Topics in forest pathology and ecology in the Sierra Nevada and the Sierra San Pedro Martir, Baja

Logging and fire exclusion policies in the Lake Tahoe Basin have increased tree densities over the past 150 years. Current tree densities range up to 450 trees/ha. Cumulative tree mortality in logged (mean = 25%) and unlogged (mean = 21%) stand types were significantly and positively correlated with tree density. The synergistic effect of bark beetles, pathogens, and stand density linked with drought events largely explain mortality in these forests.;Unlogged, mixed-conifer forests with the historical fire regime still intact were studied in the Sierra San Pedro Martir, Baja, Mexico. Average tree density was low at 160 trees/ha. Cumulative mortality was 12.7%, with the greatest amount of mortality occurring to larger trees. Most tree mortality (78%) was due to pathogens and bark beetles. Mistletoe and a bark beetle species were widespread on Abies concolor. Mistletoe severity was negatively correlated to A. concolor regeneration.;White pine blister rust (WPBR) is a heteroecious rust fungus that alternates between 5-needle pines, and Ribes to complete its life-cycle. In mixed-conifer forests, WPBR prevalence on Pinus lambertiana was correlated with the nearness of Ribes and influenced by environmental conditions favorable for rust infection. Disease was spatially aggregated with new infections occurring annually. In subalpine forests, disease was not correlated with the presence of Ribes. In this exposed location, disease may be episodic rather than chronic. When conditions are favorable, wind allows for widespread dispersal of spores. Demographic effects of this disease on its pine hosts include juvenile mortality and reduced cone production.;In two locations in the Sierra Nevada, we found the prevalence and severity of dwarf mistletoe (DWM) on A. concolor was not or weakly correlated to host density, but severity was positively correlated to host size. On Pinus jeffreyi, DWM prevalence and severity were positively correlated with host density. Individuals of all sizes were susceptible to DWM, with less than expected becoming infected in the seedling-10 cm diameter class. Both aggregated and random spatial patterns were found for DWM, suggesting that the degree of infection and logging history are important in the spatial dynamics of DWM species.