Characterizing ground and surface fuels shortly after the 2012-2016 drought across Sierra Nevada mixed-conifer forests

Emilio Vilanova, Lauren E. Cox, John J. Battles and Jodi N. Axelson, Department of Environmental Science, Policy, and Management, University of California, Berkeley, Berkeley, CA, Leif A. Mortenson, USDA Forest Service, Placerville, CA, Beverly M. Bualon, USDA Forest Service, Sonora, CA, Jamie M. Lydersen, California Department of Forestry and Fire Protection, Fire and Resource Assessment Program, Sacramento, CA, Christopher. J. Fettig, USDA Forest Service, Davis, CA

Background/Question/Methods: Long-term fire suppression combined with the extended aridity caused by the 2012–2016 drought in the Sierra Nevada (SN) region of California led to a massive wave of tree mortality that affected hundreds of millions of trees, posing significant challenges for current and future wildfire risks, carbon emissions, conservation of biodiversity, land-use planning, and many other aspects of sustainable forest management. In this study we asked 1) What is the approximate biomass of surface fuels shortly after the 2012–2016 drought in the Sierra Nevada, but prior to the commencement of widespread snag fall, across a range of forest conditions?; 2) Can relatively distinct vegetation groups with different fuel load signatures be identified based on the assessment of forest structure and composition? and 3) How well does overstory structure and composition explain the variability in surface fuels?. We used data from an extensive network of forest plots across 13 different locations in the SN spanning a wide range of forest types, management conditions and jurisdictions measured shortly after the drought. A total of 462 plots and 1,386 fuel transects were measured. Results/Conclusions: We found high levels of fuel biomass for most sites (overall mean 138.2 ± 21.7 (±95% CI) Mg ha-1), especially in areas lacking recent fire or active management. Across elevation gradients, sites located above 1900 m elevation showed higher surface fuel loads (209.9 ± 81.1 Mg ha-1) largely influenced by the biomass of 1000-hour fuels or coarse woody debris (mean 132.7 ± 79.3 Mg ha-1). We found relatively weak relationships between overstory structure and composition and ground and surface fuel loads, but total basal area and tree density were the most important predictors. Overstory data helped to distinguish four vegetation clusters from which significant differences in fuel loads were also detected. We found two potential ‘extremes’ in the range of fuels loads across the SN. On the higher side, we find Giant Sequoia (Sequoiadendron giganteum) dominated stands where large 1000-hour fuels occupy the biggest proportion. On the other, ponderosa pine (Pinus ponderosa) dominated stands were overall characterized by lower fuel loads with the largest proportion occupied by duff and litter. It is anticipated that the massive tree mortality event will further increase fuel loading across this region for which continuous monitoring is needed. Finally, a better understanding of baseline fuels conditions can help inform on the recovery and management of drought-impacted forests in the SN.