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A DENDROECOLOGY-BASED FIRE HISTORY OF COAST REDWOODS (SEQUOIA SEMPERVIRENS) IN CENTRAL COASTAL CALIFORNIA

  • Author(s): Striplen, Charles Joseph
  • Advisor(s): Huntsinger, Lynn
  • et al.
Abstract

This dissertation focuses on fire history reconstructed from select coast redwood stands in the Santa Cruz Mountains, California based on the fire scar record in specimens collected between 2008 and 2013. The research is one component of a larger multidisciplinary examination of indigenous burning practices in the Central California Coast region (Lightfoot et al. 2013). Research presented herein exhibits concordance with results from related studies, including analyses of soil phytolith content (Evett and Cuthrell 2013), faunal assemblages (Gifford-Gonzalez et al. 2013), and microscopic pollen and charcoal evidence (Cowart and Byrne 2013). Fire history research was conducted in three coastal watersheds in Santa Cruz and San Mateo counties, California using standard dendroecological techniques, as well as a novel statistical approach developed to address undated, floating chronologies.

A total of 103 coast redwood samples were collected from 95 sample trees in 19 plots within the study area. The fire return intervals recorded from the dated redwood samples in this study were relatively frequent. Fire information was estimated for three focal management eras: the native and ranching eras (1600-1850), intensive commercial logging (1850-1950), and the modern fire suppression/sustainable harvest era (1950-2013). Results from dated fire scars indicate that fires were less frequent in the native and ranching period (mean FRI 7.6 years; range 1-29) than the intensive logging period (mean FRI 3.1 years; range 1-11), as well as the modern period (mean FRI 4.6 years; range 1-12).

However, use of a generalized linear mixed model (GLMM) on undated, floating chronologies indicated that the probability of fire may have been quite high in the earlier period, and that three independent variables were significant predictors in assessing the annual probability of the occurrence of fire in the study area: physiographic zone; position on slope; and linear distance to pre-colonial, native habitation sites. The GLMM also indicates that fire probabilities are not distributed uniformly in study watersheds. Trees located in close proximity to native residential sites had a high probability of being burned than those farther away (42-69% vs. 17-38%). Similarly, top of slope fire were more likely in all watersheds and physiographic zones, though varying in degree.

The season of fire occurrence was determined for 85% of the fire scars. Dormant or late season fires accounted for a combined total of 87% of all fires for the entire period of record (1350-2013; 55% dormant, 33% late) - indicating that historic fires most likely took place between approximately mid-August to late March. Early season (approx. April to August) fires accounted for 13% of fires. In the 1600-1850 period, combined dormant and late season fires accounted for 91% of fires (64% and 27%, respectively), with 9% of fires occurring in the early season. During the intensive logging period (1850-1950) combined dormant and late season fires accounted for 85% of fires, and 15% in the early season. In the modern era (1950-2013), dormant and late season fires still account for the majority of fires (86%), but with a marked shift into the drier late season (mid-August - September), which now account for 43% of fires. Early season fires represent 14% of fires in this period.

Though this study faced significant challenges (i.e. low sample density for earlier period specimens, large study area, experimental use of a GLMM), these data reveal interesting and potentially useful patterns of historic fire occurrence in the Santa Cruz Mountains, especially with respect to human influence over coastal fire regimes. All sources of information indicate that coastal Santa Cruz Mountains experienced far more ignitions than would be expected under a lightning-driven fire regime (roughly 4 strikes per century), and that human activity is strongly linked to fire frequency in throughout observable time periods. There is ample opportunity to improve on this data with future work in efforts to inform and refine modern approaches to resource management in this region.

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