Fire is recognized as one of the primary disturbances in mixed conifer forests in the Sierra Nevada, but there has been comparatively little research on the variability of fire regimes across major environmental and compositional gradients and the role it plays in patterning forests from stand to landscape scales. Reconstructions of historic forest conditions are used to determine the influence of fires on the structure and composition of mixed conifer forests, and to determine the degree of change in the forests since fire suppression.

Forest structure data (size, age, composition and spatial pattern) was collected on a systematic sampling grid of 62 plots over 10km2 in the mixed conifer zone in Yosemite National Park. Fire scars were collected in 9ha plots centered on each grid point to determine the fire history for the study area. Historic forest structure was reconstructed by subtracting radial growth from each tree core equal to the number of years since the last recorded fire.

Analysis of the fire scar data suggests that the area experienced very frequent, low to moderate severity fires. The average fire return interval (FRI) for individual trees was 14 years, while the composite average FRI for the entire study area was 5-8 years. The majority of samples recorded their last fire before 1900. Fires occurred primarily at the end of the growing season and into the dormant season, after tree growth stopped for the year. There was also a strong correspondence between years of widespread fire and drought drought years identified by the Palmer Drought Severity Index.

Analysis of the vegetation data indicate a wide range of structural and compositional changes that have occurred due to fire suppression. The pre-settlement canopy was composed mostly of ponderosa pine and sugar pine, with lesser amounts of incense cedar. On mesic sites, there is a significant component of white fir and Douglas-fir in the overstorey. The post fire-suppression understorey is mainly incense cedar and white fir. Very little ponderosa pine or sugar pine occurs as seedlings or saplings in the understorey. The density of the forest has increased significantly from an average of 140 trees per hectare in 1899 (time of last fire) to an average of 1000 trees per hectare in 2002. In contrast, there was relatively little change in the basal area of the forest from an average of 101m2/ ha in 1899 to an average of 109m2/ ha in 2002. At the same time there has been a compositional shift from fire tolerant species (ponderosa pine) to fire intolerant species (white fir). The increase in density is due to the lack of fire over the last 100 years.