Multi-scale assessment of wildfire use on carbon stocks in the Sierra Nevada, CA

Background

The active use of wildfire to meet forest management objectives is an important tool to increase the scale of forest restoration in dry, historically frequent-fire forests. While there are many benefits of reintroducing fire to these forests, the impact of wildland fire use policies in frequent-fire forests on aboveground carbon stocks has not yet been studied. In this study, we begin to fill this knowledge gap by assessing how fire frequency and severity affected aboveground carbon dynamics in two basins in the Sierra Nevada with a history of wildfire use over the past 20 to 50 years, compared to a nearby basin that has remained largely unburned.

Results

Across two spatial and temporal scales, live carbon stocks in wildfire use areas decreased by on average 22–48% (depending on the scale and basin), while dead carbon stocks changed little or decreased in areas managed with wildfire. The unburned basin held higher amounts of carbon stocks than the burned basins, and on average, these stocks remained relatively constant over the study period. Fire severity appeared to exert a stronger influence on carbon change than frequency, with areas that burned at high severity resulting in the largest losses in aboveground carbon, regardless of the number of fires that the area had experienced.

Conclusions

We found that greater wildfire use over several decades comes at some costs to carbon stocks as trees—which store the greatest amount of carbon in forested systems—are killed or consumed and forest patches transition to shrublands and meadows. In our study area, these conversions from forest to non-forest types were in relatively small high-severity patches and likely represent areas that were historically maintained as non-forested shrublands, grasslands, or wetlands by an intact fire regime. Additionally, we found that the surviving carbon stocks may be more resistant to future disturbances. Restoration of dry mixed-conifer forests solely by reintroducing a characteristic fire regime is a slow process, but increasing wildfire use in areas where it is feasible is an important tool that managers should consider to address our growing forest restoration needs.