Fuel characteristics and forest structure in lodgepole pine forests on the Pumice Plateau result from complex interactions among site characteristics and disturbance agents [5,6]. Fire regime characteristics in these forests are thus variously affected by climate patterns and weather events, as well as insects and diseases that affect mortality, succession, and fuel characteristics, including mountain pine beetle, lodgepole pine dwarf mistletoe, Pandora moth, and root diseases [28,32,50,76]. Because of the unique character of central Oregon's lodgepole pine forests, historical fire regimes cannot be extrapolated from lodgepole pine forests elsewhere [39].

Sites having an understory dominated by antelope bitterbrush could carry fires at about 20- to 80-year intervals even in the absence of other disturbances [39]. On other lodgepole pine sites in the Eastern Cascades, fires of any severity were limited by low surface fuel loadings because harsh soil conditions supported only sparse vegetation [26,28,81], and shrub and herbaceous fuels were typically too scattered and discontinuous to carry surface fires [5]. On these latter sites, fires were likely carried by partially decayed, downed logs from trees killed by mountain pine beetles or previous fires [5] and might only carry fire immediately following mountain pine beetle mortality, after decades of woody fuel accumulation and decay, or during extreme fire weather [26,39,55,80].

When receptive fuels were available or weather was conducive, fires in lodgepole pine stands were often ignited by fires in adjacent forests. Sites with antelope bitterbrush may have been intentionally burned by American Indians, and cultural fires in adjacent plant communities may have spread into lodgepole pine stands (see Historical Fire Ignition). Fires would have been most likely carried during the dry summer months or during extreme fire weather (i.e., dry and windy) (see Historical Fire Season).

Lodgepole pine forests in the Eastern Cascades occur within a larger matrix of ponderosa pine and dry mixed-conifer forests where historical fire regimes were characterized by frequent, mostly low-severity fires, some of which spread into lodgepole pine stands where they had more mixed- and high-severity effects [55]. Two large-scale, systematic fire history studies [33,55] used tree-ring reconstructions to describe historical fire regimes over large areas on the Pumice Plateau:

• Across 10,393-ha of dry conifer forest on the Deschutes National Forest, low-severity, surface fires were frequent (10-33 years) and often extensive, and severe fire was rare and isolated within the analysis period (1650-1871) but occurred in all forest types. In this landscape, lodgepole pine dominated 22 plots, 19 of which occurred in pumice basins. Each of these 22 plots had incomplete fire records, and it was thought that these stands may act as barriers to fire spread, and that fires only intermittently spread through them under specific conditions such as (1) bark beetle attack, subsequent fuel accumulation, and understory development coinciding with severe fire weather, (2) lightning storms igniting multiple fires in the same year across pumice basins, or (3) spotting from embers carried by wind across pumice basins [55].
• Across 85,750 ha of dry conifer forest (mostly ponderosa pine) on the Fremont-Winema National Forest and surrounding areas on the Pumice Plateau, fires larger than 20,000 ha occurred every 9.5 years, on average, from around 1700 to 1918. During this time, seven fires larger than 40,469 ha occurred during periods of extreme drought. High-severity, stand-replacement fire occurred primarily in lodgepole pine stands and patches adjacent to lodgepole pine stands [33].

Details from stand-level observations and analyses are included in the sections that follow.

Historical Stand Structure and Fuels

Photo Credit

Photo © Michael Long, iNaturalist.org, some rights reserved.

Figure 4—Lodgepole pine stand on the Deschutes National Forest, La Pine, Oregon (CC BY-NC).

