By Mike Hillis, Cohesive Strategy Team
Denise Pengeroth, Helena National Forest
Rose Leach, Confederated Salish and Kootenai Tribes
Region One lands capable of growing large diameter, durable snags were identified using PNVs. The percentage of those lands within mature and old forests was identified using satellite imagery and compared against the percentage of mature and old forests present in pre-fire suppression/pre-logging periods. The current levels of mature/old forest were comparable to those present during historic periods. FIA analyses were used to determine, within those mature/old forests, if there have been shifts from old forest to mature forest. The results indicated that westside old growth ponderosa pine forests have undergone massive shifts from old to mature forest, with comparable losses in large diameter, durable snags. Conversely, FIA-based results showed no measurable change in the distribution of old and mature forest or snag densities at high elevations, both east and west of the continental divide. Mature and old forests that had been partially-logged, and/or were adjacent to roads were identified using TSMRS and the Region One roads layer. Using snag attrition factors from ongoing research, it was determined that snag attrition from past partial-removal logging, and woodcutting accounted for an additional 28% reduction in snags in Region One. Non-lethal and moderate-severity wildfires were found to be an important natural process in both the recruitment of snags and the long-term durability of snags. An analysis of past fires suggested that low and moderate-severity fires have been drastically curtailed since 1940. This further places snag populations and snag-dependent species at risk in Region One.
Snags and their management have become a major conservation issue in managed forests. While it’s been long recognized that snags provide essential wildlife habitat, research increasingly underscores the need to understand snags as part of a larger interacting ecosystem (Bull et al. 1997, Duncan 1999, Duncan 2002, Rose et al. 2001). We are challenged to expand our thinking to the landscape level and ecological processes that sustain snags over time – to place snags in an ecological context.
Status in Region One - Woodpeckers are given special status in Region One as Management Indicator Species (MIS) in Forest Plans or as sensitive species on the Region One Sensitive Species List (Bosworth 1999). Management Indicator Species are used to measure the effects of management on representative wildlife habitats with the objective of ensuring that viable populations of species are maintained. Indicator species are generally identified for those species groups whose habitat is most likely to be changed by Forest management activities. Indicators are selected for emphasis in planning and are monitored to assess management effects on them and other species with similar habitats (Forest Service Manual (FSM 2620.5, 36 CFR 219.19). Sensitive species are those plant and animal species for which population viability is a concern due to significant current or predicted downward trends in population numbers or in habitat capability that would reduce a species’ existing distribution (FSM 2670.5).
Cavity users that are considered MIS species in Region One include: hairy woodpeckers that represent the snag-dependent species group, pileated woodpeckers that represent the old-growth dependent group, and American martens that represent the interior forest/mature tree-dependent group. Cavity/decayed tree users that are listed as sensitive in Region One (Bosworth 1999) include: Townsend’s big-eared bat, fisher, flammulated owl, white-headed woodpecker, and black-backed woodpecker.
The goal of this assessment is to provide a framework for snag management at the project and broad-scale level. Specifically our objectives are to:
Provide a context for future snag management that includes:
Consideration of a diversity of tree species and size classes in various stages of decay,
Recognition of natural disturbances, and
Spatial considerations
Address changes in age class distribution as it affects snag densities and snag recruitment
Address effects of partial logging on snags
Address effects of woodcutting on snags
Provide a tool for understanding and evaluating cumulative effects
Identify future research needs
Importance of snags to wildlife- Snags provide essential habitat for wildlife. We generally think of snags as providing nesting and foraging habitat (Thomas et al.1979); however snags meet a variety of behavioral and physiological needs for a variety of species. These include but aren’t limited to singing, viewing, perching, estivating, communicating, escaping, hibernating, resting, and observing (Davis 1983). Snags are also important to wildlife once they fall and become down woody material. Down woody material provides habitat for a variety of species including foraging sites, hiding and thermal cover, denning, nesting, and travel corridors (Rose et al. 2001). As down woody material further decays it plays an important role in nutrient cycling, soil fertility, and erosion control, among other functions (Maser et al. 1979, Tarrant and Maser 1988, Maser et al. 1988, Rose et al. 2001). Decaying wood is a key feature of productive and resilient ecosystems and overall is extremely important to wildlife.
Decaying Wood- Tree decay is an important ecological function that creates key habitat elements for wildlife (Rose et al. 2001). Living trees with decay, hollow trees, and dead trees are expressions of mortality agents (e.g. fire, disease, and insects) that create a range of snag species and sizes (Bull et al. 1997) that are utilized in different ways by different species. While tree species may be important - some habitat functions of wood are provided by certain tree species - more important characteristics that affect wildlife use of wood include decay characteristics, size, and wood density (Rose et al. 2001).
Living trees with decay typically last longer than snags although the portion of the tree suitable for use may be smaller than in a snag of the same size (Bull et al. 1997). Living trees with decayed heartwood are selected by some woodpecker species and may retain nesting characteristics for several years because of the sound outer sapwood. Trees with large dead branches and broken tops also provide a distinct habitat. Bull (1980) concluded that pileated woodpeckers selected nest snags that had an abundance of stem decays. She further concluded that such snags often had broken tops. These two attributes of stem decay and broken tops are frequently cited as characteristics that make snags particularly desirable to cavity-users (McClelland 1977). Because silvicultural treatments in the ‘60’s, ‘70’s, and ‘80’s often “sanitized” stands to remove trees with stem decays or broken tops, Bull and Partridge (1986) explored methods for inoculating trees with stem decays and removing tops from otherwise healthy trees. Lockman (Region One Pathologist, Missoula, MT,unpublished data) explored similar methods for introducing stem decays into trees in western Montana.
Hollow trees have advanced heartwood decay that produces structural elements from small patches to large cylinders to entire hollow trunks (Rose et al. 2001). Black bears and pine martens use hollow trees for denning, pileated woodpeckers use hollow trees for roosting, and Vaux’s swifts use these structures for nesting and roosting (Bull et al. 1997). Large hollow trees are uncommon in managed forests.
Dead trees are created through one of several factors or a combination of factors and will exhibit structural characteristics as described above. How a tree dies determines its use by wildlife (Bull et al. 1997).
