Annosus Root Disease in True Firs in Northern and Central California National Forests

True fir stands in California (3.5 million acres) were surveyed during 1979-1980 to estimate the prevalence of infection by Heterobasidion annosum, and to determine stand and tree characteristics associated with the occurrence of annosus root disease. Approximately 4 percent (1.46 billion board-feet) of the live true firs were estimated to be infected by H. annosum; dead firs associated with the fungus were estimated to be 25 percent (68 million board-feet) of the total dead firs during a 1.5-year period. Annosus root disease was estimated to infest 18 percent of the surveyed area (632,000 acres), indicating a high prevalence in fir stands. The fungus occurred most often in pure fir stands, red fir stands, in stands where the basal area of live true fir exceeded 20 m per hectare, and in stands having patterns of chronic mortality. Larger, older trees, near stumps, had high rates of infection. Annosus root disease, caused by Heterobasidion annosum (Fr.) Bref. (Fomes annosus [Fr.] Karst.), causes root decay and mortality in conifers throughout California (Bega and Smith 1966). In red fir (Abies magnifica A. Murr.) and white fir (Abies concolor [Gord. & Clend.] Lindl.), root decay often leads to bark beetle attack (Cobb and others 1974) or to windthrow (Smith 1978). Forest productivity can be seriously reduced by such losses. The presence of annosus root disease in California forests has been known for decades, but an objective estimate of its prevalence is lacking. Statewide estimates of the prevalence of annosus root disease in true fir forests, the amount of tree mortality associated with 1 Presented at the Symposium on Research and Management of Annosus Root Disease in Western North America, April 18-21, 1989, Monterey, California. 2 Staff Research Associate IV and Professor, respectively, Department of Plant Pathology, University of California, Berkeley, California. infection by the fungus, and the stand and tree characteristics related to its occurrence should be useful for the evaluation of the impact of this root pathogen upon forest productivity. In the past, root disease surveys generally focused on sites of tree mortality, rather than on entire forests. Surveys incorporating aerial photography (Hanson and Lautz 1971, Johnson and Wear 1975, Williams and Leaphart 1978, Byler and others 1979, James and others 1984) detected standing dead trees and stand openings suggestive of mortality centers. Surveys of states and regions have often incorporated data from several sources collected by different methods and with various levels of precision (Smith 1984). Ground surveys generally have involved transects through forest stands (Bloomberg and others 1980, Filip and Goheen 1984), studies of tree mortality in plantations (Hadfield 1970, Filip 1979), or scouting surveys near roads (Bega and others 1966). Hamilton (1984) discussed the need to sample both live trees and dead trees to estimate mortality rates. The survey reported here had two objectives: to estimate the prevalence (the proportion of trees infected out of the population of both live and dead trees, at a given time) of annosus root disease in true firs on the National Forests of northern California, and to describe stand, site, and tree characteristics associated with the occurrence of annosus root disease in stands with true firs. Estimates were, therefore, necessary both for stands having evidence of root disease and also for apparently healthy stands showing no tree mortality on aerial photographs. The methods discussed below are a compromise among the needs to survey the forest as a whole, without reference to tree mortality, to sample a sufficient number of recently dead fir trees to allow a useful count of fir mortality, and to provide sufficient opportunities to collect data at locations where annosus root disease occurs.