Drivers of fire severity in repeat fires: implications for mixed-conifer forests in the Sierra Nevada, California

Background

While the reintroduction of recurring fire restores a key process in frequent-fire adapted forests, the ability to significantly shift the structure and composition of departed contemporary forests has not been clearly demonstrated. Our study utilized an extensive network of field plots across three short-interval successive fires occurring in the northern Sierra Nevada, California. We evaluated the influence of plot-level forest structure and composition, topography, and weather on fire severity in a third successive fire (i.e., second reburn). Additionally, we assessed the range of forest structural conditions that emerge following multiple low- to moderate-severity fires, whether these conditions were associated with fire severity in a third fire, and how they compare to historical estimates for these forests.

Results

Across plots that burned in multiple low- to moderate-severity fires, our findings indicated that post-fire outcomes in these systems are variable, resulting in a range of structural conditions following a first reburn (i.e., second fire). Areas with high levels of dead biomass burned at significantly higher severity in the third fire compared to those with higher shrub cover. Following a second fire, many plots exceeded historical estimates of stand structure metrics for yellow pine and mixed-conifer forests of the Sierra Nevada, particularly for coarse woody debris load, with some plots exceeding historical natural range of variation (NRV) estimates for live tree density. In plots with a history of varying fire severity in the initial and second fires, we found that snag basal area was associated with higher fire severity in the third fire.

Conclusions

Low- to moderate-severity fire has the ability to restore ecosystem processes and reduce future fire severity in the long term, but our results suggest that it can also create fuel conditions that drive higher fire severity in successive fires. Our study demonstrates that vegetation and fuel conditions existing prior to the initial first-entry fire can largely influence post-reburn outcomes.