regime dominated by frequent surface fires. Fulé et al. [62] stated that for ponderosa pine throughout its distribution, "uncharacteristic large, high-severity fires pose one of the greatest risks to ecosystem integrity in the 21st century". Across the West, fire records have documented increases in the frequency of large wildfires (>1,000 acres (400 ha)) since the mid-1980s (P < 0.001). From 1987 to 2003, the annual average number of large wildfires was almost four times that from 1970 to 1986, and the annual area burned had increased more than six times [53,175].

Some claim that because fire severity varies in time and space [86,166], present-day fire regimes of ponderosa pine ecosystems in the Northern Rocky Mountains are still within the historical range of variability [23,85,86]. Baker et al. [22,24] suggest that for much of the western United States, current frequencies of stand-replacement fires in ponderosa pine and other dry forests are either within the historical range of variability or longer than the historical range, depending on location. Baker [22] reconstructed historical fire-rotation intervals for ponderosa pine and other dry forests of the western United States based on fire history studies. He then used Monitoring Trends in Burn Severity data from LANDSAT to determine occurrences of severe fire in low-elevation dry forests from 1984 to 2012. For ponderosa pine forests in northwestern Montana and central Idaho, there was a trend of fire-rotation intervals that were within the range of historical variability. For ponderosa pine forests in eastern Washington; northern Idaho; and southwestern, west-central, and central Montana, there was a trend of fire-rotation intervals that were longer than those that occurred historically [22].

Grazing
Although the effects of livestock grazing on fire regimes in ponderosa pine ecosystems in the Southwest are well documented (e.g., [50,51,90,92]), less is known of the effects of livestock grazing on fire regimes in the Northern Rocky Mountains. In general, grazing pressure and resulting changes to plant community composition and fuels were less severe in the Northern Rocky Mountains than in the Southwest [131]. However, pioneering ranchers in eastern Idaho and western Montana favored midelevation ponderosa pine-Douglas-fir communities as summer rangelands, and advocated livestock grazing for fire hazard reduction. Barrett [36] suggested that as a result, fire-return intervals lengthened in the early 1900s in ponderosa pine-Douglas-fir stands grazed by livestock.

Climate Change
Climate is a strong driver of the occurrence of severe fire seasons throughout the western region, although fire exclusion, changes in the number of human-caused ignitions, expansion in the ranges of invasive species, grazing practices, and changes in land use such as logging may dampen or enhance the effects of climate [153]. Increased temperatures, earlier spring snowmelt, and longer fire seasons [175] in the already dry ponderosa pine zone have resulted in large fires that occur more frequently than in presettlement times [9,62,175]. Attiwill and Binkley [19] report that there "is little doubt that climate change is a major cause of the increase in mega-fires. In many parts of the world, climates have shifted to drier conditions...and prolonged fire seasons". Stephens et al. [163] caution that in forest types adapted to frequent, low- to moderate-severity fire, large, stand-replacement fires can limit tree establishment and may result in type conversions.

Studies using 20th century fire data [100,139,175] and modeling [25,100,154] across large areas of the West to evaluate interactions between fire, climate, and management actions (e.g., fire suppression) found that increases in fire size and severity were associated with climate warming. One study [175] used agency fire records for the western United States to evaluate changes in wildfire activity and fire-climate interactions from 1970 to 2003. Spring and summer temperatures, length of the fire season, and timing of snowmelt were all highly correlated with frequency of large fires (>1,000 acres (400 ha)) from 1987 to 2003. Across the West, the Northern Rocky Mountains accounted for 60% of the increase in large wildfires since the 1980s. Increased frequency of large fires was associated with increased spring and summer temperatures and early spring snowmelt (P < 0.001) [175]. The authors note that changes in fire size and frequency are an indication of the impacts climate change may have on fire regimes across the West [153,175].