Primary/Secondary Users- Primary cavity users excavate their own cavities; secondary cavity users occupy those cavities already created. Primary cavity excavators use snags differently. Some species can only excavate soft wood; others only excavate hard wood. Hence it’s important to distinguish between TYPES of snags. (Thomas et al. 1979). For example, pileated woodpeckers and black-backed woodpeckers excavate trees with hard exterior sapwood shell and decaying heartwood. Weaker excavators, e.g. red-breasted nuthatches and chickadees, select trees with softer exterior wood such as those created by armillaria root rot and other saprophytic fungi (Rose et al. 2001). Woodpeckers usually excavate a new cavity each year (Bull et al. 1997) therefore old cavities are continuously available for secondary cavity users.
Specialization of Cavity-users- Snags provide key habitat elements wildlife and often species-specific relationships exist relative to particular characteristics and components of standing dead trees (Rose et al. 2001). The following examples illustrate some of these relationships.
Woodpeckers– Primary Cavity Excavators: Woodpeckers represent the primary excavator group and generally come to mind in snag discussions. The variability among woodpecker species in their use of snags is broad. Pileated woodpeckers select nest trees with sound sapwood and decaying heartwood. The average size of the nest tree is 28” diameter breast height (dbh) and pileateds tended to select for stands with grand fir (Bull and Holthausen 1993) and ponderosa pine trees for the actual nest (Bull 1987). McClelland (1977) found that western larch snags were also preferred for nesting in Montana. However, pileated woodpeckers roost tree requirements differed from nest tree requirements. Nest trees tended to be solid wood while roost trees were hollow (Bull et al. 1992).
White-headed woodpeckers tend to excavate nests in broken-topped snags and stumps in trees that are completely dead and the nest is generally close to the ground with moderate to extensive decayed wood (Raphael and White 1984, Milne and Hejl 1989). In west-central Idaho, Frederick and Moore (1991) found nests in snags averaging 22” dbh. The white-headed woodpecker is generally associated with pine and mixed pine/fir forest (Dixon 1995).
Black-backed woodpeckers prefer to nest in recently burned areas. Research conducted in Montana (Hutto 1995, Caton 1996, Hitchcox 1996, Hejl and McFadzen 2000, Powell 2000) suggests Black-backed woodpeckers may require recent burns for long-term survival. Minimum dbh required for nesting is 12 inches (Thomas et a. 1979). Powell (2000) concluded that while black-backed woodpeckers forage in areas infested with both bark beetles and woodborer beetles, they strongly select woodborers over bark beetles while foraging. In burned forests, black-backed woodpeckers prefer large, thick-barked conifers including ponderosa pine, Douglas-fir, and western larch, over small, thin-barked species such as subalpine fir, Englemann spruce, and lodgepole pine (O’Connor and Hillis 1999, Hejl and McFadzen 2000). Hejl and McFadzen (2000) concluded that large, thick-barked trees are more likely to retain green cambium after a severe fire, which provides attractive habitat for bark beetles and woodborers. The cambium of thin-barked trees like lodgepole pine or subalpine fir, conversely, is more likely to be charred by severe fires, making those thin-barked conifers generally less suitable for bark beetles, woodborers, and black-backed woodpeckers
Secondary Cavity Users – Birds:- Other birds such as saw whet owls, use existing cavities with a minimum dbh of 12 inches (Thomas et al. 1979, Bull et al. 1997). Larger owls such as the great gray owl, great horned owl, and barred owl use platforms that are often created when snags break (Bull and Duncan 1993, Bull et al. 1997). Vaux’s swifts nest and roost in large (27 inch dbh) hollow trees (Bull and Collins 1993, Bull et al. 1997). Brown creepers nest between a partially detached piece of bark and the trunk of a dead tree (McClellan 1977).
Secondary Cavity Users - Mammals: Mammals also use cavities. For example, bats generally roost under the loose bark of snags; however recent studies show they also use cavities (Bull et al. 1997). Marten use hollow trees for denning and shelter. Black bears will den in hollow trees (Thomas et al. 1979, Bull et al. 1997). The average size of black bear den trees is 43 inches (Bull et al. 1997).
Disturbance ecology of snags- While snags are technically just dead trees, the circumstances in which they occur, and the characteristics they exhibit when dead, are the result of complex ecological disturbances (Ritter et al 2000). The Region One Snag Protocol (Ritter et al 2000) categorized naturally-occurring snag densities, based on Forest Inventory Assessment data (FIA), using a combination of Vegetation Response Units (VRUs) and Fire Regimes. Harris (1999) categorized snag densities by habitat group based on FIA data. These relationships are described in the following examples (from Ritter et al 2000):
Example #1- Droughty ponderosa pine-Douglas-fir/frequent, non-lethal fire regime- This community is characterized by predominately large diameter, uneven-aged stands of ponderosa pine. Ponderosa pine snags occur at low densities (1-2 snags/acre), and individual snags persist (i.e. remain standing after death) for very long periods of time (50 plus years). Snag densities tend to vary little over time at the landscape scale, and occur somewhat continuously over time.
Example #2- Cool, lodgepole pine or spruce-fir forests/infrequent, stand-replacing fire regime- This community is characterized by a mosaic of even-aged stands of lodgepole pine. Lodgepole pine snags occur at high densities after fire, but do not persist for long periods of time (<20 years). As a result, snags in this situation occur as infrequent, periodic “pulses” (Lyon 1977).
Example #3- Mixed coniferous forests, mixed-severity fire regime- This example is a mix of several categories described in Ritter and others (2000), and ecologically falls somewhere between the previous two examples. This category has enormous variability, and is subsequently the most difficult to describe in terms of snag densities. Fires typically include both non-lethal and stand-replacing severity depending on many, highly variable conditions over time. In northern Idaho such forests typically include a mix of grand fir, white pine, western larch, and Douglas-fir, which may also include western redcedar and hemlock in the more mexic sites. In western Montana, such forests typically contain mixes of larch, Douglas-fir, or grand fir. In eastern Montana, such forests typically occur at the interface of bunchgrass/sagebrush communities and include a mix of Douglas-fir and lodgepole pine.
