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Additional key words: oak decline, SOD
We catalogued failures in coast live oak (Quercus agrifolia) in natural stands at six locations in Marin County, California. Phytophthora ramorum canker (sudden oak death) was present at relatively high levels in these stands. We noted characteristics of each failure; estimated the date range in which the failure occurred; evaluated the condition of the tree with respect to P. ramorum infection, colonization by Hypoxylon thouarsianum and other decay fungi, beetle boring, and tree defects; and noted stand characteristics in the immediate vicinity of the failure. Nonfailed trees in the stand were used as a control population to which failed trees were compared. Results from natural woodlands were compared to coast live oak failures recorded in the California Tree Failure Report Program (CTFRP) database. Failures recorded in the CTFRP database date back as far as 1987 and occurred primarily in urban trees rather than in natural stands.
Stands of coast live oak that have been impacted by P. ramorum canker show rates of tree failure that are higher than recent historical failure rates. Among trees that failed between about July 2001 and December 2002, 83% showed symptoms of P. ramorum infection. Branch, scaffold, bole, and root crown failures showed a strong association with advanced symptoms of P. ramorum canker. By definition, advanced or late symptoms of P. ramorum canker include evidence of wood degradation by H. thouarsianum and/or various wood boring beetles. Early P. ramorum canker symptoms, in which the only sign of infection is bleeding cankers, were not associated with an increased likelihood of failure.
Bole failures were the most common type of failure in these P. ramorum-affected woodlands. For the years 1992 through 1996, we estimated that bole failures occurred in about 0.5% of the trees each year. The incidence of bole failures increased to 5% per year for the period from mid-2001 through the end of 2002. Among recent failures (2001-2002), 39% of the bole failures and 30% of the scaffold failures occurred in live stems. The majority of observed root and root crown failures also occurred in live trees. Root failures were only observed in 4% of the failed trees in this study. In contrast, root failures are the most common failure type reported in the CTFRP database, making up 39% of reported failures.
Wood decay was the most consistent and important factor influencing failure potential. Decay was present and rated as a contributing factor in almost all failures. Fruiting bodies of H. thouarsianum and other wood decay fungi, decay columns, and canker rot symptoms were significantly more common among failures than among nonfailed controls. Also, several variables related to decay were highly significant in both recursive partition and multivariate logistic regression models. Beetle boring was also significantly more common among failures than among nonfailed trees. Other factors associated with increased failure potential include overtopping of the tree canopy, local alteration of the stand canopy due to dead or failed trees, multiple trunks, multiple branches arising from the same point, and asymmetric canopy shape. Failures in live and dead trees were largely influenced by the same factors.
Based on our results, we present a set of preliminary guidelines for evaluating failure potential of coast live oak in woodlands affected by P. ramorum. The most important risk factors in these guidelines are factors related to the amount of decay present in the tree.