Across the West, fire season is longer than it was historically due to increased temperatures, earlier snowmelt, and a greater proportion of winter precipitation falling as rain. Fire season has increased by >75 days since 1985 [53,175]. Sommers [153] predicted that "based on projections of generally warmer climate in many regions of the West, we can expect the frequency of large fires and severe fire seasons to continue to increase, but the strength of this effect will depend to a large extent on how changing climate affects the intensity, variability, and dominant phases of key ocean-atmosphere circulation patterns." Modeling predicts a short-term increase in stand-replacement fire for dry forests of the Northern Rocky Mountains, with longer-term reductions in vegetation productivity where moisture becomes limiting. These fuel reductions may result in less frequent fires in most years, but fires may be severe in wet years. Rocca et al. [139] report with "high confidence" that climate change will increase fire risk and severity in the short term because fuels will be drier for longer periods. They caution that their short-term projections for changes in fire regimes due to climate change have higher confidence levels than their long-term projections. Management that leads to heterogeneous landscapes, including restoration activities, could alter the trajectories predicted in table 9 [139].

Table 9—Forecasted change in fire risk (probability of fire occurrence) and fire severity (effects of fire on forest overstory) over the short and longer term in ponderosa pine and Douglas-fir habitat types. Short-term predictions assume current fuel structures; longer-term trajectories consider recovery from the first fire and likely fuel production over one or more fire cycles [139].
Change Confidence level
Short-term increase in fire risk (likelihood of next fire) high
Short-term increase in fire severity during the next fire high
Longer-term decrease in fire risk (fire frequency) low
Longer-term increase in fire severity low

LIMITATIONS OF INFORMATION

Incomplete fire records and different methodologies and scales can confound comparisons among fire studies. Barbouletos et al. [26] note "forest managers will never have sufficient information to make fully informed management decisions". However, available information on historical stand structure and fire-return intervals can be used as a basis for "initiating restoration efforts on large areas of ponderosa pine stands that have been fire-excluded" [26].

Fire records for the Northern Rocky Mountains have been incomplete and inconsistent over time. From 1950 to 1959, wildfire summary reports did not account for 30% to 39% of Forest Service, U.S. Department of Agriculture lands in Idaho and for 60% to 69% of Forest Service lands in Montana [150]. Because older digital archives were erratically populated in databases, fire records for the U.S. Department of Interior are considered complete only back to 1980, and Forest Service fire records only to about 1992. Moderate Resolution Imaging Spectroradiometer (MODIS) active fire and burned area land cover products date back to about 2000. Although intended to represent all fires >998 acres (404 ha) in the western United States, some fires are missed or incompletely mapped due to poor scene quality (e.g., clouds or smoke), tree canopies blocking surface fires, patchy burn patterns, mismatches in pre- and postfire data, and/or rapid postfire recovery of vegetation [150].

As of 2016, published fire history studies were sparse or lacking for ponderosa pine communities in some areas of the Northern Rocky Mountains. These include the western front of the Northern Rocky Mountains in northeastern Washington and western Idaho, and outlier ranges east of the Continental Divide such as the Big and Little Belt mountains and Big and Little Snowy mountains (figure 4). Vegetation surveys on the Colville National Forest showed fire-scarred ponderosa pines were common in Douglas-fir habitat types with bluebunch wheatgrass, pinegrass, or mountain snowberry understories [177]; however, only two fire history studies [145,146] of ponderosa pine communities of northeastern Washington had been published as of 2016. Only unpublished studies [112,113] were available for areas east of the Continental Divide.

Methodology affects estimates of fire-return intervals. Point fire intervals, which are based on fire scars from individual trees or a small area, generally overestimate mean fire-return intervals because some low-severity surface fires do not leave fire scars and some fire scars may be lost due to weathering, decay, or overburning of an older scar [153]. Composite fire intervals, which are based on master fire chronologies of individual fire-scarred trees over a designated area, are highly dependent on the size of the study area. Composite fire intervals tend shorten as the study area increases and more trees are sampled. This is primarily because fires burn only part of the study area are calculated as if they burned the entire area. Arno and Peterson [16] caution that for fire-return intervals calculated from a master chronology, "it must be remembered that the data (including mean fire intervals) represent only the occurrences of fire somewhere in the fire area". Based on work on the Bitterroot National Forest (Table A2.1 ), they found areas of about 1 acre (0.5 ha) yielded the most accurate estimates of fire-return intervals in low-elevation ponderosa pine and Douglas-fir-ponderosa pine habitat types [16].

