Chaparral Fire History and Fire-Climate Relationships in the Transverse Ranges of Southern California, USA

Abstract

There is vigorous debate regarding possible changes in the spatial and temporal attributes of chaparral fire regimes within southern California. We employed a novel approach to reconstruct a multi-century record chaparral fire history and to evaluate the effects of climate on these fire regimes across three southern California National Forests. The research in this dissertation is presented as three related studies. The first focused on using fire scars and tree rings from isolated stands of bigcone Douglas-fir (Pseudotsuga macrocarpa), which we demonstrate as reflective of the temporal and spatial patterns of fire in the surrounding chaparral. We found many extensive fires were apparent in both the pre-and post-twentieth century period indicating that such events were a natural component of the system. The second study applied the same approach but the spatial extent of the project was expanded to examine fire histories at a regional scale. Our results confirm that widespread fire events have, for centuries, likely played a critical role in shaping the fire regime of southern California chaparral landscapes. We found that such events occurred on a multi-decadal interval and that interval lengths have nearly doubled since the turn of the century. The third study examined the relationship between antecedent climate and wildfires in chaparral landscapes across southern California. We found that acute drought, driven by antecedent cool season precipitation in the previous winter and spring, was a reliable indicator of increased wildfire activity in the past; however, we now find a contemporary system influenced by antecedent climate in the two years prior to the fire event and no immediate connections to climatic drivers is apparent in the year of the event. The broader results from these three studies indicate that some changes in fire return intervals have occurred in the modern era but widespread fires have been and remain an integral part of chaparral fire regimes. We hypothesis that land use in the 20th century has altered vegetation structure and composition so much so that chaparral fire regimes now respond differently to climatic cues than they had for the past 200-300 years.