Enlargement of canopy gaps associated with a fungal pathogen in Yosemite Valley, California

The enlargement of 21 canopy gaps associated with the root pathogen Heterob asidion annosum (Fr.:Fr.) Bref. (= Fomes annosus (Fr.) Karst.) in the mixed - conifer forest of Yosemite Valley was monitored between 1971 and 1998. Me an expanded gap area was 232 m(2) (range 38-802 m(2)) in 1971 and 1455 m(2) (range 150-4216 m(2)) in 1998. The pathogen is primarily spread among tree s via root contacts; therefore, the spread and impact of H. annosum from th e stump(s) where infection originated (gap makers) were characterized by de scribing the area of potential root contact as a variable-area plot. The po tential root contact zone delineated the area that would be encompassed by mortality if all trees potentially in contact with the original gap makers had died. Of the 21 plots, the actual area of tree mortality exceeded the p otential root contact zone in only two gaps. Enlargement rates around gap p erimeters were uneven, ranging from 0 to 1.33 m/year. Incense-cedars (Caloc edrus decurrens (Torr.) Florin) tended to die more slowly than ponderosa pi nes (Pinus ponderosa Dougl.), often 10-30 years after adjacent pines had di ed. Gap enlargement was not dependent on host density, basal area, or gap-m aker diameter, and tended to stabilize over time, probably because of funga l competition in root wood and lack of sufficient suitable host material al ong gap margins.