The resilience of many dry and frequent-fire forests in the western U.S. is compromised by elevated fire hazard, which is driven by both climate change and fuel buildup from a legacy of fire suppression. Fuels reduction treatments (e.g., mechanical thinning and/or prescribed burning) are often used to reduce the probability of uncharacteristically severe fire that is misaligned with the adaptations of dominant tree species. While much is known about the immediate and short-term (e.g., 0-3 years after treatment) effects of fuels reduction treatments on fire hazard, less is known about longer-term (e.g., 15-20 years) effects that align more with the historical fire-return interval. Such information is critical to understanding the duration over which treatments can decrease the likelihood of severe fire. We addressed this research gap by measuring fuel profiles in 136 plots across 8 units in the East Cascades Fire and Fire Surrogates study site in Washington (USA), 15-19 years after four treatments (thin-only, burn-only, thin+burn, control) were implemented. We asked: how do fuel profiles (amount, arrangement, and composition of biomass) and stand structural characteristics (basal area, density, and composition of trees) in 2019 differ across the experimental treatments, and how do fuel profiles in each treatment compare to pre-treatment values in 2000?
Results/Conclusions
In the long term, fine (≤ 7.6 cm diameter) woody surface fuel biomass was significantly (p < 0.10) positively associated with pre-treatment values, and was greater in the thin-only than control treatment. Coarse (> 7.6 cm diameter) woody surface fuel biomass was greater than controls 15-19 years post-treatment for each active treatment, and duff depth was greater in the thinning treatment. The only treatment to show persistent reductions in live basal area was thin+burn. Collectively, our results suggest that increased surface fuel loads from thinning treatments can persist for nearly two decades. These insights are consistent with short term results from thinning experiments, and from chronosequence studies of time since wildfire. Long term studies on fuels reduction experiments are critical to providing insight for treatment options that can foster resilience in dry fire-prone forests.