Justification for Managing Below Levels of Down Woody Debris as Specified Within the Willamette Late-Successional Reserve Assessment in the 2007 Plantation Thin, Mt. Hood National Forest, Clackamas River Ranger District
1/25/07
Proposed Action: In the Collawash and Roaring River Late-Successional Reserves, commercial thinning is being designed to accelerate the development of mature and late-successional stand conditions. These habitats are important because they are habitat for late-successional and old-growth related species, including the northern spotted owl. Trees would be thinned at a relative density of 20 to 35. Trees would not be cut if they are greater than 20 inches in diameter (at a height of 4.5 feet). Hardwood trees across a range of size classes would be favored, including large trees that occupy mid-canopy and higher positions.
Skips (no treatment pockets in the unit) would be created that would vary in size and would comprise a minimum of 10% of each unit. Skips would be 0.25 to 1.25 acres or larger where appropriate based on site-specific features. Where riparian reserves overlap late-successional reserves, the protection buffers adjacent to streams may be counted as skips. Gaps are comprised of openings and heavily thinned pockets. Three to 10% of the resultant stand would be in heavily thinned patches (i.e., less than 50 trees per acre), or in openings up to ¼ acre in size, to maximize individual tree development, encourage some understory vegetation development, and encourage the initiation of structural diversity.
An average of 5 trees per acre would be girdled and 2 per acre would be felled after harvest to enhance the current and short-term down woody component in the stands. In addition, up to 3 trees would be inoculated or topped to potentially create snag habitat in the future. Some of the gaps would be planted to enhance the tree species diversity in the units.
Current Condition of Proposed Treatment Units: The LSR stands within the 2007 Plantation Thinning project occur within the RoaringRiver (207A) and Collawash (RO 210) LSRs and comprise approximately 955 and 354 acres, respectively. The stands proposed for treatment occur within the Western Hemlock zone. These stands are plantations between 32 and 61 years old and are considered mid-seral stands. Canopy closure is 74-98% with relative densities greater than 80. All the units were originally clearcut back in the 1940’s to 1970’s and a few have been commercially thinned. Few snags or residual trees were left in the stands after harvest. Currently, trees are beginning to experience growth suppression and some mortality is occurring; mainly in the trees smaller than 10” DBH.
1998 North Willamette LSR Assessment Strategy: At the time this LSR assessment was developed the Mt. Hood National Forest LSR Team reviewed the work of Andrew Carey to determine the amount of down wood that should be retained to maximize small mammals that prefer old growth conditions. Carey (Carey and Johnson, 1995 found that small mammal densities were maximized at 15-20 percent coarse woody debris. He also stated that 5-10% cover would not allow the mammals to reach their potential abundances. For this reason the LSR assessment team on the Mt.HoodNational Forest set thresholds of down wood for stands that were to be treated silviculturally. Younger mid-seral stands had a minimum of 10 percent down wood as a requirement and older mid-seral stands less than 80 years were required to meet a 15 percent cover of down wood objective.
Spotted Owl Habitat: Nesting and roosting habitat may function as foraging habitat but there are times when there are insufficient large nest trees with the correct structure for foraging habitat to be nesting habitat. The most limiting of these habitats is nesting habitat, since it requires large trees usually over 31 inches in size that have broken tops or side entrance cavities. This is a specialized structure that requires many years to attain. Often legacy trees left over from fire or windstorms create the conditions for these nest trees.
Kit Hershey’s research on spotted owl nesting structure (Hershey, Kit OSU 1995) recognized the need to create mixed species, multilayered stands, with trees, snags, and down woody debris of various ages and sizes and that tree diameter distributions should follow the reverse-J shaped curve found in old forest habitat where there are many small trees and a progressively fewer number of large trees. Hershey also points out the nest sites had more densely multilayered canopies than random sites as evident from the greater density of tree species, especially trees <21 inches dbh and <123 feet in height. Hershey’s thesis points out that, “Nest sites also had greater horizontal and vertical diversity in stand structure than random sites in all physiographic provinces. Greater horizontal structural diversity at nest sites was evident by the higher density and basal area of understory trees (trees <21 in. dbh) and was especially prominent in the Klamath hardwood understory. The following is a histogram found within Hershey’s paper displaying this concept.
North’s (North, Malcolm et al 1999) work on spotted owl foraging habitat pointed to both snags and height class diversity as important factors in the use of a stand for spotted owl foraging. North states that, “stands with high tree height diversity would have low perches available for hunting and yet enough open space to facilitate sub-canopy flight and prey capture.North cautions that, “stand structure other than snag volume and tree height class diversity should not be discounted because components of old growth are dynamically linked.” Large diameter trees are needed first in a stand before high volume snags are produced and these in turn become big course woody debris pieces. A high diversity of tree heights is usually found in stands with a range of tree diameter sizes and a mix of shade tolerant and intolerant species. Singular focus on 1 or 2 stand characteristics could skew management from considering the dynamics of a complex old-growth ecosystem in its entirety.”