Exceptions- The previous examples provide descriptions of what snag densities may look like in naturally-functioning ecosystems. The variability of snag densities, however, is huge. When detailed information is collected for a given landscape, a very different picture on snag occurrence and snag function may emerge. For instance, the Trail Creek drainage in the Beaverhead National Forest is a lodgepole pine-dominated forest at 7000’. One might conclude from the previous example #2 that snags in that lodgepole pine-dominated forest would only occur as short-lived pulses after in-frequent, stand-replacing fires. Research by Losensky (2002) found that within this landscape characterized by a stand-replacing fire regime, intermediate, moderate-severity fires had occurred at least back to 1720 at an average of 35-year intervals. While the stand was composed of predominately 100-year old, even-aged lodgepole pine, which originated from a severe stand-replacing event in the 1880’s, it contained scattered, very large 200-300 year-old lodgepole pine, many of which exhibited multiple fire scars. In addition, there were many large, scattered, old lodgepole pine snags that were still standing since they were killed in the 1880’s fire. Consequently, with such fine-scale data, we find evidence of both the expected “pulses” of high density snags that occurred after the 1880’s event, and evidence of continuous, large diameter lodgepole pine snags occurring at low densities. The fire history literature is full of such “exceptions” (Gabriel 1976, Arno 1976, Barrett 1991) that suggest that snag occurrence is highly variable and difficult to predict. While the snag densities described in Ritter and others (2000) are useful for characterizing historic conditions at very large scales, they may seldom reflect real conditions when applied to smaller scales.
Pulse versus continuous populations of snags- Where present, larch and redcedar snags can stand for very long periods after death (50-100 years for larch, and 100+ years for redcedar). Other species of snags, such as subalpine fire, stand for relatively short periods (Ritter et al 2000). Ritter and others (2000) described snag populations as occurring in either: 1) “pulses” of snags following stand-replacing burns; or 2) as “continuous” populations of scattered individuals, such as ponderosa pine snags that occur in old growth ponderosa pine stands. In mixed fire regimes, where larch or other durable snag species are present, snags can occur as both pulse and continuous populations. For instance at the lower, warmer end of the subalpine fir habitat series, stand-replacing fires can created dense pulses of subalpine fir, lodgepole pine and spruce. Scattered large diameter larch may survive such fires at low densities and provide a fairly continuous population of low density live trees and larch snags.
Loss of “pulse” snag recruitment- Hillis and others (2002a) concluded based on fire history data that habitat for the black-backed woodpecker, defined as fire-killed stands within 1-6 years of the fire, declined 82% between 1940 and 1987. While that assessment did not address the availability of large diameter, durable snags, they concluded that one component of habitat for snag-dependent species, the dense “pulses” of snags that result from high severity burns, had been severely compromised by fire suppression.
Snag durability- Snags are highly variable in terms of how “durable” they are, defined in this paper as how long they remain standing after death. Ritter and others (2000) explained the durability of larch and redcedar (50-100 years +) by those species’ inherent resistance to rot. Conversely, subalpine fir snags seldom stand more than 10 years after death, presumably because they have little resistance to rot. In ponderosa pine snags, Smith (1999) and Smith and Arno (2000) found that snag durability was the result of a multitude of factors including age (the older the more durable), size (snags greater than 23” dbh were substantially more durable than trees less than 23”), and potentially pitch content (the more pitch imbedded in the butt log, the more durable). Smith’s first two findings were not surprising. Observers have long concluded that large, slow-growing (i.e. old-aged) ponderosa pine snags stand for long periods after death (USDA 1994). In western Montana, many ponderosa pine snags are still standing after succumbing to the fire of 1910 (Hillis et al 2000).
Smith and Arno’s findings (2000) on the relationship of pitch content to snag durability could not be strongly correlated, due to a multitude of other variables in the data. The suggestion, however, that snag durability is dependent on pitch content due to exposure to non-lethal fires, is both interesting and alarming. Smith and Arno explained pitch content resulting from periodic “pitching,” as the tree’s natural response to a basal injury, usually the result of exposure to non-lethal fires. Trees with high pitch content often have multiple fire scars (Smith 1999). Since ponderosa pine is not considered a rot-resistant wood, high pitch content, resulting from multiple exposures to non-lethal fires, might well explain (perhaps more so than size or growth rate) why ponderosa pine snags, and likely other species of snags, are capable of standing for a century or more after death. Consequently, this raises the management concern that in landscapes where fires are routinely suppressed, the loss of frequent, non-lethal fires may also have serious implications on snag durability.
Exposure of live trees to non-lethal fires, which is the basis of Smith’s findings on durability, is not limited to low elevation, ponderosa pine cover types. Many researchers (Gabriel 1976, Arno et al 1993, Barrett 1997, Barrett et al 1997, Losensky 2002) concluded that even in high elevation forests influenced by infrequent, stand-replacing fires, intermediate, low and mixed-severity fires were common. Consequently, large diameter western larch, Douglas-fir, and western redcedar typically contain fire scars from non-lethal events, even in landscapes that are most influenced by stand-replacing events. Broad-scale assessments (Ruediger et al 2000) often erroneously conclude that fire suppression has had little effect on mid and high elevation stand-replacing fire regimes, due to the long (stand-replacing) fire return intervals in those landscapes. The Interior Columbia Basin Ecosystem Management Plan (USDA/USDI 2000), however, concluded that low and mixed-severity fires have been substantially reduced within mixed severity and stand-replacing fire regimes. In Region One when fire history analyses are conducted in landscapes characterized by stand-replacing fire regimes, we find substantial evidence of intermediate, frequent, non-lethal and mixed-severity fires in those landscapes (Gabriel 1976, Arno et al 1993, Barrett 1997, Barrett et al 1997, Losensky 2002). From a snag durability standpoint, therefore, the loss of low and mixed-severity fires appears to be a concern in most fire regimes.
Eastside durability and importance of Douglas-fir- In northern Idaho and western Montana, Douglas-fir snags are categorized along with spruce, subalpine fir, and lodgepole pine as low value snags that are neither durable nor preferred by cavity-users (Ritter et al 2000). In eastern Montana, however, Douglas-fir is often the preferred snag species for cavity-users. In droughty, eastside sites, Douglas-fir often occurs in mixed-severity fire regimes, has apparently good resistant to low/moderate-severity fire (as exhibited by multiple fire scars), and stands for long periods after death (Losensky 2002). Gilbert (2000), however, notes that in burned areas on the Lewis and Clark National Forest limber pine snags remain on the landscape longer than Douglas-fir snags. His findings indicate that limber pine snags are very persistent with 67% of the snags still standing at 53 years post-fire. Douglas-fir snags, on the other hand, had 2% of the snags still standing at 50 years post-fire. However, in an assessment of wildlife use of snags, Gilbert indicates that Douglas-fir – and ponderosa pine – appear to be the preferred species.