Stand structure and fuel characteristics in lodgepole pine forests of the Eastern Cascades are largely shaped by low-fertility soils and interactions among disturbances like insects, disease, and fire [5,6,26]. On lodgepole pine sites with understories dominated by antelope bitterbrush, tree cover may reach 35% to 60%, shrub cover may reach 20% to 60%, and herb cover may reach 30% or more [86]. Antelope bitterbrush is a shade-intolerant, highly flammable, woody shrub that acts as a ladder fuel and facilitates passive crown fire, and it would have carried fire in these stands [39]. Other lodgepole pine associations in the Eastern Cascades tended to be fuel-limited because, in the absence of disturbance, stands had a low density and cover of trees (typically <40% cover of lodgepole pine), relatively sparse understories (typically <20% cover each of shrubs and herbs), and little litter or other surface fuel accumulation [5,26,76,86]. In these stands, shrub and herbaceous fuels were typically too scattered and discontinuous to carry surface fires (fig. 4) [5] and, in the absence of extreme weather, fires were more likely to have been carried by partially decayed, downed logs from trees killed by mountain pine beetles, previous fires [5], or other disturbances.

Canopy Fuels

Canopy fuels are limited in Eastern Cascades lodgepole pine associations, although crown fires have been noted to burn through the crowns under high winds [5]. Tree cover was sparse in all plots (n=32) studied by Heyerdahl et al. (2014), with all but one having <50% canopy cover and about half with <30% canopy cover [39]. Historically, density of conifers >15 cm diameter at 1.37 m height (dbh) exceeded 120 trees/ha on less than 5% of the area covered by dry forests in the Eastern Cascades [33]. This pattern was probably quite stable across the landscape over time, with occasional (50-100 years) region-wide decreases in basal area (30%-50%) resulting from widespread insect attack, fire, or a combination of the two [6,39]. Canopy fuels can also be altered by non-lethal disturbance. For example, infection by dwarf mistletoe and associated brooming can alter the crown height, crown base height, and live crown ratio [32,76], and it frequently occurs after mountain pine beetle outbreaks in lodgepole pine forests [9].

Lodgepole pine stands in the Eastern Cascades are typically multi-aged with a clumped distribution of cohorts, resulting from complex interactions among disturbance agents [5,6,26], with more clumping in the youngest age classes than the oldest [81]. Stands might also be even-aged, such as those studied by Stuart (1983) [81] and Gara et al. (1985) [26], which appeared to have established after a stand-replacement fire in 1840 [6].

Surface Fuels

Stands with an understory of antelope bitterbrush historically had sufficient fuels to carry a surface fire. Although fire initially reduces the abundance and biomass of antelope bitterbrush, it also stimulates regeneration, and populations can recover to prefire levels via a combination of sprouting and germination from seed caches, especially where fire creates canopy gaps. Antelope bitterbrush individuals can be long-lived, but productivity and recruitment decline with increasing shrub age, as well as increasing tree canopy cover, so that without disturbance, antelope bitterbrush becomes senescent and dies off ([39], and references therein).

In other stands, shrub and herbaceous fuels are often too scattered to effectively carry fire [5,76], and annual accumulation of needles, branches, and downed, suppressed trees is generally small and insufficient to carry fire [81]. Late-successional lodgepole pine stands that have not burned for 60 years or more have substantial bare soil exposed, except in areas of high tree density where litter layers may be 1 to 2 cm thick [5]. In addition, fuel beds of short pine needles, such as those of lodgepole pine, are relatively dense, and fire spread and intensity are diminished in high-density fuel beds ([14], and references therein).

Because they lack surface fuels, these stands may act as fuel breaks where wildfires can be stopped or suppressed except under unusual fire weather [5]. Summer fires—both prescribed and wild—have been observed to spread widely across adjacent forests without entering lodgepole pine stands [26]. Fires that did spread into lodgepole pine stands traveled along corridors of partially decayed, downed logs [3,26], which comprise the most continuous vector for surface fire [5,76], although they often have higher moisture content than other fuel classes.

In order for fire to carry in these stands there must be either enough downed, connected, decayed logs (decay class 4: rind mostly gone, heartwood rotten), or a combination of decayed logs, a moderately deep layer of litter, and some aerially supported fuels [81]. These conditions are most likely to occur during the first few years after mountain pine beetle-caused mortality, or approximately 50 years after a mountain pine beetle outbreak [26,39,55,80]. Thus, mortality-causing disturbances are required to create sufficient fuels to carry surface fire [6].

