Using Bigcone Douglas-Fir Fire Scars and Tree Rings to Reconstruct Interior Chaparral Fire History

Abstract

Bigcone Douglas-fir (Pseudotsuga macrocarpa [Vasey] Mayr) is a long-lived, fire-adapted conifer that is endemic to the Transverse Ranges of southern California. At the lower and middle reaches of its elevational distribution, isolated stands of bigcone Douglas-fir are surrounded by extensive stands of chaparral. Our dendrochronology investigations have revealed that these ancient trees commonly record multiple past fires as fire scars in their lower boles. We hypothesized that the fire-scar record found within and among bigcone Douglas-fir stands reflects the temporal and spatial patterns of fire in the surrounding chaparral. We compared the fire scar results with independent, twentieth century fire atlas data to assess our interpretations. Using fire scars and ring-growth changes, we reconstructed fire history in Los Padres National Forest and investigated changes in fire regime characteristics over the past several centuries. Our analyses confirm that the tree-ring record can be used to accurately reconstruct past fire occurrence and extent patterns both within bigcone Douglas-fir stands and surrounding chaparral stands. 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. However, many smaller fires were also evident in the tree-ring record, and more of these types of events occurred during the nineteenth century (and earlier) than during the twentieth century. We also identified a shift after the late nineteenth century to potentially more severe fires within and among stands, and by inference the surrounding chaparral. These findings suggest that land management policies, rates of human-set fires, or climatic variations may have played a role in shaping the contemporary fire regime, and that this recent period is different in some respects from the pre-twentieth century regime. Replication of this work in other mountain ranges, in addition to comparisons with climate and human histories, will provide valuable insights into our understanding of the relative roles of humans versus climate in changing bigcone Douglas-fir and chaparral fire regimes.