Mt.HoodNational Forest LSR Strategy for the 2007 Plantation Thinning Project
Down Woody Debris: The following diagram displays a histogram of the current condition of down woody debris within the surrounding landscape of the project area and is based on field data taken within the WestsideLowlandConifer-HardwoodForest stand types in the Middle Clackamas Watershed. Surveys were completed in the summer of 2006 and used data collected using the Bate et al. (1999) survey method. Down wood data was collected using the Line Intersect Method. The analysis area is currently estimated to be comprised of 43% large tree structure, 37% small-medium tree structure and 20% in the open and sapling/pole stage. It should be noted that a small portion of the 2007 Plantation Thin LSR proposed harvest units (i.e Collawash LSR units) are located just south of the Middle Clackamas Watershed. However, it is felt that due to the proximity of these units to the sampled areas and the similiarity of past management, this histogram is representative of the current conditions of these units as well.
Figure 1: Distribution of the total area (excluding areas treated since 1996) of the WestsideLowlandConifer-HardwoodForest Vegetative Condition among down wood percent cover classes for down wood 3” diameter.
Figure 2 displays the estimated Desired Future Condition for down woody debris percent cover levels within the RoaringRiver and Collawash LSRs. This histogram is based on the percentage of structural classes that is the goal for the LSRs contained within the North Willamette LSRA. This goal is 70% late-seral structure. In addition the histogram was devised from the unharvested areas of the Westside Lowland Conifer-Hardwood Vegetative Conditions inventory plots as contained in DecAid. The assumption here is that the levels of down woody debris found within unharvested plots and summarized in data presented in DecAid is utilizing the most current science and is the basis for the DFC of the RoaringRiver and Collawash LSRs.
Figure 2: Distribution of the unharvested area of the WLCH Vegetative Conditions among down wood percent cover classes for down wood 4.9” diameter, based on 247 unharvested plots.
By comparing the two histograms above it is apparent that overall the current landscape in the Middle Clackamas Watershed (including all land allocations) has levels of down woody debris similar to the estimated DFC of the LSR’s as described above. In fact, it appears that there is a higher percentage of the landscape in the high percent cover classes of 16%+ (i.e. tails of histogram) in the current landscape (4%) as compared to the DFC of percent cover classes of 16”+ of 1.5%. In addition over 50% of the current landscape is currently at the 50% (i.e average 6% cover in primarily decay class 1-4) or above tolerance level for down woody debris (Mellen et al 2006). Currently the landscape does not appear to be limiting in this structure. (I.E. It should be noted here that the LSRA direction is to manage for moderate to high levels of down woody debris within the late-successional reserves.)
Late-Successional Habitat -The Roaring River LSR (207A) is 73,282 acres in size and is comprised of approximately 44% late-successional habitat. Late-successional as defined in the LSRA are stands dominated by trees over 21” dbh. Canopy closure is at least 40%, with over 60% considered closed stands. Approximately 24% of this LSR is in mid-seral and are defined as stands with at least 40% canopy cover and dominated by trees in the 8-21” dbh size class. Many of these stands are very dense and have only one or two canopy layers. About 29% is in early-seral and is defined as stands that currently function as openings up through closed sapling/pole stands. The remaining 3% of this LSR is in non-forest. The Collawash LSR (210) is 16,170 acres and is comprised of 74% late-successional, 4% mid-seral, 21% early seral, and 1% non-forest.
The desired future condition of late-successional stands is 70% within the ClackamasBasin as noted in the North Willamette LSRA.
Roaring River LSR– As demonstrated above, this LSR currently only has 44% late-successional habitat and is well below the DFC of 70%. In addition, areas of fragmentation caused by timber harvest exist in the eastern and southern edge of the Roaring River LSR (i.e. locations of the proposed harvest units). These areas have many stands less than 80 years of age that may benefit from young stand thinning designed to accelerate development of late-successional characteristics (USDA 1998).
