AUTHORS: Iver T. Hull, Stephanie L. Berry, Lisa A. Shipley – School of the Environment, Washington State University; Chris O. Loggers, Colville National Forest, U.S. Forest Service, Kettle Falls, WA
ABSTRACT: Inland northwestern forests of the United States were historically structured to support light fuel loads and frequent low-severity fires. For over 100 years, fire suppression has shifted these forests to have thick, continuous overstories, dense tree stocking, and heavy fuel loads, which increase the chances of catastrophic wildfires that threaten human safety and further alter natural forest communities [1]. To restore historic conditions, promote healthy forests, and reduce wildfire risk, a group of state, federal, and private forest managers in northeastern Washington State have used commercial thinning to remove a portion of the overstory canopy and reduce fuel loads in the Colville National Forest. In addition to unnatural wildfire risks, closed canopy, densely-stocked forests limit light penetration to the forest floor, which can limit production of herbaceous and woody understory vegetation. Therefore, these thinning projects have the potential to improve productivity of understory forage resources for native herbivores that reside in these forests, including mule deer (Odocoileus hemionus), white-tailed deer (Odocoileus virginianus), Rocky Mountain elk (Cervus elaphus), and moose (Alces alces). To examine the effects of these thinning projects on the quality and quantity of forage resources for mule deer and white-tailed deer, we measured biomass, plant species composition, and nutritional quality of understory vegetation over four seasons in 78 stands that ranged from 0 – 100% canopy closure, 1 – 20 years post-thinning, along with unharvested stands. Within these stands, we also measured diet quality and composition, nutrient intake, and nutritional carrying capacity using bite-count methods with tractable, hand-raised deer.
Our preliminary results indicate that, non-conifer forage biomass increased as canopy cover declined (Fig. 1) and with time since thinning (Fig. 2). Additionally, we found that digestible energy and protein in deer diets decreased with high canopy cover. The amount of time spent active, based on accelerometer data calibrated with behavioral observations, showed that deer spent more time active as canopy cover increased and less time active as available biomass increased. Deer also spent a lower proportion of each day traveling with time since thinning. Deer bite and harvest rates increased with years following thinning and greater available biomass. Likewise, dry matter intakes increased with higher available biomass. Results from this project suggest that deer benefit from fuel reduction thinning projects. Higher available forage led to higher nutrient consumption and daily digestible energy and protein intakes. Daily intake of digestible protein and energy are directly related to deer fitness because they affect pregnancy, twinning rates, and fawn survival, which are good metrics for population sustainability [2]. Our data also indicate that the benefits of thinning increased until 15 – 20 years post-thinning, when canopies begin re-closing and light penetration again becomes reduced. Results from this project can help identify how deer adapt to changing, human-influenced habitats, aid in management decisions, and promote interdisciplinary natural resource management that supports biodiversity and sustainability.
