Between the years 2012 and 2016, extreme drought and subsequent increase in bark beetle populations led to the death of more than 129 million trees across the Sierra Nevada. State land managers have since aimed to reduce the densities of vulnerable forests because forests with fewer tress require less water, allowing these ecosystems to survive frequent or extreme droughts and related issues like bark beetle epidemics. This management strategy, known as forest thinning, is accomplished through techniques such as prescribed burning and can reduce the negative impacts of climate change and increase the overall resilience of forest ecosystems.

In a 2019 study partially funded by the Southwest CASC, researchers with the University of California, Davis and the USDA Forest Service Pacific Southwest Region examined forested areas in the Sierra Nevada which had been treated by forest thinning or prescribed burning. Researchers collected data from both treated and untreated forests, then compared the impacts of the 2012-2016 drought and beetle outbreaks on four major species: ponderosa pine, sugar pine, white fir, and incense cedar. The results showed that the use of forest thinning treatments led to a decrease in tree mortality from drought and bark beetle infestations for some tree species. For example, among untreated areas of ponderosa pine, the chance that any one tree would die during the study period was 45%. In treated stands, the chance was reduced to 30%.

This study also emphasizes the importance of implementing forest thinning treatments before a drought begins. Finally, an important management consideration going forward may include expanding treatments to include a broader range of tree species found in the Sierra Nevada, which could serve to increase the overall resilience of forests across the region.

This research was funded in part by the Southwest CASC project, Post-Fire Conifer Regeneration Under a Warming Climate: Will Severe Fire Be a Catalyst for Forest Loss?