Snag attrition factors- Most Forest Plans in Region One have prescriptive standards for retaining snags following vegetative treatments. Only a modest amount of monitoring has been done to determine how well those standards have been met. Results from those monitoring efforts were generally disappointing. Monitoring of snag attrition during logging on the Lolo and Flathead Forests concluded that snag retention standards were seldom met (USDA 1987, USDA 1989). Monitoring was done on the Lolo National Forest (USDA 1992) repeatedly after the site was scheduled for logging to determine exactly what activities resulted in snag attrition. They concluded that attrition occurred from multiple activities including: 1) inadequate marking; 2) inadequate contractual protection; 3) felling; 4) yarding; 5) fuels treatment; and 6) woodcutting after logging was completed. They further concluded that even if the effort to avoid snag attrition during logging was rigorous and continuous, a substantial percentage of snags was lost by the time all logging-related activities were completed and access roads were closed or obliterated. In situations where access roads were left open, snag attrition continued indefinately.
Bate and Wisdom’s findings on snag attrition from logging and woodcutting- Bate and Wisdom (in prep) studied snags in Oregon and Montana to determine the effects of logging and woodcutting on snags. They compared untreated stands against stands that had been harvested with a variety of salvage, single-tree selection, or other partial removal treatments, and found only a third as many snags in the partially-logged stands. They also compared snags in roaded and un-roaded landscapes, and found only a third as many snags near roads. They attributed this reduced snag density to woodcutting. Snag attrition was highest within 50 meters of roads. The effect of the road generally became insignificant beyond ~200 meters, if there were no other roads in the vicinity. Often, however, in heavily-roaded landscapes, Bate and Wisdom found that even as snag attrition diminished beyond 200 meters from one road, snag attrition started to increase again beyond 200 meters because the transect approached another road. Consequently, particularly in Oregon, Bate and Wisdom found few areas where snag densities hadn’t been affected to some degree by woodcutting. Snag attrition due to woodcutting was also highest within 40 km of towns. Bate and Wisdom defined “towns” as having a population of greater than 1000 people.
Management questions - Ritter and others (2000) concluded that the snags most at risk were large diameter, seral species including ponderosa pine and western larch west of the continental divide and Douglas-fir east of the continental divide. Wisdom and others (1999) had comparable findings for ponderosa pine and larch snags west of the continental divide. Ponderosa pine, larch, and Douglas-fir tend to provide the best nesting habitat for pileated woodpeckers and other cavity users (McClelland 1977, Bull 1980). Biologists in mesic habitats in Idaho (Davis, Clearwater NF biologist, Orofino, ID) found similar levels of high woodpecker use in western redcedar snags. Although redcedar is a climax species, unlike ponderosa pine or larch, the Interior Columbia River Basin Assessment (USDA/USDI 2000) concluded that species dependent upon old growth redcedar were also at some risk. For these reasons, this analysis is focused on the availability of ponderosa pine, larch, and redcedar snags within mature and old forests, west of the continental divide, and Douglas-fir snags within mature and old forests east of the continental divide.
Snags also occur as “legacy trees” following stand-replacing disturbance (Ritter et al 2000). This analysis did not address the status of snags in young stands.
The management questions pertinent to maintaining ponderosa pine, larch, and redcedar snags within mature and old forests, west of the continental divide, and Douglas-fir snags within mature and old forests east of the continental are summarized below, and are the focus of this assessment.
1) On forested lands capable of growing large diameter trees and durable, snags, how has the percentage of mature and old forest changed due to logging and/or fire suppression, both east and west of the continental divide?
2) In mature and old forests, how has partial removal logging affected snag densities?
3) In mature and old forests, how has woodcutting and road access affected snag densities? And
4) Since snag durability appears related to live tree exposure to non-lethal fires, how has fire suppression affected the recruitment of durable snags?
Management question #1- Within potential habitat, or forested lands capable of growing large diameter trees and large, durable snags, how much existing habitat, defined as mature and old forest, remains, and how has this level changed due to logging and/or fire suppression? This question was addressed with the following five steps:
1) Potential habitat, i.e. lands capable of growing large diameter trees and large, durable snags, was identified on the Westside of the Continental Divide by Potential Vegetation Types (PNVs) where ponderosa pine, western larch, or western redcedar are found and in elevations less than 6200’, and on the eastside PNVs where Douglas-fir is found and in elevation less than 7400’. Those PNVs included ABGR1, ABGR2, ABLA1, ABLA3, PICEA, PIPO, PSME1, PSME2, PSME3, THPL1, THPL2, and TSHE (for definitions see PNV metadata at website http://www.fs.fed.us/r1/cohesive_strategy/index.htm). Lands coded as any of those PNVs within the elevation limits were mapped and acres summarized for both east and west of the Continental Divide. This data layer was labeled potential habitat.
2) Within potential habitat, existing habitat, i.e. lands having large diameter trees and durable snags were identified using Satellite Image Land Classification (SILC) categories:1) SILC1 and SILC3 size classes 3, 4 and 5 (greater than 9”); 2) crown closure classes [1] greater than 40% west and 25% east of the continental divide; and 3) showing no regeneration timber harvest, as coded in the Timber Stand Management and Resource System (TSMRS) data base, since the SILC layer was done (not coded 4113, 4114, 4131, 4132, 4133, or 4134). Lands meeting these criteria were mapped and acres summarized for both east and west of the Continental Divide. This data layer was labeled existing habitat (definitions and PNV and pileated metadata at website http://www.fs.fed.us/r1/cohesive_strategy/index.htm ).
3) Historic records (Losensky 1993, Losensky 1995, and Hessberg et al 1995) were reviewed to determine the percent of the landscape that would have been in a mature/old age class in historic periods.
4) The level of existing habitat was divided by the potential habitat both east and west of the continental divide. These percentages were compared against the percentages recorded in historic periods.
5) Existing FIA queries (described in Hillis and others 2002b and 2002c) were reviewed to determine within lands mapped as existing habitat if there have been shifts between the distribution of mature and old forests.
Management question #2- Within existing habitat, how has partial removal logging affected snag densities? This question was addressed by the following two steps:
1) Existing habitat was segregated using the TSMRS database by lands that had not been previously harvested, and those that had had some type of partial removal harvest, TSMRS coded as 4151, 4152, 4210, 4211, 4220, 4230, 4231, 4232, and 4240 (definitions and PNV and pileated metadata at website http://www.fs.fed.us/r1/cohesive_strategy/index.htm ).
