Although fire is a fundamental ecological process in western North American forests, climate warming and accumulating forest fuels due to fire suppression have led to wildfires that burn at high severity across larger fractions of their footprint than were historically typical. These trends have spiked upwards in recent years and are particularly pronounced in the Sierra Nevada-southern Cascades ecoregion of California, USA and neighboring states. We assessed annual area burned and percentage of area burned at high and low-to-moderate severity for seven major forest types in this region from 1984 to 2020. We compared values for this period against estimates for the pre-Euro-American settlement (EAS) period prior to 1850 and against a previous study of trends from 1984–2009. Our results show that total average annual area burned remained below pre-EAS levels, but that gap is decreasing (i.e., c. 14% of pre-EAS for 1984–2009, but 39% for 2010–2020 [including c. 150% in 2020]). Although average annual area burned has remained low compared to pre-EAS, both the average annual area burned at high severity and the percentage of wildfire area burned at high severity have increased rapidly. The percentage of area burned at high severity – which was already above pre-EAS average for the 1984–2009 period – has continued to rise for five of seven forest types. Notably, between 2010 and 2020, the average annual area burned at high severity exceeded the pre-EAS average for the first time on record. By contrast, percentage of area that burned at low-to-moderate severity decreased, particularly in the lower elevation oak and mixed conifer forest types. These findings underline how forests historically adapted to frequent low-to-moderate-severity fire are being reshaped by novel proportions and extents of high-severity burning. The shift toward a high severity-dominated fire regime is associated with ecological disruptions, including changes in forest structure, species composition, carbon storage, wildlife habitat, ecosystem services, and resilience. Our results underscore the importance of finding a better balance between the current management focus on fire suppression and one that puts greater emphasis on proactive fuel reduction and increased forest resilience to climate change and ecological disturbance.

The primary source of fire severity data came from the "Vegetation Burn Severity – 1984 to 2017” geospatial data layer (USDA 2018). For the area of analysis, we the same perimeter used by Mallek et al. (2013) and Miller et al. (2009), and is based on the Sierra Nevada ecoregion as defined by the Sierra Nevada Ecosystem Project (SNEP 1996) and the Sierra Nevada Forest Plan Amendment (SNFPA; USDA 2004). We included fires greater than or equal to 80 ha in size (see Methods for specifics). For the 2018–2020 fire years, we estimated burn severity using Google Earth Engine following Parks et al. (2018c, 2021). For the 2018–2020 fires we used the attached zip file of fire perimeters.

• Mallek, C., H. Safford, J. Viers, and J. Miller. 2013. Modern departures in fire severity and area vary by forest type, Sierra Nevada and southern Cascades, California, USA. Ecosphere 4.
• Parks, S. A., L. M. Holsinger, M. A. Voss, R. A. Loehman, and N. P. Robinson. 2018c. Mean Composite Fire Severity Metrics Computed with Google Earth Engine Offer Improved Accuracy and Expanded Mapping Potential. Remote Sensing 10.
• Parks, S. A., L. M. Holsinger, M. A. Voss, R. A. Loehman, and N. P. Robinson. 2021. Correction: Mean Composite Fire Severity Metrics Computed with Google Earth Engine Offer Improved Accuracy and Expanded Mapping Potential. Remote Sens. 10, 879, 2018. Remote Sensing 13.
• SNEP. 1996. Sierra Nevada Ecosystem Project: Final report to Congress.in C. f. W. a. W. Resources, editor. University of California, Davis, California, USA.
• USDA. 2004. Sierra Nevada Forest Plan Amendment, Record of Decision.in P. S. Region, editor. USDA Forest Service, Vallejo, California, USA.
• USDA. 2018. R5 VegBurnSeverity - Metadata.in P. S. R. USDA Forest Service, editor., USDA Forest Service, Pacific Southwest Region, California.