During the first few years after a mountain pine beetle outbreak begins, there is a greater probability of high-intensity fire than without beetles because of increased fine fuel in the crown and on the forest floor. Logs with attached needles and branches may burn soon after falling, or in years of extended drought. Fire potential then declines over time until the beetle-killed trees fall, typically in a widely spaced jackstraw pattern [5,6], increasing the probability of fire again.

The log-to-log pattern of fire spread was observed during wildfires and prescribed fires on the Fremont National Forest [81] and at Crater Lake National Park [3,26,76], and it was inferred for historical fires based on the distribution and ages of fire scars in adjacent lodgepole and ponderosa pine stands on the Fremont National Forest [26]. Observed fires were carried by large, partially decayed logs (decay class 4, greater than 1000-hour time lag), with heat transfer primarily by conduction. In some areas, fire spread through litter and/or aerially supported 1-to-10-hour time lag fuels in small patches. Fire spread was extremely slow (approximately 2 to 5 m/hr) and prescribed fires sometimes needed to be sustained with fuel oil and drip torches [81]. It is likely that a fire that burned around 1840 was sustained by decay class 4 logs from a disturbance that occurred around 1750 as well as an abundance of fine fuel from a mountain pine beetle outbreak that started in 1827. Similarly, a fire around 1899 was likely fueled by decay class 4 logs from the disturbances (fire and mountain pine beetles) that occurred around 1840 as well as a thicker-than-normal litter layer resulting from drought-induced needle drop [81].

Mountain Pine Beetle

Mountain pine beetle outbreaks are an important determinant of stand structure and fuel characteristics in lodgepole pine stands. When stands in the Eastern Cascades reach about 80 to 150 years old and have trees large enough to sustain brood populations (about 25 cm dbh), they become particularly vulnerable to infestation by mountain pine beetles. Trees are especially susceptible after a sustained period of low radial growth [5,28] or many decades after infection by decay fungi [28]. Outbreaks often occur in relatively high-density lodgepole pine stands, last from several years to a decade, and collapse once most preferred host trees are killed [10,28]. In an outbreak, anywhere from 33% to 66% of the stand dominants may be attacked by beetles, and about 75% of those attacked are killed [5,81], although mortality rates vary. Basal area and leaf area of the stand may be reduced by 50% or more [5,28,76].

In these low vigor stands, mountain pine beetle attack often scars trees rather than killing them [79]. Early research suggested that these scars (that damage cambium) might be mistaken for fire scars, and several publications provide information about how to tell them apart [24,57,79].

Mountain pine beetles typically target large-diameter trees with sufficiently thick bark (at least 1.5 mm) and sufficiently thick phloem to support a brood of larvae [6,71]. Nonetheless, dispersing mountain pine beetle adults may first target lodgepole pine trees suffering from injury or disease (e.g., fire damage or fungal infections) before “switching” to larger, healthier trees [25,26]. This process is not fully understood, and at least three hypotheses have been advanced to explain the process of switching attacks from a “focus tree” to a neighboring “recipient” tree [27,71].

Studies of bark beetle activity on four burned areas in the Eastern Cascades found that mountain pine beetle and pine engraver attacks began within 2 weeks after fire, and that infestation rates increased with degree of bole damage, wound height, and percent root kill [26,29]. Mountain pine beetles were most common on undamaged and lightly damaged trees, whereas pine engraver beetles were most common on moderately and severely damaged trees [29]. Within a year after fire, decay fungi infected fire-damaged root tissues [26,49]. Decades later, dispersing mountain pine beetles preferentially landed on fire-scarred and fungus-infected trees [25,26,27]. During the first few years of an outbreak, more fire-scarred than unscarred trees are killed by mountain pine beetle, and fire-scarred trees tend to have more advanced fungal decay than unscarred trees [27]. As outbreaks develop, large-diameter trees nearby those originally selected by dispersing mountain pine beetles are attacked [30]. For example, during development of a mountain pine beetle outbreak at Lookout Point, the beetles preferentially attacked large-diameter trees with advanced butt rot. In subsequent years the largest of the remaining trees were killed until the remaining trees were too small (<25 cm dbh) to support beetle populations [26,28].