2) Acres of existing habitat that had been previously harvested, were multiplied by .33 to represent the percentage of snags remaining on those lands, based on attrition identified by Bate and Wisdom (in prep). Acres of existing habitat that had not been previously harvested, were multiplied by a factor of one (representing no loss in snags). The following formula was used to determine the percentage of snags remaining, i.e. not lost to partial removal logging, within existing habitat:
% = (Acres existing, partially-logged x .33) + (acres existing, not logged x 1.0)
Acres of existing habitat
Management question #3- Within existing habitat, i.e. mature and old forests, how has woodcutting affected snag densities? This question was addressed by the following three steps:
1) Existing habitat was categorized using the roads layer (pileated metadata at website http://www.fs.fed.us/r1/cohesive_strategy/index.htm ) as: 1) within 50m of roads; 2) 50-100m of roads; 3) 100-200m of roads; and 4) >200m from roads. Acreages within each category were summarized.
2) For each of the four distance-from-road categories identified in step 1, a coefficient, extrapolated from Bate and Wisdom’s findings (in prep) was assigned to represent the percent of snags remaining in each category including .11 within 50, .33 within 50-100, .44 within 100-200, and 1.0 for greater than 200m from roads. Note that the weighted average percent of snags remaining within 200m of a road is .33, which is consistent with Bate and Wisdom’s (in prep) findings. Also note that the coefficients reflect decreasing snag attrition the farther from the road, which is again consistent with Bate and Wisdom’s findings.
3) The percent snags remaining, i.e. not impacted by woodcutting, was determined by the following formula:
(Ac < 50m x .11) + (Ac 50-100m x .33) + (Ac 100-200m x .44) + (ac >200m x 1.0)
Acres of existing habitat
Cumulative impacts from partial removal logging and woodcutting- According to Bate and Wisdom (in prep), if stands were partial cut within 200 feet of a road, we would expect predictable snag attrition from both logging and woodcutting. In order to reflect these cumulative losses, the previous formulas were combined to reflect this cumulative loss as follows:
(Ac not partially logged >200m from a road x 1.0) +
(Ac not partially logged 100-200m from a road x .44) +
(Ac not partially logged 50-100m from a road x .22) +
(Ac not partially logged 0-50m from a road x .11) +
(Ac partially logged >200m from a road x .33) +
(Ac partially logged 100-200m from a road x .1452) [2] +
(Ac partially logged 50-100m from a road x .0726) +
(Ac not partially logged 0-50m from a road x .0363) =
Equivalent acres of high value snag habitat remaining following losses
from partial removal logging and woodcutting.
* The factor .1452 is the multiple of the loss from partial removal logging (.33) times the loss from woodcutting 100-200M from a road (.44). The factor .0726 is the multiple of .33 (loss from partial removal logging) times .22 (loss from woodcutting within 50-100M of a road). The factor .0363 is the multiple of .33 times .11 (loss from woodcutting within 0-50M of a road).
Management question #4- Since snag durability can be related to live tree exposure to non-lethal fires, how has fire suppression affected the recruitment of durable snags? This management question was assessed by searching the literature for information on: 1) the frequency of low and moderate-severity fires by fire regime; 2) changes in total acres burned since 1940; 3) evidence of increased fire severity in recent fires; and 4) increases in climax species composition and condition class
Management Question #1- Within potential habitat, or forested lands capable of growing large diameter trees and large, durable snags, how much existing habitat, defined as mature and old forest, remains, and how has this level changed due to logging and/or fire suppression?
1. Potential habitat- The GIS query (see METHODS) suggested that 13,176,788 acres of Region One National Forest lands are capable of growing large diameter trees that produce durable, highly valuable snags (ponderosa pine, larch, or redcedar on the Westside, and Douglas-fir on the eastside). Those findings were summarized for both west and east of the Continental Divide. On the westside, there are 10,520,384 acres of potential habitat. On the eastside, there are 2,656,405 acres of potential habitat.
2. Existing habitat- West of the continental divide, of the 10,520,384 acres of potential habitat, the GIS query concluded that 5,128,766 acres are in the mature or old size class (>9”), and have at least 40% crown closure. These 5,128,766 acres of existing habitat represent 48.8% of potential habitat. East of the continental divide, of the 2,656,405 acres of potential habitat, the GIS query concluded that 653,529 acres are in the mature or old size class (>9”), and have at least 25% crown closure. These 652,529 acres of existing habitat represent 24.6% of potential habitat. Total existing habitat, east and west of the continental divide, is 5,782,295 acres.
3. 1900 distribution of age classes- Losensky (1993) provided a ballpark estimate of the extent of mature and old forests in pre-European settlement periods in Montana and Idaho. His findings are illustrated in Tables 1 and 2. The findings of Hessberg and others (1995) and Losensky (1995) were roughly comparable, but the scale was larger and included the Interior Columbia River Basin. For that reason, the findings of Losensky (1993) provided the most relevant historic information.
Table 1. Percent age class distribution of representative cover types in Montana in 1900
| COVER TYPE |
MATURE |
OLD |
MATURE/OLD |
| Ponderosa pine |
16.1 |
54.4 |
70.5 |
| Larch/Douglas-fir |
18.2 |
31.3 |
49.5 |
| Douglas-fir |
33.4 |
6.9 |
40.3* |
| Lodgepole pine |
9.3 |
2.7 |
12.0 |
| Redcedar |
5.8 |
61.8 |
67.6 |
| Redcedar/grand fir |
10.2 |
29.5 |
39.7 |
Table 2. Percent age class distribution of representative cover types in Northern Idaho in 1900
| COVER TYPE |
MATURE |
OLD |
MATURE/OLD |
| Ponderosa pine |
26.6 |
45.5 |
72.1* |
| Larch/Douglas-fir |
15.0 |
19.9 |
34.9 |
| Douglas-fir |
17.6 |
7.1 |
24.7* |
| Lodgepole pine |
4.0 |
3.5 |
7.5* |
| Redcedar |
6.2 |
80.7 |
86.9 |
| Redcedar/grand fir |
25.1 |
18.4 |
43.5 |
* percentages selected for comparative purposes that best represent those cover types that would have had large ponderosa pine, larch, western larch, redcedar, or Douglas-fir snags in historic periods
Determining which cover types to use for comparative purposes- Losensky’s findings show a huge range in the amount of mature and old forests present in pre-European-settlement periods, depending on the cover type. For instance, in Idaho only 7.5% of the lodgepole cover type was mature or old age classes in 1900. Conversely, 86.9% of the redcedar cover type was mature or old in 1900. The redcedar cover type, however, while locally abundant in portions of Idaho, is not a widespread cover type in Region One. Losensky (1993) indicates that in 1900, the redcedar and redcedar/grand fir cover types only represented 2.2 and .1 percent of the landscape in Idaho and Montana, respectively. By comparison, the ponderosa pine cover type represented 13.0 and 13.7 percent of the landscape in Idaho and Montana, respectively. Larch/Douglas-fir represented 9.1 and 17.3 percent of the landscape in Idaho and Montana, respectively. Douglas-fir represented 5.9 and 5.1 percent in Idaho and Montana, respectively. While lodgepole pine was also abundant at 8.1% and 13.4% for Idaho and Montana respectively, very little of the lodgepole cover type would be considered potential habitat west of the continental divide, i.e. most of it is too high or too cold to support larch, ponderosa pine, or redcedar. Consequently, 1900-era percentages of mature/old forests in the ponderosa pine, larch/Douglas-fir, and Douglas-fir cover types provide the best “benchmarks” for comparing existing conditions against historical conditions, west of the continental divide. According to Losensky’s data (Tables 1 and 2), the amount of mature/old age class in 1900 would have ranged from 72.1% (ponderosa pine in Idaho) to 24.7% (Douglas-fir in Idaho).
