Effects of young-growth management on Sitka black-tailed deer in southeast Alaska

Abstract

Commercial logging was the dominant industry in southeast Alaska during the second half of the twentieth century. Logging practices have left a landscape legacy of regenerative forest types such as clearcuts and second growth. Second-growth forest occurs about 20-30 years after a clearcut and is relatively unproductive compared to other forest types. To enhance productivity, second-growth is often thinned to a lower density of standing timber, this process is referred to as pre-commercial thinning (hereafter, thinning). Sitka black-tailed deer (Odocoileus hemionus sitkensis) are an important cultural and subsistence resource across southeast Alaska. How thinned forests affect deer is not well known. To better understand how forest management is affecting local populations of deer in southeast Alaska we examined how thinning treatments impact deer browse intensity and snow accumulation on the forest floor. In this thesis, we presented two studies that examine the effects of thinning on deer habitat quality and deer access to forage. In the first study of this thesis, we quantified browse intensity in recently thinned (≤4 years post thinning) and adjacent old-growth forests. We also explored the immediate effects of thinning and slash (felled trees left on the forest floor) on forage availability. We performed a pairwise comparison of browse intensity between thinned and adjacent old-growth forests and modeled the effects of thinned forest characteristics on browse intensity. In the second study of this thesis, we quantified maximum snow depth in thinned, unthinned second-growth, old-growth and unforested (control) habitat types. Forest structure and composition affected how snow accumulates on the ground. Snow can impede the movement of ungulates species, such as deer, and reduce available forage. We evaluated how different forest types accumulated snow in southeast Alaska to better understand the implications on winter habitat quality for Sitka black-tailed deer. To quantify browse intensity in thinned and adjacent old-growth forests, we conducted browse surveys in recently thinned stands (2017 to 2021) and adjacent middle to high volume old-growth forests. We established 50m transects and surveyed plots every five meters to quantify browse of Vaccinium sp. (blueberry and huckleberry), a preferred deer forage species. In the second study, we measured snow depths throughout thinned, unthinned, old-growth and unforested sites to identify if these forest types accumulate snow differently. We also measured forest structure variables to use as predictors when modeling maximum snow depth. Transects were 70 meters long, and snow depth and forest structure data were collected every five meters. Snow depths were measured four to six times throughout the winter. The maximum depth of each transect point was recorded and paired with the forest structure variables. For both studies, we used nonparametric tests and generalized linear mixed models to understand the interactions between forest types and their maximum snow depths or percent of a Vaccinium sp. plant browsed. From the first study, we concluded that browse intensity was significantly different in thinned and oldgrowth forests (P<0.01). We learned that thinned stands with more slash reduce browse intensity. Slash (vertical obstruction) volume and time since thinning (metrics of slash decay) best explained percent of a plant browsed. From or second study, we found that thinned forests accumulated the same amount of snow on the forest floor as unforested sites. Moreover, old-growth and unthinned sites accumulated snow on the forest floor comparably during a relatively normal snow load year. Our findings regarding browse intensity showed that thinned forests have a delayed benefit to deer because of slash abundance. Our observations regarding snow accumulation showed that thinned forests have little value to deer in a winter with deep snow accumulation. Managers can use this information to better understand the extent of forage available to deer in recently thinned forest habitat. Our findings also demonstrated that the implementation of thinning treatments that minimize slash volume and accelerate decomposition will enhance benefits for deer.