Reconstructing fire history

The devastating 2017 Sonoma Complex fires burned nearly 110,000 acres, resulted in the loss of 24 lives and an estimated $7 billion in damages. An improved understanding of fire within our natural and human-built environment is critical. Our motivation is to reconstruct the return interval of fire within differing ecosystems in Sonoma County. We present a preliminary dendropyrochronological analysis of Quercus agrifolia collected from Mark West Springs. Our goals were to determine the age of four tree cross-sections and examine them for fire scars. We sanded the samples down to 800 grit. A dissection microscope was used to count the tree rings. We identified and aged fire scars based on growth structure changes. Cross-sections ranged in age from 109-122 years. Initial analysis indicates one instance of fire scarring, which may correlate with the Porter Creek Fire in 1996. Future research will examine samples from Osborn, Los Guillicos and Galbreath Preserves. 

The SSU Quaternary lab (SQUAL) has a large repository of tree trunk “cookies” obtained from the Fairfield Osborn Preserve (N=10; mixed collection of California Bays and Coast live Oak), Galbreath Preserve (N=39; tanoak), and Shiloh Ranch Regional Preserve (N=3; Coast Live Oak). These cookies have been sanded for tree ring analysis and preliminary age counts have been undertaken on a subset of the samples. In this project we undertake more detailed laboratory analysis and potentially further field sampling if the opportunity arises. There are two specific goals.

  • Firescar analysis: The samples have not been systematically analyzed for firescar records. In light of last year’s fires an understanding of the fire return interval is critical for fire planning and preparedness. Preliminary analysis of the Shiloh cookies indicates the presence of fire scars, while rudimentary analysis of the FOP samples shows no evidence of fire activity, the Galbreath samples have not been examined for fire scar history. A systematic analysis of the samples as well as obtaining other samples from the preserves has improved our understanding of long-term fire history in these three area.
  • Dendrogeomorphic analysis: At FOP and Galbreath all trees were growing on steep slopes. The tree ring structure indicates that mass movements occurred during the lifetime of the trees. Mass movements can be related to a variety of phenomenon such as heavy precipitation and slope denudation related to fire events. Preliminary work at FOP indicated mass movements occurred at some time in the 1950s and 1970s, but the data analysis needs refinement. We wish to undertake a rigorous age count and statistical analysis of the results and inferred year of mass movement. Data was examined in relation to historic precipitation and fire history records. This information provides a longer term perspective on fire history than that currently afforded by the oral and historic documentation over the past 100 years. The information is of interest in particular to land managers.   


Project Results
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"Reconstructing Fire History in Sonoma County Using Tree Ring Analysis" poster Elise Piazza