East of the continental divide, Douglas-fir is a very important cover type for providing large diameter, durable Douglas-fir snags. Unlike lands west of the continental divide, the lodgepole pine cover type often contains scattered Douglas-fir snags, as well as scattered, very old lodgepole pine snags (Losensky 2002). Consequently, 1900-era percentages of mature/old forests in the Douglas-fir and lodgepole pine cover types provide the best “benchmarks” for comparing existing conditions against historical conditions, east of the continental divide. According to Losensky’s data (Tables 1 and 2), the amount of mature/old age class in 1900 would have ranged from 40.3% (Douglas-fir in Montana) to 7.5% (lodgepole pine in Idaho).
4. Comparison of existing habitat to the HRV- In areas west of the continental divide, using Losensky’s (1993) 1900-era findings for the ponderosa pine, Douglas-fir/larch, and Douglas-fir cover types, the mature/old age class in 1900 would have ranged from 72.1% to 24.7% . Existing habitat (mature/old stands in lands capable of producing large diameter, durable snags) represents 48.8% (from RESULTS, Existing habitat). Note that the percentage of existing habitat falls in the middle of the historic range.
In areas est of the continental divide, using Losensky’s (1993) 1900-era findings for the Douglas-fir and lodgepole pine cover types, the mature/old age class in 1900 would have ranged from 40.3% to 7.5%. Existing habitat (mature/old stands in lands capable of producing large diameter, durable snags) represents 24.6% (from RESULTS, Existing habitat). Note that the percentage of existing habitat falls in the middle of the historic range.
5. Distribution of existing habitat based on FIA data- The SILC-generated queries suggest there is little change between the historic distribution of mature/old forests and the current distribution of mature/old forests. Unfortunately, this query tells us nothing about possible shifts from old age classes to mature. Biologists often suggest (Thomas et al 1979) that there has been a shift from old-aged forests (lost due to logging) to mature forests (recruited due to fire suppression). FIA-based analyses provide some indications about how mature and old forests are distributed within lands identified as existing habitat in both low elevation, ponderosa pine-dominated forests, and high elevation, spruce/fir forests.
Hillis and others (2002b) in an assessment of large diameter ponderosa pine and flammulated owl habitat, concluded based on FIA data that large diameter ponderosa pine forests (with a minimum of 8 trees/acre >21”) only represented 11.9% of the amount of old forest that would have been present in historic periods.
Hillis and others (2002c) in an assessment on the availability of lynx denning habitat concluded that old stands in lynx habitat, the majority of which are in subalpine fir habitat types, occurred at levels slightly higher than the historic average based on data from Losensky (1993).
West of the continental divide, Hillis and others findings (2002b, 2002c) seem consistent with our general assumptions about timber harvest, i.e. timber harvest has been heavy in low and mid elevations, but relatively light in high elevations. We collected no information on the availability of mature/old forest in the mid elevation larch/Douglas-fir, redcedar, or redcedar/hemlock cover types, where larch or redcedar snags would have been abundant in historic periods. Intuitively, we might assume that the current status of old forests is somewhere between the low and high elevation forests, i.e. not as devastated as low elevation ponderosa pine forests, but not as abundant as high elevation forests. Additional FIA-based analyses are needed to determine the actual amount of old forest in the Douglas-fir/larch, redcedar, and redcedar/hemlock cover types.
East of the continental divide, all timbered lands occur at relatively high elevations, compared to west of the continental divide. Consequently, Hillis and others’ findings (2002c) for old, high elevation stands might suggest that there has been little shift from old forests to mature. This too is consistent with our general assumptions about timber harvest east of the continental divide, i.e. timber harvest has occurred at low to moderate intensities, and has been fairly concentrated in productive lodgepole pine forests, at high elevations. Additional FIA-based analyses are needed to determine the actual amount of old forest in the Douglas-fir or Douglas-fir/lodgepole cover types.
Management Question #2 - Within existing habitat, how has partial removal logging affected snag densities?
Acres of existing habitat both previously logged by partial removal harvest and not previously logged, were entered into the following equation:
% = (Acres existing, partially-logged x .33) + (acres existing, not logged x 1.0)
Acres of existing habitat
% = (481,157 x .33) + (5,301,139 x 1.0)
5,782,295
94.4% = (481,157 x .33) + (5,301,139 x 1.0)
5,782,295
This suggests that 94.4% of the snags occurring within existing habitat remain following losses from partial removal harvest. The inverse of this percentage (1 - .944) suggests that partial removal logging has accounted for a loss of 5.6% of the snags on the landscape. Keep in mind that this loss is limited to those mature/old forests that have high densities of large, durable snags. This finding in no way reflects the loss of “legacy snags” lost during regeneration timber harvest in young forests.
Management Question #3 - Within existing habitat, i.e. mature and old forests, how has woodcutting affected snag densities?
