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Determining the spatial distribution of high elevation Quercus garryana var. breweri (Brewer's Oak) and identifying the age and presence of fire scars using dendrochronology, Tejon Ranch, California
Brewer’s Oak (Quercus garryana var. breweri) is a dense, shrub-like variety of Garry Oak (Quercus garryana – referenced as Oregon White Oak or Oregon Oak), which is found primarily in the western regions of North America. Scientists and surveyors working within Tejon Ranch, the largest expanse of privately-owned land in California, have previously identified Brewer’s Oak at high elevations (> 1,500m) and are interested in better understanding the species’ fire ecology. The response of Brewer’s Oak to natural and prescribed fire events is of ecological concern to conservation scientists owing to the increased potential for changes in the natural fire regimes in the western U.S. and California. While oak woodland ecology has been, and continues to be, studied in great detail in southern California and within Tejon Ranch, little has been published on the ecology of Brewer’s Oak. This study aimed to better understand the ecological framework of Brewer’s Oak on Tejon Ranch by (1) conducting a comprehensive mapping of the species, (2) analyzing the species’ slope, aspect, and elevation profile on the Ranch, (3) dating the Brewer’s Oak by examining its tree-rings using dendrochronology, and (4) investigating whether previous fire events on the Ranch can be identified through dendrochronological analysis. To this end, an initial mapping project was completed using physical site surveying techniques, aided with GPS and LandSat 5 satellite imagery. Five research sites within the Brewer’s Oak stands were chosen for dendrochronological examination. One hundred Brewer’s Oak cross-sections were processed and analyzed in order to compile an accurate master chronological record for each research site and identify fire scars in the growth-rings. Tree-ring cross-dating techniques were used to develop an independent chronology for each site. Mapping of the Brewer’s Oak revealed an increase in the recorded extent of the species as compared to that of a 1980 timber survey, the only other survey of the species on record at Tejon Ranch. For the 100 cross-sections analyzed at the five sites, there was no evidence of fire scars and the mean year of origin was between 1878 and 1916, making this species significantly older than conservation scientists have estimated. Additionally, mean slope and aspect measurements were generated for the mapped Brewer’s Oak, revealing that over half of the species grow along north-facing slopes and on slope angles of less than 30 degrees. Mapping limitations exist in high elevation research locations and further research into more accurately mapping Brewer’s Oak should include comparative analysis between ground reference measurements with that of high resolution satellite imagery.