COS 286-6 - Long-term effects of common fuel treatments on fuel loads in Western US dry forests: knowledge gaps and research advances

Abstract: Fuel treatments are integral to forest restoration, climate adaptation, and mitigating wildfire severity in historically frequent-fire dry forests across western North America. Numerous studies have examined the short-term effects of treatments (e.g. prescribed burning and/or forest thinning) on fuel loads, but long-term effects have received less scientific attention. We conducted a two-part project to address research needs in long-term fuel treatment effects on fuel loads. First, we conducted a literature review to identify knowledge gaps about long-term effects of fuel treatments on fuel loads. Second, we leveraged existing datasets in Washington state (USA) to address some of these knowledge gaps. Our literature review highlighted several opportunities to advance knowledge of long-term treatment effects on fuel loads, including 1) expanding definitions of long-term effectiveness to include a broader suite of management goals; 2) using evaluations of effect size, variance, and statistical power to mitigate the likelihood of not detecting ecologically meaningful treatment effects in naturally heterogeneous dry forests; 3) including additional covariates beyond treatment type in statistical tests; 4) collecting longitudinal data with finer temporal resolution to explore trajectories of fuel load responses over time; and 5) reporting measures of fine-scale variability in fuel loads in addition to average responses. Long-term treatment effects on fuel loads were assessed with field data at 310 plots across 3 sites in central and eastern Washington state (USA). On average, treatments had greater beneficial long-term effects on canopy-fire potential than on surface-fire potential. Thin-plus-burn treatments had the greatest long-term effects on canopy-fire potential among treatment types. Analyses that included plot-scale estimates of pre-treatment conditions and treatment intensity produced more precise estimates of long-term effects for many response variables. The importance of treatments compared to modelled fire weather depended on response variable tested. For example, active crown fire potential was more strongly affected by treatment type and intensity whereas surface fire potential, torching, and tree mortality were most strongly affected by fire weather. In a small subset of the data with high temporal resolution longitudinal data (22 plots), woody surface fuel dynamics were non-linear, with rapid accumulations 5-10 years post-treatment followed by a decline 10-15 years post-treatment. These insights on long-term treatment effects can help managers in dry and frequent-fire forests produce more targeted and efficient forest restoration plans.