Factors in Pinus ponderosa and Calocedrus decurrens mortality in Yosemite Valley,USA

Management practices over the past 100 years, especially fire exclusion and suppression, in Yosemite Valley, Yosemite National Park, CA resulted in forest stands with basal areas in excess of 110 m² ha^-1. Accelerated attack by several species of bark beetles (Scolytidae) followed a severe drought in 1976–77. Pinus ponderosa, already under attack by root rot (Heterobasidion annosus) was affected severely, and mortality exceeded 70%. Calocedrus decurrens did not suffer comparable mortality and has replaced P. ponderosa as the dominant species. Size class distributions as well as comparisons of relative density and relative basal area from 1961, 1976 and 1983–85 also demonstrate a major change in forest composition. This change is most pronounced on alluvial landforms.