Recent Increases in Fire Severity in California Contribute Significantly to Forest Loss

Warming and drought are increasing annual burned area in the western United States, resulting in large impacts on ecosystem structure. The severity of wildfires is also increasing, raising the likelihood of severe crown fires that result in tree mortality. However, it is challenging to use traditional metrics to quantify how changes in fire severity are impacting forests. While forests play a central role in many nature-based solutions for carbon sequestration, the impact of long-term changes in fire severity on forest structure remain uncertain. In this study, we use satellite datasets of annual fire occurrence and tree cover at a 30-m resolution across California to characterize trends in fire severity and fire-driven changes in forest cover from 1985-2020. We find that the average fire severity, measured as the pixel-wise change in tree cover following fire, has risen from 13.0 ± 1.9% absolute tree cover loss (mean and 95% CI) during 1985-1990 to 16.9 ± 2.1% during 2015-2020, equivalent to a relative increase of 30.9 ± 18.5%. From 1985-2020, fires drove a 3,985 km2 loss of tree cover area. However, in a modeling scenario where burn severity was held constant using a 1985-1990 mean rather than increasing as observed, fires contributed to a loss of just 2063 km2, suggesting that increasing burn severity alone is responsible for 1,922 km2 (48.3%) of tree cover area loss in the state. In addition, the average pre-fire tree cover of burned areas rose from 43.0 ± 5.0% during 1985-1990 to 48.6 ± 2.1% during 2015-2020, suggesting that wildfires are accessing denser forests. There was substantial geographic variability in these trends, with the Central Coast and Foothills ecoregion experiencing the most rapid increases. A linear model combining annual precipitation and summer temperature variables explained some of the interannual variability (and long-term trends) in fire severity (R2 = 0.23; p = 0.02) and pre-fire tree cover (R2 = 0.35; p = 0.005). Ongoing climate warming and worsening droughts will increase burn severity, potentially resulting in tree mortality even in fire-adapted or managed forests. Wildfires that are burning more severely and into denser forests are increasingly challenging the biodiversity and carbon storage of terrestrial ecosystems in areas experiencing rapid climate change like California.