Influences of fuel reduction treatments on the nutritional ecology of deer in northeastern Washington

Iver Thorin Hull

Historically, low-elevation dry forests in the inland northwest were structured to support light fuel loads and frequent low-severity fires. More recently, fire suppression has shifted the structure of these forests to have thick, continuous overstories with densely-stocked trees, and heavy fuel loads that can lead to high-severity, stand replacing wildfires that may threaten human safety and further alter natural forest communities. In addition, closed canopy, densely-stocked forests reduce light penetration to the forest floor, which can limit production of herbaceous and woody understory vegetation that can serve as forage for native herbivores. To restore historic conditions, promote healthy forests, and reduce wildfire risk, forest managers have implemented fuels reduction treatments. To determine whether these treatments improve the quality and quantity of forage resources for deer, we measured biomass, plant species composition, and nutritional quality of understory vegetation in treated and untreated forest stands that ranged from 2 - 20 years since treatment and 2 - 100% canopy cover in the Colville National Forest of northeastern Washington. We also measured diet quality and composition, nutrient intake, and foraging mechanics of mule deer (Odocoileus hemionus) and white-tailed deer (Odocoileus virginianus) using bite-count methods with tractable, hand-raised deer. In addition, we tested the ability of LiDAR-derived metrics of understory vegetation density to determine understory vegetation biomass remotely. Our results showed that understory vegetation biomass roughly doubles with a 50% reduction of canopy cover from fuels reduction treatments, and that the biomass of plants that are most important to deer increases with time since treatment, but begins declining approximately 14 years post-treatment. Increases in understory vegetation biomass, in turn, allowed deer to harvest food faster and increase their daily intake of digestible energy, which allowed them to meet maintenance requirements of dietary digestible energy and protein in our sample sites. Our LiDAR-based estimates of understory biomass correlated to field sampling when the amount of time between sampling methods was constrained to 3 years and canopy cover was < 50%. We were able to quantify the necessary data acquisition requirements for this type of remotely-sensed estimate of understory vegetation.

Influences of fuel reduction treatments on the nutritional ecology of deer in northeastern Washington

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