Fuel characteristics after a mountain pine beetle epidemic have been well studied; however, most research has been done in the Intermountain West, with only a few studies in the Eastern Cascades [10]. A typical successional pattern for surface fuels in beetle-attacked stands is described by Agne et al. (2016) [10], which is similar to a more detailed account by Woolley et al. (2019) that was based on a chronosequence from 2 to 32 years since mountain pine beetle epidemics in south-central Oregon (1979 to 2008) [88]. The latter account suggests distinct periods in which changes occur in fuels (figs. 5-8). Canopy bulk density was low throughout the chronosequence, with substantial declines soon after beetle activity and a slow recovery over time. Surface fuels, including 10-h, 100-h, and 1000-h fuel loads, live woody fuels, and fuel bed depth increased over time. The 100-h fuel load increased over the entire 30-year period, while 1000-h fuel load leveled off 14 to 26 years following epidemic initiation. Litter and 1-h fuels changed little over time. Live woody fuels consisting of lodgepole pine seedlings and saplings establishing and growing in canopy gaps increased through the initial overstory mortality stage and began to decrease during the overstory recovery stage [88].

Photo Credit

Photo modified from Woolley et al. 2019 [88].

Figure 5—The red stage (2-4 years after attack) has a mix of recently killed (red) and living (green) trees; a 50% decrease in canopy bulk density due to mortality and needle loss; and a small increase in needle litter.

Photo Credit

Photo modified from Woolley et al. 2019 [88].

Figure 6—The gray, standing snag stage (5-13 years after attack) has a mix of gray, standing snags and living, green trees; further decrease in canopy bulk density; >85% decrease in dead basal area (fallen snags, with most snags fallen by year 13); and increases in 100-hr, 1,000-hr, and live woody surface fuels (lodgepole pine and shrub regeneration).

Photo Credit

Photo modified from Woolley et al. 2019 [88].

Figure 7—During the regeneration stage (14-25 years after attack), snags continue to fall, with few standing by year 25; there is >90% decrease in dead basal area; and there are increases in 10-hr, 100-hr, and 1,000-hr surface fuels, fuel bed depth, and live woody fuels.

Photo Credit

Photo modified from Woolley et al. 2019 [88].

Figure 8—The overstory recovery stage (26-32 years after attack) is characterized by increased overstory canopy closure, with crown bulk density approaching predisturbance levels and increased live basal area as intermediate canopy trees and saplings grow into the canopy. Both 100-hr and live woody surface fuel loads remain similar to previous stage but with high variability.

Although post-outbreak changes in fuel characteristics are well described, large-scale assessments and reviews have not found a consistent relationship between mountain pine beetle activity and wildfire occurrence (e.g., [36,53,56,60,73]). Spatial and temporal scales are important considerations when evaluating associations between mountain pine beetle outbreaks and subsequent wildfires [60]. At local scales, changes in fuel characteristics can lead to increased potential for wildfire occurrence and severity in these lodgepole pine forests [53,65], although results vary, and other factors may be more important for driving fire occurrence and severity [10].

Fewer data are available to assess these relationships at local scales, although a study of pine forests in Oregon and Washington from 1984 to 2012 found greater-than-expected levels of overlap between mountain pine beetle epidemics both preceding and following wildfire. This included evidence of larger areas of medium- and high-severity burns in locations with prior mountain pine beetle epidemics. However, the authors concluded that fire severity was probably highly dependent on other local and regional drivers—such as climate, local topography, forest composition, and wind throw—which can overwhelm the ability to detect the weaker influence of bark beetle outbreaks [60]. A study of the 2012 Pole Creek Fire in central Oregon suggests that the relationship between fire hazard and previous mountain pine beetle activity varies with time since beetle attack and factors such as topography, drought, fire weather, and previous fire and fuels management [10].