In their review, Baker and Ehle [23] discuss several uncertainties and biases that may occur when fire history data are used to estimate mean fire-return intervals or other characteristics of historical fire regimes: Point fire intervals and composite fire intervals may not accurately represent fire-return intervals at the stand level. However, Baker and Ehle [23] conclude that those methods provide a way to bracket the potential range (minimum and maximum) of fire-return intervals. The composite mean fire-return interval may be seen as a minimum, although that method assumes that the entire stand burns if any tree in the stand is scarred. Baker and Ehle [23] contend that actual mean fire-return intervals may be 5 to 10 times longer than those based on composite fire intervals. The point mean fire-return interval may be seen as a maximum, since it assumes that the average for each tree is the same as the average for the stand [23].

Historical information on landscape-level distributions and spatial scales of different fire severities is sparse [86], so determining the extent of past fires is challenging. Information on fire patterns and fire-climate interactions at landscape levels (1,000s to 100,000s of acres) is limited and largely based on inferences and extrapolations from fire histories reconstructed at finer scales [140]. Fire scars and fire atlases are both used to delineate fire extent and pattern. In a study comparing agreement between fire scar and fire atlas records in Idaho and Washington, fire scar and fire atlas data generally agreed on the extent of two of three Idaho wildfires; however, data agreement was poor on the extent of four Washington wildfires. The authors concluded that fire atlases were most useful at coarse landscape scales, but fire scars were more accurate at local scales. They advised that when fire atlases are used at local scales, they should be used with caution and validated with field data [148].

It is unclear how much resilience ponderosa pine stands that have missed several fire cycles have when wildfire re-enters. Limited information suggests that at least some old-growth ponderosa pine stands crowded by advanced regeneration in the understory may survive. In the Flathead River valley in the Bob Marshall Wilderness, the 2003 lightning-ignited Bartlett Mountain Fire burned into groves of old-growth ponderosa pine with Douglas-fir-western larch understories. The old growth had not burned for at least 70 years, and deep duff mounds had accumulated around the ponderosa pines [94,105]. Mortality of ponderosa pine in postfire year 1 averaged 16%, and researchers estimated that mortality would rise to 34% or more by postfire year 3. The authors suggested that restoration of historical fuels and stand structure would help maintain surviving ponderosa pines [94].

Postfire survival of mature and old growth ponderosa pine may be most likely in areas that burned in the recent past. In 2011, for example, the Hammer Creek Wildfire entered the 2003 Bartlett Mountain Burn. The Hammer Creek Fire was of mixed severity but burned at mostly low severity in ponderosa pine stands that burned in 2003. Duff mounds around old ponderosa pines had mostly burned off in the 2003 fire, so the fuelbed consisted of herbs and a "modest loading" of woody debris. Many 100- and 1,000-hour fuels burned with incomplete combustion. The 2011 fire killed a few ponderosa pines in reburned areas; however, most mortality was mature Douglas-firs and western larches that survived the 2003 wildfire and lodgepole pine and western larch seedlings that had established since 2003. The authors concluded that the 2011 fire "strongly altered the trajectory of the system" to sustainable ponderosa pine by killing conifer seedlings in the understory [105].

Old-growth ponderosa pine in fire-excluded, uneven-aged stands may be more resilient to stress than previously thought. Within and near the Frank Church-River of No Return and Selway-Bitterroot wildernesses, old-growth ponderosa pine on recently burned sites did not exhibit higher levels of stress than old-growth ponderosa pines on long-unburned sites. The study was conducted on four unlogged, low- to midelevation old-growth ponderosa pine-Douglas-fir communities. Stress variables measured included differences in needle chemistry, fine root production, and mycorrhizal infection rates. However, single or multiple fires reduced ponderosa pine growth compared to growth on sites that had not burned for at least 70 years. Growth on recent burns and long-unburned sites was documented for both the short term (5-10 years) and long term (70-94 years). On sites that had burned within the last 70 years, negative growth responses were not significantly associated with time since fire [98]. However, the long-unburned stands were denser and had more understory Douglas-fir and grand fir than burned stands. The authors cautioned that without fire, old-growth ponderosa pines in long-unburned stands would ultimately be replaced successionally by Douglas-firs and firs [99].

See the FEIS Species Reviews of Columbia ponderosa pine and Rocky Mountain ponderosa pine for detailed information on the general ecology and fire ecology of ponderosa pines in the Northern Rocky Mountains. The Fire Study Vegetation response to restoration treatments in ponderosa pine-Douglas-fir forests of western Montana provides information on prescribed fire and thinning fuel treatments applied on the Lubrect Experimental Forest in western Montana.

APPENDICES


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