Changes in fuel loads following severe drought and bark beetle outbreaks in the central and southern Sierra Nevada

The disturbance ecology of many conifer forests in western North America has deviated drastically from historical conditions largely as a result of fire suppression, extreme drought events, and bark beetle outbreaks. These deviations were exemplified in 2012–2015 when the worst drought in over a millennium occurred throughout parts of California. During and following the drought, a western pine beetle (Dendroctonus brevicomis) outbreak occurred in the central and southern Sierra Nevada causing severe ( >90% in some areas) ponderosa pine (Pinus ponderosa) mortality. The objectives of our study are to determine changes and variations in fuel loading over time, and to determine predictive variables of fuel loading following the outbreak. A network of 180 11.3-m fixed-radius plots were installed across three elevation bands on the Eldorado, Stanislaus, Sierra and Sequoia National Forests to monitor tree mortality levels, changes in tree species composition, and changes in fuel loading. Fuels data across the plot network were measured in 2017 and 2019 using modified Brown’s transects, and measurements will be repeated again in 2021 and 2023. Preliminary data show total surface fuel loading has increased from 2017 to 2019 across all national forests and elevation bands, with the largest increase on the Sierra National Forest. Better understanding of changes in fuel composition following bark beetle outbreaks in Sierra Nevada forests provides important context for land management decisions, especially as outbreaks likely increase in frequency and severity moving forward.