A review by Shaw et al. (2022) [73] suggests that fires in lodgepole pine-mountain pine beetle stands in the red phase have the potential for higher fire intensity, faster rate of spread, lower crowning thresholds, greater consumption of fine dead branches and needles, and more crown fire than predicted by fire behavior models that do not include mountain pine beetle effects (e.g., [65]). Decreased moisture contents and changes in foliar chemistry increase flammability of needles in mountain pine beetle-attacked trees. Laboratory tests found foliar moisture contents were lowest for red needles (12% on average), highest for green needles (109% on average), and most variable for needles of recently attacked trees. Time to needle ignition was strongly negatively related to time since beetle attack. Ignition times varied from 41 seconds for green needles to as little as 11 seconds for red needles [45].

After mountain pine beetle attack, the period of increased crown fire risk depends on the proportion of the overstory affected at a given point in time [73]. An examination of 11 lodgepole pine sites on the Deschutes National Forest in central Oregon and the Salmon-Challis National Forest in central Idaho suggests that preoutbreak forest structure and percent tree mortality influence crown fire behavior while dead needles are in the canopy (i.e., the red stage). This relationship varies with spatial heterogeneity among trees, where clumpy arrangements result in greater canopy fuel consumption and higher crown fire intensity compared to random or uniform spatial patterns [41]. The potential for active crown fire likely decreases during the gray stage and as snags fall, due to decreases in canopy bulk density [10,73]. As fallen logs decay and live woody fuels (i.e., seedlings, saplings, and shrubs) increase, the potential for surface fire increases [3,26,81]. The 1,000+ hour fuels take about 50 to 80 years to be sufficiently decayed to fuel fire [81], at which point they can sustain slow-moving, smoldering fires [6]. As suppressed trees grow into the overstory, crown fire potential may also increase with increased canopy bulk density [10].

An analysis of wildfires and subsequent mountain pine beetle outbreaks in pine forests of Washington and Oregon from 1984–2014 found a clear association of increased mountain pine beetle activity during the first 4 postfire years, especially on living trees growing in high-severity burn areas [60]. These findings are consistent with the general consensus from prescribed burn studies and conceptual models that fire can predispose host pines to subsequent mountain pine beetle attack for the first few postfire seasons. Although fires may not always facilitate subsequent mountain pine beetle epidemics, especially when mountain pine beetle populations are in the low-density endemic phase, they may contribute to mountain pine beetle population increases when they coincide with other drivers that favor successful colonization of host trees (review by [60]).

Historical Fire Ignition

Literature reviews (e.g., [4,6]) suggest that most fires in the Eastern Cascades were ignited by lightning in the dry summer months. While American Indians also ignited fires in the areas covered by this synthesis, it is unclear how cultural burning affected fire regimes in lodgepole pine communities.

Historically and currently, frequency of thunderstorms and associated lightning varies regionally, interannually, and seasonally [1]. Although lightning is frequent in the Eastern Cascades, it may be relatively less common in lodgepole pine stands on the Pumice Plateau. Fires in lodgepole pine forests are more likely to have originated from flaming fronts spreading from adjacent ponderosa pine or mixed-conifer stands. From around 1950 to 1980 very few lightning strikes occurred in the flat terrain of late-seral lodgepole pine forests in the northern part of the Fremont National Forest [81].