Acres of existing habitat within 0-50 meters of roads, 50-100 meters of roads, 100-200 meters of rods, and greater than 200 meters of roads, were entered into the following equation:
% = (Ac < 50m x .11) + (Ac 50-100m x .33) + (Ac 100-200m x .44) + (ac >200m x 1.0)
Acres of existing habitat
% = (418,531 x .11) + (691,958 x .33) + (754,591 x .44) + (3,763,440 x 1.0)
5,782,295
75.6 = (46,038) + (228,346) + (332,020) + (3,763,440)
5,782,295
This suggests that 75.6% of the snags occurring within existing habitat remain following losses from woodcutting. The inverse of this percentage (1.0 - .756) suggests that woodcutting, facilitated by road access, has reduced snags by 24.4%.
Cumulative impacts from past partial removal harvest and woodcutting- The combined effects of snag attrition from past partial removal logging and woodcutting, as described in METHODS are reflected in the following formula:
(Ac not partially logged >200m from a road x 1.0) +
(Ac not partially logged 100-200m from a road x .44) +
(Ac not partially logged 50-100m from a road x .22) +
(Ac not partially logged 0-50m from a road x .11) +
(Ac partially logged >200m from a road x .33) +
(Ac partially logged 100-200m from a road x .1452*) +
(Ac partially logged 50-100m from a road x .0726) +
(Ac not partially logged 0-50m from a road x .0363) =
Equivalent acres of high value snag habitat remaining following losses
from partial removal logging and woodcutting.
% = {(3,675,429 x 1.0) + (652,136 x .44) + (549,334 x .22) + (311,423 x .11) + (88,011 x .33) + (102,455 x .1452) + (142,624 x .0726) + (107,108 x .0363)} ÷ 5,782,295
.72 = 4,175,642 ÷ 5,782,295
This suggests that 72% of the snags occurring within existing habitat remain following losses from partial removal logging and woodcutting. The inverse of this percentage (1.0 - .72) suggests that partial removal logging and woodcutting has accounted for a loss of 28% of the snags on the landscape. Again, keep in mind that this loss is limited to those mature/old forests that have high densities of large, durable snags. This finding in no way reflects the loss of “legacy snags” lost during regeneration timber harvest in young forests.
Management Question #4- Since snag durability can be related to live tree exposure to non-lethal fires, how has fire suppression affected the recruitment of durable snags?
Reduced pitch content in live trees due to reduced exposure to non-lethal fires was assessed by searching the literature for information on: 1) the frequency of low and moderate-severity fires by fire regime; 2) changes in total acres burned since 1940 ( the date at which reliable data on acres burned is available, and also the approximate date at which fire suppression became very effective ); 3) evidence of increased fire severity in recent fires; and 4) increases in climax species composition and condition class.
1. Frequency of low and moderate-severity fires within all fire regimes- Fire return intervals are characterized for all fire regimes in Montana and Idaho (Fisher and Bradley 1987, Smith and Fisher 1997, Jones and Barrett in press). Fire return intervals for low elevation, droughty, ponderosa pine cover types are described in those publications as being very short, i.e. 10-30 years. Conversely, fire return intervals for high elevation spruce/fir forests are categorized as being very long, i.e. 200 years plus. In the latter category, fire return intervals are based on the mean stand-replacing events that replace forests in those high elevation sites. Intermediate, non-lethal fire events, while common, are largely ignored in those publications. For instance, in the Bighole drainage in Montana, Losensky (2002) found strong evidence of intermediate, mixed-severity fires at 35-year intervals in a fire regime classified as having 244 year fire return intervals. Barrett (1997) found similar fire return intervals of 25 to 100 years in the Beaverhead National Forest, again in lodgepole-dominated landscapes where the stand-replacing fire return intervals were long. In western Montana, Arno and others (1993) found that within lodgepole pine cover types (typically associated with stand-replacing fires), a majority of their plots showed more evidence of mixed-severity fires than stand-replacing fires. Barrett (1997) further concluded that 70% of historic fires in lodgepole pine were low-to-moderate severity. Barrett and others (1997) concluded that fire frequency along the Idaho-Montana state line was quite high between 1500 and 1880. Their fire frequency was several times higher than the 133 to 244 mean fire return intervals mapped for the majority of that landscape. Gabriel (1976) described a mix of non-lethal and mixed-severity burns that had occurred at 20 to 40 year intervals in the Bob Marshall Wilderness, including the presence of lodgepole pine with multiple fire scars. Gabriel’s findings are particularly interesting since they were largely derived within even-aged lodgepole pine stands in the Bob Marshall that originated from stand-replacing events, even though those stands had substantial evidence of intermediate disturbances. In stark contrast to the previous findings, Romme and DeSpain (1989) concluded that intermediate, mixed-lethal fires were very uncommon in very high, cold plateaus such as Yellowstone National Park.
Several explanations for why intermediate fires are often ignored in the fire regime literature are offered. Barrett (1997) suggests that in habitats characterized by understories of elk sedge or pinegrass, the fuels are often too light to induce scarring during mixed-severity or non-lethal fires. Gabriel (1976) infers that scattered fire scar evidence of intermediate events if often missed or ignored, in favor of more conclusive, widespread evidence of major stand-replacing events. Barrett (1993) further concludes that large events like the fire of 1726 consume all previous evidence of mixed-severity events, thus effectively “wiping the slate clean.”
2. Changes in wildfire occurrence - Overall, fire occurrence has declined substantially in Region One since European settlement and the onset of fire suppression. Barrett and others (1997) concluded that within the interior Columbia Basin, big fires historically occurred at the rate of 1 to 2 per decade prior to 1900. Compare that to the last 62 years since 1940 in which 1988 and 2000 were the only years in which the acreage burned came close to historic levels. Hillis and others (2000a) found an 82% reduction in total fires (all severities) in Region One from 1940 to 1987 compared against the historic mean. After large fires burned in 1988 and 2000, however, the reduction in large fires for the 1940-2000 period dropped to only 28% below the historic mean.
3. Increase in fire severity of 1988, 2000, and 2001 fires - Williams (1996) predicted that a loss in low and moderate-severity fires due to fire suppression, would be accompanied by an increase in fire severities in future fires. Recent fires in 1988, 1994, 2000, and 2001 seem to suggest fires are burning at higher-than-normal severities. The ~230,000 acre Canyon Creek Fire of 1988 burned almost totally at stand-replacing fire severity, whereas the mix of fire regimes in the area would have suggested that “normal severity” should have included more of a mix of severities (Losensky pers comm.). The 1994 Henry Creek Fire of 1994 burned at stand-replacing severity within non-lethal and mixed severity fire regimes. Both the Lolo and Bitterroot Fires of 2000 (USDA 2001, USDA 2002) burned at hotter-than-normal severities at low elevations, although both areas burned at a more normal mix of severities at higher elevations.
