Dry forest fuel and biodiversity management in the Pacific Northwest

Abstract

Wildfire suppression and exclusion of Indigenous fire have caused profound changes in many historically frequent-fire ecosystems of the world. In dry and historically frequent-fire forests of interior western North America (‘dry forests’), fire deficits have led to uncharacteristically high fuel loading and continuity. To address these changes and reduce potential for severe wildfire, managers commonly apply fuel treatments such as thinning and/or prescribed burning. Fuel treatments can reduce the potential for severe wildfire, open stand structure, and improve habitat for open-forest organisms at local spatial scales in years immediately following treatment implementation. However, many key knowledge gaps remain about the long-term (>10 year) dynamics of fuel profiles and stand structures following fuel treatment, and about the spatial impacts of fuel treatments on dry forest biodiversity. My first chapter is a literature review briefly synthesizing the state of knowledge of long-term treatment effects on fuel profiles and stand structure and exploring opportunities for new research directions. I found that sufficient literature existed to develop general expectations about fuel and stand structural responses 1-2 decades following fuel treatment types of thin-only, burn-only, and thin-plus-burn. However, I also found several knowledge gaps including: substantial variability of responses reported within treatment types remained largely unexplored and contributed to large statistical uncertainties, the common practice of comparing long-term results against pre-treatment references may not always have reflected management goals, and low sampling frequency in published long-term studies contributed to uncertainty about continuous trajectories of responses. My next two chapters began to address these knowledge gaps. My second chapter used a long-term experimental site to explore the fine-scale (plot-level) drivers of fuel profile and stand structural variation 15 years following burn-only, thin-only, and thin-plus-burn treatments. I found that pre-treatment conditions were positively associated with long-term responses for most studied response variables, and that treatment intensity was negatively associated with long-term basal area, tree density, and potential for active crown fire. My third chapter used a small but intensively sampled dataset to explore trajectories of fuel profiles and stand structure for 15 years following thin-plus-burn treatments. I found that woody surface fuel showed nonlinear patterns of response to treatment, with the post-treatment surface fuel loads often peaking above pre-treatment fuel loads and the timing and magnitude of responses varying with the size of the fuel component. However, basal area and tree density showed sustained reductions relative to pre-treatment values, and sustained increases in among-plot heterogeneity. In my fourth chapter, I focused on songbird responses to thin-plus-burn treatments, exploring edge effects of treatment and landscape contexts of canopy cover. Most songbird species classified as dense-forest favoring showed higher occupancy of interior positions of un-treated stands, and one species classified as open-forest favoring showed higher occupancy of interior positions of treated stands. Songbirds showed a range of magnitudes and directions of responses to landscape context of canopy cover, indicating that no single management strategy is likely to provide habitat for the full range of species. Collectively, this work provides a range of insights for managers interested in dry forest restoration at broad temporal and spatial scales.