Currently the LSR is well below the DFC of late-successional habitat. The structure currently lacking in this habitat stage is large tree structure, multi-species of trees and a multi-layered assemblages of trees. As described above, the landscape appears to be within the DFC for down woody debris and presumably snags. The goal of the proposed action is to accelerate development of these structures that are currently lacking in this LSR
Collawash LSR - Although the Collawash LSR already meets the goal of 70% late-successional habitat, much of this habitat, especially in the north end of the LSR is currently fragmented by early seral stands created from past timber harvest. These intervening stands are mostly young stands, most of which are not even providing dispersal habitat. Silvicultural treatments that would accelerate the development of dispersal habitat in the short-term and late-successional habitat in the long term would aid in reducing the effects of the fragmentation in these areas (USDA 1998). The units proposed for treatment within this LSR are between 40-50 years. They either are not quite dispersal habitat or are low quality habitat for dispersing owls. It is likely these stands are currently contributing to the current fragmented nature in the northern part of the LSR. The goal of the proposed action is to accelerate the development of high quality dispersal habitat within these young managed plantations and decrease the amount of fragmentation currently present in this LSR.
Resource Trade-offs of Proposed Action
One trade-off is whether to maximize the habitat for small mammals (i.e. spotted owl prey) in the early stages of this project; or whether to emphasize the large-tree structure and multispecies/ multilayered assemblages of trees that would also benefit the northern spotted owl by producing more nesting/roosting habitat. Although the spotted owl requires a good amount of down wood to encourage prey species (northern flying squirrels) it also needs large trees (over 20 inches) a dense canopy closure (above 60%) and a multi-storied stand. The limiting factor for spotted owls is large-tree structure and a multi-storied/multi-species assemblage of trees within these LSRs. It does not appear to be down woody debris.
Although a high amount of down wood could be produced by retaining more tree density which would increase suppression mortality and down wood, the objective is better met by creating a more open stand through silvicultural treatments that would create a flush of understory growth to create the multi-storied stand and increase tree size and growth. Both understory shrub species and conifer regeneration contribute to a multi-storied layer.
In the no treatment option, coarse woody debris is being managed at its highest potential level, but we are failing to manage for structure. In the no treatment option, the overstocked condition of these stands would result in stands with reduced vigor, smaller diameters, increased mortality, and increased susceptibility to stressors such as insects, diseases and weather. In addition, uniformity of these plantations would likely remain unchanged in terms of species composition, vertical or horizontal structure for several decades.
Although the goal of reaching 15 percent down wood cover as stated in the LSRA is desirable to maximize small mammal densities it may not always be feasible when creating conditions that are conducive to accelerating other stand characteristics of old growth. In addition, managing for 15% down wood cover only occurs on less than 5% of the landscape based on historical range of variability. One hurdle to developing all of the desirable characteristics is economics. The economics analysis contained in the supporting documentation for this project reviews two methods of timber harvest using a relative density of 30 to determine if it were economically feasible to thin the stands and either leave down trees (in a Stewardship Contract methodology where goods can be traded for services) and using a standard timber sale contract (where trees could not be left on the ground but would have to be created after the sale was over). In most scenarios it was not economically feasible to create down wood at the 10 and 15 percent cover standards.
Because of the difficulty in reaching the higher percentage of down wood throughout the majority of the stands, two alternative objectives were determined that will achieve the goals of the NWF Plan standards and guidelines but differently than originally proposed in the North Willamette LSRA. The first alternative method to achieving more down wood is to rely on the proposed skips to increase the amount of down wood in the moderate to long term. The second strategy is to manage the young plantations within these RoaringRiver and Collawash LSRs within a range of down woody debris levels based on the DFC shown in the above histogram. This will meet the 30-50% tolerance level for all of the species and the 80% for some. This strategy still depends on natural mortality to provide the down wood and may take several decades to achieve. Currently there are many stands in the surrounding landscape that are currently fully functional as high-density small mammal habitats and are above the historical range of variability for moderate to high levels of down wood. Currently down wood is not as limiting asother structures such as large-tree structure, multi-layered and multi-species assemblages of trees.
Comparision of No Action & Action Alternative Using Data Generated from FVS Runs
Table 1: Current Condition of Snag and Down Woody Debris Levels in 2007 Plantation Thin Units Proposed for Treatment Within the Roaring River LSR– Two Stand Types (Based on age)
Average Current Stand Age / Average Current Trees Per Acre (TPA) / Basal Area (BA) / Quadratic Mean Diameter / Common Tree Species / Current DWD Levels 3”DBH at small end (percent cover) / Current Snag Levels 10” DBH32-49 yrs. / 641 TPA / 440 BA / 11.2 / DF and WH / 4.9% / 1.8 snags/acre
50-61 yrs. / 513 TPA / 551 BA / 14.0 / DF and WH / 6.4% / 1.3 snags/acre
Table 2: Estimated Condition Post-harvest of Snag and Down Woody Debris Levels in 2007 Plantation Thin Units Proposed for Treatment Within the Roaring River LSR – Two Stand Types (Based on age): Proposed Action = 30 RD with 10% skips and 3-10% heavy thin (40tpa) and gaps.