4. Increases in climax species composition and changes in condition class- The loss of fires, including intermediate non-lethal and mixed-severity fires, has resulted in an increase in climax conifers. Hartwell and others (2000) concluded that after a century of fire exclusion in high elevation portions of the Bitterroot National Forest, subalpine fir increased from 13% to 33% of the basal area. During that same interval, lodgepole pine increased from 31% to 44% and spruce increased from 7% to 8%. Not unexpectedly, Douglas-fir, the early-seral species on those high elevation sites, declined from 10% to 4%.
Schmidt and others (2002) categorized stands by condition class. Stands where no fire return intervals had been missed were designated condition class 1. Stands where one fire return interval had been missed were designated condition class 2. Stands where two or more fire return intervals had been missed were designated condition class 3. Hillis and others (2002b) found that 70% of old growth ponderosa pine flammulated owl habitat west of the continental divide was in condition class 3, suggesting many fire return intervals had been missed in that area.
Summary of management question 4:
1) Non-lethal and mixed severity fires were historically common, even at mid and upper elevations, and resulted in a substantial amount of basal scarring on live trees;
2) The occurrence of fires, lethal and non-lethal, has declined drastically since the onset of fire suppression;
3) The fires of today are hotter than they were historically with proportionately less low and mixed-severity fires suggesting more trees are being killed and less are being scarred; and
4) Increased climax conifers and elevated condition class suggest the fires of the future will be even hotter than those of the last few years.
Overall, snag attrition is cumulative - i.e. loss from partial harvest and woodcutting, and changes in age distribution and durability are multiplicative.
Changes in age classes, east and west of the continental divide- West of the continental divide, there is conclusive evidence of a massive shift from old growth stands to mature stands in low to mid-elevation ponderosa pine-dominated landscapes. As a consequence large durable snags have decreased substantially on those landscapes. On cooler mid-elevation landscapes dominated by Douglas –fir/western larch, and western red cedar, we infer that there’s been a similar shift from old growth to mature stands. Subsequently large durable snags would have experienced a significant reduction from historic levels. Further analysis is needed in westside Douglas-fir/western larch and western red cedar-dominated landscapes to further quantify changes in the distribution of old and mature stands.
On the eastside of the continental divide, there’s no evidence of any substantial shift in age classes that would affect snag densities. However, data are limited for the eastside and more focused analyses are needed.
Losses from partial removal harvest and woodcutting – Losses from partial removal harvest and woodcutting are substantial and widespread. However, our analyses underestimated snag losses because:
Mature and old growth stands were lumped because SILC cannot accurately differentiate mature from old growth stands across Region One.
Snag densities are substantially higher in old growth stands than mature stands.
In a landscape of mature and old growth stands timber harvest will disproportionately target old growth stands.
Therefore by lumping mature and old growth stands in the analysis we will underestimate the losses in snags from woodcutting and partial removal logging.
Losses from regeneration harvest - Early seral stands historically provided a high density of snags. Findings from Bate and Wisdom (in prep) conclude that regeneration timber harvest retains few snags and in no way mimics the historic distribution of snags. Losses of snags due to regeneration logging were not assessed in this analysis.
Losses in durability from fire suppression – Loss of low to moderate severity fires have been dramatic and widespread. These fires are critical for the recruitment of large snags and for increasing snag durability. While the Forest Service has recognized the need to reintroduce fire into non-lethal fire regimes, the Forest Service has not fully embraced the need to consider reintroduction of fire that includes all natural-occurring severities at historically normal acreages within historically normal fire return intervals.
Reliability of satellite-generated findings- Potential significance of the error inherent in SILC. Satellite imagery works well for detecting major changes in vegetation, i.e. forest from non-forest, dense forest from clearcuts, etc. In an analysis of habitat availability and connectivity for American martens (Hillis et al in prep), SILC data accurately predicted habitat patterns at relatively small scales. Conversely, in an analysis of old growth ponderosa pine habitat, SILC data substantially underestimated actual habitat, because it could not accurately differentiate old ponderosa pine forests from mature forests. In that case, however, when the results were: 1) compared against fine-scale data; and 2) adjusted to accommodate the error; then 3) the results were very meaningful compared against the historic levels of old ponderosa pine forest, i.e. little of the historically-available habitat remained. Because SILC data is polygon-based, it can detect changes in patterns and patch sizes, a critical requirement of many wildlife analyses.
Reliability of FIA-generated findings - FIA is an excellent tool for summarizing abundant forest components such as tree size class, tree species, or tree stocking categories. Based on statistical analyses (Leach 2002) such data is accurate down to a 5th code hydrologic unit (~100,000acres). For rare or highly variable forest components. i.e. old ponderosa pine trees or large, durable snags, FIA data may only be reasonably accurate at much large scales (~1,000,000 acres). Since FIA is plot-based, it cannot be used for assessing spatial arrangement or patch sizes. Wildlife analyses that combine SILC-based queries with FIA-based analyses are likely to provide the best answers.
Reliability of turn-of-the-century data for determining HRV- Critics of historic “snapshot data” often contend such data may not represent historically normal conditions. For instance, they contend the effects of the little ice age make historic data useless for comparative purposes. Losensky’s (1993) historic percentages of mature/old forest are consistent with our understanding of the fire regimes for his respective cover types (Arno et al 1994, Jones and Barrett in press). Additionally, fire history data (Gabriel 1976, Barrett 1997, Losensky 2002) indicated fires occurred at “normal” intervals back through the little ice age. Consequently, while the little ice age represents a significant perturbation in world weather, it apparently had little effect on historic age class distribution, as it was recorded in the early 1900s.
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[1] 40% and 25% crown closures were selected for westside and eastside forests, respectively, to exclude very open “forest savannah” situations, not generally considered suitable for nesting pileated woodpeckers
[2] Since the effects of logging and woodcutting are cumulative, the factor .1452 is the multiple of the loss from logging (.33) times the loss from woodcutting (.44). The factor .076 is the multiple of .33 (loss from logging) times .22 (loss from woodcutting within 50-100m of roads). The factor .0363 is the multiple of .33 (loss from logging) times .11 (loss from woodcutting within 0-50m of roads).
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