Information on the use of fire by American Indians in the Eastern Cascades is limited, although fire use appears to have been widespread. The extent of its influence on vegetation across the landscape is unknown [78]. Historical accounts indicate that the Paiute and Tenino tribes, who resided in the Deschutes River Valley, regularly burned to aid in hunting, improve pasture for wildlife, increase seed yield of harvested plants, and improve berry yield [15]. Evidence suggests that fire was used systematically in Klamath Marsh to encourage growth of wokas (yellow pond lily), a culturally significant resource for Klamath Tribes ([33], and references therein). Native people of the southern Cascade Range used fire to promote production of food and basketry materials, to help gather insects, to improve hunting conditions, and for ceremonial purposes. They may have used fire to keep the forest understory clear of shrubs to facilitate hunting [70].

Historical Fire Season

Little information was available about historical fire season in lodgepole pine stands in the Eastern Cascades. In stands that have adequate fuel connectivity, summers are typically hot and dry enough for fuels to dry, and weather conditions occur that can allow fires to ignite and carry [5]. Fires were likely more limited by fuels than weather, although the relative importance of these two factors varied through time and across space [39]. Fire history studies in adjacent ponderosa pine and montane mixed-conifer forests suggest that wildland fires could ignite as early as May and as late as October [15], but fire was predominantly recorded in latewood or during dormancy (i.e., late summer and into fall) [91].

Fire season varied with elevation in the Eastern Cascades. Growing season fires were more frequent in drier, low-elevation forests than in more mesic or high elevation forests, probably because fuels dry earlier at low elevations. Fires may not start or spread into higher elevation forests until late summer when fuels are dry enough to carry fire [14,82].

Fire season may have varied with ignition source. Most lightning fires were ignited in the dry summer months, whereas ignitions by American Indians may have roughly followed an elevational sequence that corresponded to plant availability, beginning with low-lying areas in May and early June and higher areas in late August through September [16].

Historical Fire Frequency

Fire frequency is not well documented for Eastern Cascades lodgepole pine forests. However, these stands rarely go a century without a major disturbance from fire or insects [5]. These disturbances are linked, in that fires make stands more susceptible to insect attacks and insect attacks alter fuel characteristics in affected stands.

Historical fire intervals estimated from LANDFIRE succession modeling average 111 years for stand-replacing fires (with a minimum of 20 years and maximum of 200 years), 113 years for mixed-severity fires, and 56 years for all fires [46]. Mean fire intervals reported in the literature range from about 24 to 150 years for all fires. Estimates of historical fire frequency found in the published literature are as follows:

• For lodgepole pine forests within Crater Lake National Park, Agee (1981) estimated an historical fire interval of about 60 years based on little data [3,5,6].
• On the Fremont National Forest, Stuart (1983) evaluated 52 scarred lodgepole pine trees and found evidence of fires around 1840 and 1899 and multiple mountain pine beetle outbreaks between 1827 and 1975. He also located at least one stand that had been fire-free for about 350 years [81]. Agee (1993) speculated that mean fire intervals were about 60 to 80 years in the area, and that stands surrounded by higher productivity forests were at the lower end of that range [5].
• Geiszler (1981) documented fire-scar data on lodgepole pines in eight locations in the northern part of the Fremont National Forest and found evidence of 11 fires between 1856 and 1945 [31]. However, later studies suggest that scarring from mountain pine beetles may have been mistaken for fire scars [79].
• Across 79 plots within the lodgepole pine series on the Pumice Plateau, Volland (1985) documented multiple fires in 11 plots within dry lodgepole pine associations, and 5 plots within riparian lodgepole pine associations. Plots in dry lodgepole pine had evidence of 15 historical fires, and fire intervals for low- to moderate-severity fires ranged from 18 to 98 years and averaged 34 years. Plots in riparian lodgepole pine had evidence of 7 previous fires, and fire intervals for low- to moderate-severity fires ranged from 12 to 38 years and averaged 24 years. Plot data suggest a stand-replacement interval of 66 to 120 years in dry lodgepole pine and 75 to 132 years in riparian lodgepole pine [86].
• At their Potholes study site in central Oregon, a reconstruction by Heyerdahl et al. (2014) suggested a 60-year median fire interval from 1650 to 1900 for lodgepole pine stands, ranging from about 26 to 82 years for all fire types. Understories are dominated by antelope bitterbrush, resulting in different surface fuel characteristics than other lodgepole pine forests on the Pumice Plateau (see Historical Stand Structure and Fuels). The intervals recorded at Potholes were long enough for antelope bitterbrush to regain sufficient cover and height between fires to facilitate fire spread across the site and into the canopy in a mosaic pattern [39].
• In a lodgepole pine stand in the Elkhorn Mountains, an unpublished fire history study suggests a mean fire interval ranging from 66 to 150 years between 1504 and 1900 (Bork 1984, cited in [52]).

