Impacts of Phytophthora ramorum on oaks and tanoaks in Marin County, California forests since 2000Author(s): Brice A. McPherson; David L. Wood; Maggi Kelly; Sylvia R. Mori; Pavel Svihra; Andrew J. Storer; Richard B. Standiford
Source: In: Frankel, Susan J.; Kliejunas, John T.; Palmieri, Katharine M. 2010. Proceedings of the Sudden Oak Death Fourth Science Symposium. Gen. Tech. Rep. PSW-GTR-229. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. pp. 210-212
Publication Series: General Technical Report (GTR)
Station: Pacific Southwest Research Station
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DescriptionThe forests of Marin County were among the first in coastal California to be affected by the Phytophthora ramorum epidemic. Although initially observed in 1994 in tanoaks (Lithocarpus densiflorus) and 1995 in coast live oaks (Quercus agrifolia), it is evident from studies of disease progression that the pathogen was present at least several years prior to the first recorded tree mortality (Rizzo and Garbelotto 2003; McPherson and others 2005). The causal agent had not been identified in March 2000 when we established 20 plots in two Marin County sites, China Camp State Park, (CCSP, coast live oaks and California black oaks, Q. kelloggii) and the Marin Municipal Water District watershed (MMWD, all three species). Plots were between 320 m² and 3600 m², with a mean of 1234 (SE = 199) m². The goal of the study was to monitor disease progression (McPherson and others 2000) and, in particular, to understand the phenomenon of the abundant beetle attacks observed on bleeding trees. Plots were evaluated four times per year until March 2003, then twice annually thereafter. Through March 2008, every stem > 5 cm diameter at breast height (DBH) was evaluated for signs and symptoms of sudden oak death (SOD): bleeding, beetle attacks, and the presence of the fungus Hypoxylon thouarsianum. In 2001 and 2007, we recorded the basal area for every woody species with stem DBH > 5 cm found within 0.08-ha subplots placed within the larger plots.
Mortality of each of the three species has increased steadily since 2000. Calculated infection and mortality rates were similar for coast live oaks and black oaks, but considerably greater for tanoaks (table 1). Mortality not attributable to P. ramorum serves as an estimate of background mortality. The proportion of the total living trees that were symptomatic, recorded at 1-year increments, increased gradually for coast live oaks and black oaks, whereas tanoaks exhibited a rapid increase from 2000 to a peak in 2004, then declined. This decline is likely a result of decreasing numbers of live stems in these heavily impacted sites.
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CitationMcPherson, Brice A.; Wood, David L.; Kelly, Maggi; Mori, Sylvia R.; Svihra, Pavel; Storer, Andrew J. ; Standiford, Richard B. 2010. Impacts of Phytophthora ramorum on oaks and tanoaks in Marin County, California forests since 2000. In: Frankel, Susan J.; Kliejunas, John T.; Palmieri, Katharine M. 2010. Proceedings of the Sudden Oak Death Fourth Science Symposium. Gen. Tech. Rep. PSW-GTR-229. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. pp. 210-212
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