Sudden Oak Death Infection Levels and Associated Forest Characteristics
Danny L. Fry1, Brandon Collins2, Brice A. McPherson2, Maggi Kelly3, Andrew J. Storer4, David L. Wood5, and Richard B. Standiford2
Accurate assessment of the sudden oak death epidemic in California requires reliable information on disease impact at both local and landscape scales. Surveys of forested landscapes must be unbiased, efficient, reliable, and capable of covering relatively large areas. We are using the point-centered quarter plotless method to estimate the extent of sudden oak death in two forests with different compositions of susceptible host species in California. Parallel transects were laid out through the forests, with sampling nodes placed every 100 m. The nearest host tree in each cardinal direction was assessed, for a maximum of 4 trees per node. Each tree was also assigned coordinates using global positioning system (GPS). Variables recorded were tree size and basal area, bleeding status, presence of bark and ambrosia beetles (Scolytidae), reproductive structures of Hypoxylon thouarsianum, and foliage condition. Additional information about the woody vegetation composition and density was obtained using prism sweeps around each node.
We conducted surveys in 2001 in China Camp State Park (Marin Co.), where coast live oak, Q. agrifolia, and California black oak, Q. kelloggii, are the known oak hosts. Coast live oak is the most abundant oak species in China Camp, which has probably been infected since the early 1990s. In 2001-2002, we ran a similar survey in Soquel Demonstration State Forest (Santa Cruz Co.), where Phytophthora ramorum is suspected to have become established more recently than in China Camp. Tree species in Soquel known to be susceptible include Q. agrifolia, Shreve oak, Q. parvula var. shrevei, and tanoak, Lithocarpus densiflorus. Shreve oak is the most abundant oak species in the Soquel site, which also features extensive redwood stands. The course of the disease in Shreve oak is poorly understood. Sites were similar in dominance by one species, 85% coast live oak in China camp and 76% tanoak in Soquel. Although much less abundant at China camp, black oak was similar to coast live oak in the relative proportion of symptomatic trees and the presence of associated organisms. Approximately 16% of the trees were observed with at least one bleeding canker, evidence of wood boring beetles, and fruiting bodies of H. thouarsianum. This was not the case for Soquel where the oaks and tanoaks were symptomatic in different proportions. Tanoak had the highest proportion of symptomatic trees with beetles and Hypoxylon (approximately 1%) while the few black oaks sampled were asymptomatic.
Binary logistic regression was used to determine if any variables predicted the presence of bleeding cankers on trees. For both sites, diameter at breast height (DBH) and basal area of associated live trees were positive significant predictors, although oak density and oak basal area were not. Basal area of dead oak trees was also a significant predictor at China Camp but not at Soquel. Additionally, basal area of dead non-oak trees was found to be significant at Soquel. Although it has been suggested that associated tree species, such as bay laurel, may be important in the dispersal of P. ramorum, bay laurel alone was not a significant predictor. The association of dead oak basal area with bleeding trees may indicate a longer presence of the pathogen in China Camp.
This research was designed to evaluate the usefulness of the point centered-quarter method by comparing apparent infection and mortality levels with those obtained from disease progression plots in Marin Co. For China Camp, the estimates for apparent infection and mortality appear to be in good agreement with those obtained from disease progression plots, for both coast live and black oak. Estimates of the impact of the disease on the trees in Soquel indicate lower levels of infection and mortality for tanoak than in Marin Co. disease progression plots. Inadequate understanding of Shreve oak symptomology precludes comparable analysis.
1Department of Environmental Science, Policy and Management, University of California, Berkeley, 145 Mulford Hall, Berkeley, CA 94720-3114; dfry@nature.berkeley.edu
2Department of Environmental Science, Policy and Management, University of California, Berkeley, 145 Mulford Hall, Berkeley, CA 94720-3114
3Department of Environmental Science, Policy, and Management, and CAMFER, 145 Mulford Hall, University of California, Berkeley, CA 94720-3114
4School of Forest Resources and Environmental Science, Michigan Technological University, 1400 Townsend Dr., Houghton, MI 49931
5Department of Environmental Science, Policy, and Management, Division of Insect Biology, University of California, Berkeley, CA 94720
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