Fire intervals for all lodgepole pine series sites appear to vary based on the composition of the surrounding forest types. Where they are adjacent to ponderosa pine, white fir, Shasta red fir, or mountain hemlock, the fire frequency approaches the frequencies of the surrounding forest types [76,86], with areas surrounded by higher productivity forests burning at higher frequencies [5].

Historical fire interval estimates from the southern Cascade Range are somewhat shorter (around 38 to 67 years) and more comparable to upper montane California red fir forests (39-60 years) [14,83] than those reported for lodgepole pine in central Oregon. However, lodgepole stands north and east of Mt. Shasta may be more like those of central Oregon because they, too are most extensive on pumice and andesite flats [77]. In the eastern portion of Lassen National Park, on Prospect Peak, longer intervals between fires occurred on more xeric west and south-facing slopes (Taylor 2000). Apparently, the more xeric sites occurring in the rain shadow of Lassen Peak have less fuel production and continuity [21].

Historical Fire Intensity and Severity

While the fire regime of seral lodgepole pine forests is typically considered to be high-severity and stand replacing, lodgepole pine forests in the Eastern Cascades are characterized as having a moderate or mixed-severity fire regime that varied across space and time [6,39]. The presence of fire scars and general lack of cone serotiny in these lodgepole pine forests suggest a mixed- or moderate-severity fire regime [6,39] consisting of low-intensity, low-severity surface fires and high-intensity, high-severity surface and crown fires [4]. Across three lodgepole flats on the Pumice Plateau, fires occurred up to five times per century, were a mix of low-severity and stand-replacing fire, and created a mosaic of multi- and even-aged stands [55]. Recurring moderate- and mixed-severity fires interacting with beetles, disease, and wind events created variable, multi-aged forests at both plot and landscape scales, where about a third of the stands were removed and replaced at intervals of about 60 years [6].

Several lines of evidence support the model of extensive, mixed-severity fires historically, at both site (~800 ha) and plot (~1 ha) scales. At the Potholes site studied by Heyerdahl et al. (2014), widespread synchrony in fire-scar dates prior to 1900 suggests extensive low-severity fires; however, these scar dates were also synchronous with cohorts of tree recruitment, suggesting that individual fires included patches of both high- and low-severity fire [39].

Evidence of low-intensity, low- to moderate-severity fires includes the presence of fire scars on lodgepole pines (e.g., [31,39,81]). Low-intensity fires would have been fueled by the thin litter layer, sparse understory vegetation, and logs burning slowly across the forest floor [3,5] and likely resulted in mostly low or moderate severity effects. Fires burning along downed logs can scar or fatally scorch nearby saplings and trees, depending on the arrangement and loading of logs relative to living trees [59]. The heat from burning logs can also penetrate the soil, damage roots, and kill trees that are otherwise unaffected by the fire. Where fuel discontinuities occur, fires will often extinguish, and fires moving into lodgepole pine stands from adjacent forest types would typically be extinguished on their own due to lack of surface fuels [5].

