Pathogens and plant community structure in an old-growth mixed-conifer forest of the Sierra Nevada, California

The Teakettle Ecosystem Experiment consists of an old-growth mixed-conifer forest with over 30,000 mapped trees in the Sierra Nevada Mountains, California. Multiple research projects emphasize the impacts of various old-growth restoration techniques. As is typical in western coniferous forests with fire suppression for the past 100+ years, Teakettle forest stands have become denser with a shift from shade-intolerant fire-resistant species (Pinus lambertiana and P. jeffreyi) to younger shade-tolerant fire-susceptible species (Abies concolor, A. magnifica, and Calocedrus decurrens). We utilized the forest to investigate the impacts of these stand changes on patterns of tree mortality, canopy gaps and pathogens in the absence of wildfire.; Overall mortality as measured by standing dead trees was found to be 8.7% of all stems 5cm dbh or greater. Mortality levels were proportional to the population size for the five conifer species with no significant difference between shade tolerant and intolerant species. Mortality was significantly higher than expected for large diameter trees (>100cm dbh) except for Jeffrey pine. Mortality was higher than expected in dense stand areas and lower than expected in open areas. All dead trees and small diameter dead trees (5--20cm dbh) were highly clustered within plots and within areas of high stand density.; Canopy gaps were analyzed using repeat aerial photography. Within the forest, the area in canopy gaps increased from 9.7% in 1940 to 43.1% in 2001. Gap locations were significantly related to shallow depth to bedrock. Annosum root rot (caused by Heterobasidion annosum) and bark beetles were significantly associated with canopy gaps.; Of all white firs, 25% were infected with the white fir dwarf mistletoe (Arceuthobium abietinum f. sp. concoloris). Average dwarf mistletoe rating (DMR) was 0.701. Average dwarf mistletoe index (DMI) was 2.834. Incidence, DMR and DMI all increased with increasing stem diameter and tended to increase with decreasing stand density. Bark beetles were significantly attracted to non-infected trees over mistletoe-infected trees for all but the smallest diameter white firs (5--20cm dbh).