Variability in California's Fire Activity during the Holocene, across Space and Time

Over the past several decades, there has been an increase in wildfire activity in California. These wildfires have occurred during a period of warmer temperatures and lower precipitation than the averages of the 20th century. To improve understanding of the response of fire activity to warm and dry periods, periods in the past with similar climates were examined. A suitable method to examine paleofire is charcoal that is preserved in lacustrine sediment. To examine long-term changes in fire in California, 16 sedimentary charcoal records were collected, including four records analyzed over the past few years at UCLA, as well as 12 existing records that were collated from a database of charcoal data. These records are predominantly located in the Sierra Nevada range and the Klamath mountains. Records of charcoal accumulation at these sites were compared to determine the response of fire to known warm and dry periods during the Holocene, such as the Medieval Climate Anomaly (MCA) and the Holocene Thermal Maximum (HTM). The data collected shows there was no statewide rise in fire activity during the MCA or the HTM, but several regional trends were seen in the data. Sites in the Klamath mountains generally had a rise in charcoal influx over the course of the Holocene, and this rise was likely correlated with increasing forest coverage over time. The UCLA sites located in the Sierra Nevada mountains experienced a rise in charcoal influx during the last 2000-3000 years. The lack of statewide signal of increased burning during events such as the MCA or HTM indicates that the fire conditions seen in the present may be without precedent during the period of the Holocene. However, there are important caveats to interpretation of the results, as charcoal accumulation data is subject to several factors, of which only one is the presence of a specific fire event. Despite these limitations, this study allowed for the determination of regional responses of fire during the Holocene and allowed us to examine how those responses compared to what is seen during the present