High-severity fire was spatially limited in the Eastern Cascades, although cohorts of lodgepole pine recruitment coincident with disturbance years (i.e., fire, windthrow, and beetle outbreak) indicate high-severity fire in some areas [22,26,39]. Across 85,750-ha of dry forests on the Pumice Plateau—mostly ponderosa pine and dry mixed conifer with areas of lodgepole pine—patches of high-severity fire were concentrated in and around lodgepole pine stands between 1700 and 1918. For example, timber inventory records indicate stand-replacing fire accounted for 6% of forest area burned in 1918, when widespread fires burned more than 78,900 ha in the Eastern Cascades. Most stand-replacing fire (81%) that year occurred in lodgepole pine, and where it occurred in ponderosa pine or mixed-conifer stands, it was often adjacent to lodgepole pine stands. The largest patch of stand-replacing fire was 710 ha of lodgepole pine in the western edge of Klamath Marsh [33].

Fire behavior simulations suggest active crown fire was not common but may have occurred in spots where shrub cover, especially antelope bitterbrush, and sufficient wind carried fire into the canopy [5,39]. Strong winds are probably required to force fires to crown through late-seral lodgepole pine flats [5]. Many lodgepole pine forests in the Eastern Cascades show evidence of high-severity crown fires [5]. The 1988 Prophecy fire at Crater Lake—a prescribed natural fire that started in California red fir forest—was pushed by a strong westerly wind through a 100-ha patch of lodgepole pine forest as a crown fire, even though the crowns were not touching or continuous across much of the area [5]. Crown fire potential may be higher in stands attacked by mountain pine beetles while foliage is retained on trees [45].

High severity fires, in general, are more likely after disturbances alter fuel characteristics in lodgepole pine forests, such as at various intervals following mountain pine beetle outbreaks. Contemporary observations show that pumice basins dominated by lodgepole can have high-severity fire 1) immediately following mountain pine beetle-caused mortality; 2) after decades of woody fuel accumulation and decay; and 3) during extreme fire weather [3,26,33,81]. See the section on mountain pine beetles for more information about their effects on fuel characteristics over time.

Historical Fire Pattern and Size

Information regarding historical fire patterns and sizes was lacking for lodgepole pine forests in the Eastern Cascades. Most evidence suggests that fires were patchy, depending on configurations of surface fuels, which varied with understory composition and disturbance history, and suggests an historical mixed-severity fire regime.

Lodgepole pine stands with an understory of antelope bitterbrush likely burned in a mosaic pattern. This mosaic pattern would have allowed for postfire regeneration of antelope bitterbrush by creating canopy gaps while maintaining some unburned plants as seed sources and stimulating sprouting from undamaged portions of surviving plants ([39] and references therein).

A reconstruction of historical fire perimeters and the pattern of fire sizes across 10,393 ha of dry conifer forests on the Deschutes National Forests suggests that lodgepole pine-dominated pumice basins likely influence spatial variation in fire regimes where they are embedded in mixed-conifer landscapes. Because lodgepole pine stands tended to act as barriers to fire spread, small fires were more common in forests where these stands were common, whereas large fires prevailed in areas lacking these stands [55].

Historical Fire-Climate Relationships

Little information is available regarding historical fire-climate relationships in lodgepole pine forests in the Eastern Cascades, although drought is known to exacerbate mountain pine beetle outbreaks [28,81], which, in turn, influence fuels and the likelihood of fire in these stands (see Mountain Pine Beetle).

Fires may have been more common during drought in lodgepole pine stands on the Pumice Plateau; however, data are lacking. Between 1700 and 1918 seven fires, each >40,469 ha, occurred during extreme drought (PDSI <- 4.0) in dry conifer forests on the Pumice Plateau [33]. In lodgepole pine stands on the Fremont National Forest, major periods of reduced growth, a likely indication of drought, are evident from 1835–1856, 1881–1885, 1890–1899, 1920–1937, and 1967–1968. Two fires occurred during these periods of slow growth, and mountain pine beetle outbreaks occurred around 1840, during the mid-1920s, and after 1968 [81]. In contrast, fires reconstructed from tree rings at Potholes did not consistently occur during years with warm, dry summers, nor were they synchronous with climatically driven years of widespread fire across the region [39].