Longer-term impacts of fuel reduction treatments on forest structure, fuels, and drought resistance in the Lake Tahoe Basin

Abstract

Sierra Nevada mixed-conifer forests have undergone significant changes in structure and composition and are increasingly vulnerable to altered disturbance regimes and climate-related extreme events. Fuel reduction treatments, including thinning and follow-up surface fuel treatments, can reduce this vulnerability by creating forest structural and woody fuel conditions that not only allow forest stands to mitigate wildfire, but also alleviate individual tree stress. However, direct observations that quantify these longer-term effects are lacking. This study compares observed changes in forest structure, tree species composition, and downed woody fuel loads across three distinct time periods: pre-treatment, 1 yr post-treatment, and 10 yr post-treatment. Additionally, using tree ring data, we assessed whether treatments affected individual tree resistance to a severe statewide drought (2012–2015). Thinning treatments were able to effectively reduce tree density and basal area, increase the retention of both larger-sized and shade-intolerant trees, and mitigate tree mortality. Treatments were also associated with significantly lower coarse woody fuel and snag basal area. Snag basal area and time since treatment were related to the accumulation of fine and coarse woody surface fuel loads. Tree ring information indicated that treatments improved drought resistance as well, especially in units with lower residual live basal area. This study complements previous studies on fuel reduction thinning by demonstrating that these treatments have lasting effects on forest structure, which also confers a degree of drought resistance.