Differential susceptibility of white fir provenances to the fir engraver and its fungal symbiont in northern California
The fir engraver, Scolytus ventralis LeC., attacks white fir, Abies concolor (Gord. and Glend.) Lindl., and other true firs, Abies spp., in western North America. The biology, attack behavior, and ecology of this bark beetle were recently summarized by Berryman and Ferrell (1988). During the summer flight season, the attacking beetles bore into the cambial zone of fir boles, introducing a pathogenic brown-staining fungus, Trichosporium symbioticum Wright. Resistant firs react to the invasion by forming a resinous necrotic wound in the phloem and outer sapwood, which contains the spread of the fungus and repels or kills the beetles. In such interactions, the tree usually survives. This reaction is less intense or absent in susceptible firs, resulting in reproductive success of the beetles, and severe damage or even death of the fir.Sporadic outbreaks of the fir engraver, associated primarily with droughts, have caused widespread mortality of true firs in nearly every decade of this century in western North America.In the drought years of 1987-88, four 26-year-old white fir provenance test plantations at Camino, California, located at 1028 m elevation in the central Sierra Nevada, sustained considerable levels of mortality.Subcortical examination of a subsample of the dead firs indicated that all had reproductively successful gallery systems of the fir engraver. Drought continued until winter 1992-1993. Surveys beginning in fall 1988 revealed pronounced differences in this mortality, among both plantations and provenances, and also within plantations. This paper describes the observed patterns of susceptibility and resistance in relation to known patterns of geographic variation in white fir in western North America and discusses studies underway to understand the mechanisms responsible for these patterns.