The outsized role of California’s largest wildfires in changing forest burn patterns and coarsening ecosystem scale

Highlights • We evaluated trends for 1,809 fires that burned 1985–2020 across California forests. • Top 1% of fires by size burned 47% of total area burned across the study period. • Top 1% (18 fires) produced 58% of high and 42% of low-moderate severity area. • Top 1% created novel landscape patterns of large burn severity patches. • These large fires create new opportunities for managing forest resilience. Although recent large wildfires in California forests are well publicized in media and scientific literature, their cumulative effects on forest structure and implications for forest resilience remain poorly understood. In this study, we evaluated spatial patterns of burn severity for 18 exceptionally large fires and compared their cumulative impacts to the hundreds of smaller fires that have burned across California forests in recent decades. We used a burn severity atlas for over 1,800 fires that burned in predominantly conifer forests between 1985 and 2020 and calculated landscape metrics to evaluate spatiotemporal patterns of unburned refugia, low-moderate-severity, and high-severity post-fire effects. Total annual area burned, mean annual fire size, and total annual core area burned at high severity all significantly increased across the study period. Exceptionally large fires (i.e., the top 1% by size) were responsible for 58% and 42% of the cumulative area burned at high and low-moderate severities, respectively, across the study period. With their larger patch sizes, our results suggest that exceptionally large fires coarsen the landscape pattern of California’s forests, reducing their fine-scale heterogeneity which supports much of their biodiversity as well as wildfire and climate resilience. Thus far, most modern post-fire management has focused on restoring forest cover and minimizing ecotype conversion in large, high-severity patches. These large fires, however, have also provided extensive areas of low-moderate severity burns where managers could leverage the wildfire’s initial “treatment” with follow-up fuel reduction treatments to help restore finer-scale forest heterogeneity and fire resilience.