The ecological effects of forest thinning treatments on three sciurid species in the Lake Tahoe Basin
AuthorHeckendorn, Katrina Ann
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Forest thinning is a management strategy that is being implemented in coniferous forests in the Lake Tahoe Basin in institutional efforts to decrease fire severity, protect property, and restore pre-settlement structure and composition to those forests. Contemporary forests in the Lake Tahoe Basin are denser, less structurally heterogeneous, and have accumulated more fire fuels as compared to historic forest conditions. While forest thinning efforts might be expected to benefit the ecosystem as a whole by returning the forest to more historic states, mixed effects might be expected on individual species that have distinct habitat requirements. Some species of small mammals, important components of conifer communities, may benefit from more open forests, while others may be adversely affected. To determine salient ecological effects associated with forest- thinning activities in the Lake Tahoe Basin, we analyzed the responses of three representative species of sciurids (squirrels and chipmunks). We assessed: (1) the responses of the species to the changes in vegetation following forest thinning, (2) the responses of the same species to disturbances caused by actual entry and cutting during forest thinning, and (3) the differences in the responses of two population characteristics exhibited by the species--density and annual turnover of individuals. A priori we expected: a deleterious forest -thinning effect on northern flying squirrels (Glaucomys sabrinus), due to their arboreal nature and apparent preference for old growth forests; a relatively neutral forest-thinning effect on long-eared chipmunks (Tamias quadrimaculats), because they are semi-arboreal and found in both open and moderately dense forests; a beneficial forest-thinning effect on golden-mantled ground squirrels (Spermophilus lateralis), since they are terrestrial and found in open forests and forest edges.To assess responses of the three representative species to forest-thinning practices, we conducted a mark-recapture study of small mammals and vegetation surveys over five years. We created linear mixed effects models and used them to determine the correlations betweenvegetation characteristics, treatment, and population parameters for each species. We used annual turnover of individuals of each species as a proxy measure for survival and emigration after forest-thinning. Higher annual turnover of individuals indicated lower survival and higher emigration rates. We assumed higher densities and lower annual turnover of individuals indicated more stable and persistent populations.We found that none of the representative species were affected by forest thinning over the five year period of the study despite significant decreases in shrub cover, canopy cover, small tree densities, medium-sized tree densities, and snag densities. It is possible that there are delayed treatment effects; therefore, continued monitoring is necessary. To avoid possible problemswith using population density exclusively to determine small mammal responses to forest thinning, future small mammal monitoring projects should focus on population dynamics measured over multiple years.