Moisture relations, genetics, and the production of defensive terpenoids by white fir (Abies concolor Gord. and Glend.) in response to the bark beetle-vectored fungal pathogen Trichosporium symbioticum Wright

Fifty-eight white fir (Abies concolor) trees growing at a USDA-FS research plantation and representative of 17 provenances were inoculated with the pathogenic fungus, Trichosporium symbioticum during two years of below-normal precipitation (1990 & 91) and a year with above-average precipitation (1993). The resultant fungal wound lesions (approx. 1,500 total) were removed from the trees after three weeks and then analyzed by gas chromatography. The results showed that not only did monoterpenoids increase in response to fungal challenge, but that the size and vigor of the response is apparently related to moisture availability. For example, the lowest amounts of total wound-lesion monoterpenoids were produced in the fall of 1990 (10.0 mg/g). The amounts of total monoterpenoids double in the early summer of 1991 to about 25.0 mg/g (a year of increased precipitation over 1990 but, still below normal). The early summer 1993 inoculation produced the most dramatic resin content increase with total monoterpenoids again doubling to an average of 45 mg/g. Wound-lesion sizes were similar during the 1990/91 inoculations (150 sq mm), however, double in size during 1993 (300 sq mm). Phloem moisture content was also lowest in the 1990 inoculated phloem (41%), 45% in 1991 and 50% in 1993. Pre-dawn moisture stress measured just prior to tree inoculation was highest during 1989/90 at 15.1 bar and only 9.5 bar in 1993. Genetic comparisons indicate that the most susceptible provenances (central California) tend to produce the lowest amounts of monoterpenoids compared to surrounding provenances. Total monoterpenoids comprised mostly of α-pinene exhibited a clinal pattern in concentration among Hamrick and Libby's (1972; 1980) 5 morphological divisions of white fir. Of 1,127 trees surveyed during a fir engraver beetle infestation at the USDA-FS Camino plantation (1988–93), the central California trees exhibited the highest mortality. Central California provenances contained the lowest total monoterpenoids. These results suggest that a major factor in the ability of conifers to produce defensive monoterpenoids in response to a pathogenic fungus may be determined by genetics as well as moisture availability. Because a similar percentage of trees were also killed from Oregon and contained the largest amounts of total monoterpenoids during 1990, 91, and 93, the results also suggest other factors besides genetic ability for total monoterpenoid production alone influences host tree survival.