Responses of Dry Forest Understory Diversity to Thinning Intensity and Burning: The Importance of Time, Space, and Analytical Approach

Mechanical thinning and prescribed burning are commonly used to restore dry, mixed-conifer forests that historically experienced frequent fire. Although these treatments successfully reduce fuel loads, their ability to achieve ecological objectives, such as promoting native plant diversity, is less certain. My thesis research examined how temporal and spatial scales of observation and the approach to analyzing scale-dependent data influence our understanding of understory vegetation responses to thinning intensity and burning. I used long-term data on understory species richness from a restoration experiment in central Washington. I found no effect of thinning intensity alone and scale-dependent responses to burning. For example, annual richness increased over time in burned plots, particularly at small spatial scales, suggesting enhanced recruitment from early-established populations and little perennial expansion. I also found that the analytical approach used to address common challenges of large-scale, long-term experiments, such as variation in pre-treatment conditions and the loss of sample units over time, can affect the conclusions drawn from these experiments. However, careful specification of research questions and consideration of data limitations can yield insights into these conclusions. This research highlights long-term ecological benefits of prescribed burning and the need for measurements over time and among spatial scales, as well as the careful evaluation of analytical approaches, to clarify whether fuel-reduction treatments meet the ecological objectives of dry forest restoration.