USING FORAGING DYNAMICS TO ANSWER LANDSCAPE MANAGEMENT QUESTIONS: THE NUTRITIONAL ECOLOGY OF BLACK-TAILED DEER
Ulappa, Amy Colleen
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Black-tailed deer (Odocoileus hemionus columbianus) and other forest herbivores depend on abundant and nutritious understory vegetation found in open-canopy and young, early-seral forests. Forage management activities such as logging, thinning and using silvicultural herbicides have the potential to alter the quality and quantity of forage available to herbivores. Therefore, I compared nutrient intake and estimated nutritional carrying capacity of black-tailed deer between stands that received herbicide treatments paired with those that did not in Douglas-fir/western hemlock forests of western Washington, and investigated how these responses change as forests age from early seral stages to canopy closure (2 – 20 years post-harvest). I measured understory vegetation biomass, plant species nutritional quality, and overstory characteristics of each forest stand. Additionally, I measured diet composition, diet quality and nutrient intake of tractable black-tailed deer within each paired stand. I found that herbicide use decreased the amount of understory biomass useable for foraging deer and decreased their daily digestible energy intake, especially in the first 3 years of stand establishment. Even so, early seral habitat, regardless of herbicide use, provided more forage and higher daily energy intake for deer than closed canopy stands (14 yrs +) or mid-seral stands. Using data from these experiments, I also investigated how the resolution of data collection (e.g., plants vs. plant parts) influences estimates of plant selection and nutritional carrying capacity, and how well measures of forage abundance and quality predict nutrient intake of black-tailed deer. I found that treating plant parts (i.e., stems and leaves) as separate food items did not influence how plant species were categorized as selected, neutral or avoided, but increased estimates of nutritional carrying capacity. In addition, measures of forage resources that included both biomass and quality or selection status by deer better predicted daily digestible energy intake of tractable black-tailed deer.