Patterns of mortality in an old-growth mixed-conifer forest of the southern Sierra Nevada, California

Mortality patterns in an old-growth, mixed-conifer forest, in the absence of wildfire, were investigated at the Teakettle Experimental Forest from 2000 to 2002. We tested the hypothesis that after a century of fire suppression, pathogen- and insect-associated mortality (between episodic droughts) would be significantly greater on ingrowth trees (i.e., smaller-diameter, shade-tolerant species in high-density clusters). Using a survey of over 30,000 mapped trees, overall mortality, as measured by standing dead trees, was 8.7% of all stems >= 5 cm dbh. Mortality levels were proportional to the population size of the five dominant conifer species, white fir (Abies concolor), red fir (A. magnifica), incense cedar (Calocedrus decurrens), Jeffrey pine (Pinus jeffreyi), and sugar pine (Pinus lambertiana). There was also no significant difference in mortality between shade-tolerant and shade-intolerant species. All dead trees were clustered within plots. Mortality was significantly higher than expected for large-diameter trees (> 100 cm dbh) with all conifer species combined and for each individual species, except Jeffrey pine. Small-diameter dead trees were grouped in high-density clusters. Mortality was less than expected among small-diameter trees (5-20 cm dbh) for all species combined, red and white fir, and sugar pine. Mortality for all conifers was higher than expected in areas of high stand density and lower in areas of low stand density. Mortality of small-diameter trees was clustered and particularly high in areas of high stand density. Our data suggest pathogen- and insect-associated mortality is significantly greater in areas of high stand density but it is not higher for shade-tolerant species. Furthermore, mortality is higher than expected for large-diameter trees, suggesting an acceleration of old-tree mortality under current fire suppression conditions.