Large wildfires in California cause significant socioecological impacts. These fires impact water quality, biodiversity, carbon sequestration, and other ecosystem services; further, they pose a risk to lives and property and require increasing expense and resources be dedicated to fire suppression. Twenty-first century fire activity is projected to increase with climate change; however, predictions are uncertain because humans can amplify, buffer or even override climatic effects on fire activity. Here we test the hypothesis that historic changes in socioecological systems - from the Native American burning practices in the 1600s up to the current period of fire suppression - drove shifts in Sierra Nevada fire activity and modulated fire–climate relationships. We developed a four century long index (1600–2015) of fire activity. This index was created by merging a tree ring–based record of Sierra Nevada fire history for 29 lower montane forest sites with a 20th-century record based on annual area burned. Our fire record was then compared to proxy records of temperature and moisture and to documentary records of socioecological systems.
Results/Conclusions
The mean and median fire intervals during the past four centuries were 17.7 and 13.5 years, respectively; however, interannual and multidecadal variability in fire activity was observed. Despite the strong influence of climate variability on Sierra Nevada fire activity, fire regime shifts corresponded with socioecological change, not climate change. Further, socioecological conditions amplified and buffered fire response to climate. Fire activity was highest and fire–climate relationships were strongest after Native American depopulation (mission establishment, ca. 1776) reduced the self-limiting effect of Native American burning practices on fire spread. With the Gold Rush and Euro-American settlement (ca. 1865), fire activity declined, and the strong multidecadal relationship between temperature and fire decayed. After the implementation of fire suppression policy (ca. 1904), this temperature-fire relationship disappeared. The amplification and buffering of fire–climate relationships by humans underscores the need for parameterizing thresholds of human- vs. climate-driven fire activity to improve the skill and value of fire–climate models for addressing the increasing fire risk in California.