Characterizing ground and surface fuels across Sierra Nevada forests shortly after the 2012–2016 drought

Abstract

The 2012–2016 hotter drought in the Sierra Nevada, California, USA led to the mortality of millions of trees. This disruption to the disturbance regime is an example of how climate extremes can exacerbate current and future wildfire risks. In this study, we used data from an extensive network of forest plots across 13 different sites in the Sierra Nevada to characterize ground (i.e., duff) and surface fuels and their potential drivers shortly after the drought (2016–2018), but before snag (standing dead tree) fall associated with this massive tree mortality event occurred. Overall, we found high biomass of fuels for most sites (138.2 ± 21.7 Mg ha−1, Mean ± 95 % CI), especially in areas lacking recent fire or active management, with values up to five times higher than estimates for pre-settlement conditions for mixed-conifer forests in the region. Four major groups of forest overstory structure were identified with some distinct fuel characteristics. These ranged from a cluster of ponderosa pine (Pinus ponderosa Douglas ex Lawson)-dominated plots with the highest density of snags, lowest live tree basal area, and lowest total ground and surface fuel biomass (mean = 90.1 Mg ha−1) to a group of giant sequoia (Sequoiadendron giganteum (Lindl.) J. Buchholz)-dominated plots with the highest live tree basal area and highest total fuel biomass (mean = 547.6 Mg ha−1). Although we found relatively weak relationships between forest characteristics and fuel loads, their inclusion in a model selection framework that also considered biophysical variables and disturbance history explained>80 % of the observed variation in litter + fine woody debris loads. The baseline fuel conditions described here will not only inform the management of drought-impacted forests in the Sierra Nevada, but also help identify key drivers of fuel succession in a changing environment.