Large sugar pine (Pinus lambertiana) vigor and mortality in a fire-excluded forest of the central Sierra Nevada

In recent decades rapid environmental change has led to significant shifts in forest dynamics across the western United States. In particular, fire exclusion has led to denser forests with higher competitive stress, and climate change has increased temperatures, and affected water availability by altering snowmelt and evapotranspiration. As a result, tree vigor for many species has declined and contributed to increased rates of tree mortality, especially for large and old trees. Large trees provide important ecological services, but are rare on the landscape due to past logging activities. Therefore, forest managers have focused restoration efforts to improve old-growth forests conditions. However, the impact of environmental stress on tree vigor and mortality is a complicated process and more information about the relative importance of climate and competition is needed. This study investigated tree vigor and mortality for large sugar pine (Pinus lambertiana) in response to climate and competition at the Stanislaus-Tuolumne Experimental Forest in the Sierra Nevada. This mixed-conifer forest has experienced a long period of fire exclusion and increased warming that has likely contributed to greater vulnerability of large sugar pine to pathogen (e.g. white pine blister rust) and bark beetle attacks. Tree vigor was examined by analyzing annual measurements of growth (i.e. basal area increment, BAI) and resin duct defenses. Chapter 1 examined the response of large sugar pine growth and defense to climate (i.e. temperature, precipitation, climatic water deficit), and retrospective competition using generalized linear mixed models. Chapter 2 modeled the relative importance of growth, defense, and competition on the probability of large sugar pine mortality using logistic regression. Reduced BAI in large sugar pine was more strongly associated with lower January temperatures, less precipitation from the previous October through December, higher interspecific competition, and higher intraspecific competition (R2 = 0.81, RMSE = 9.96). Resin duct size was most associated with water deficit, precipitation from the previous October- current April, and total competition (R2 = 0.66, RMSE = 0.022). Resin duct total area was associated with water deficit, precipitation from the previous October- current April, total competition, and the interaction of total competition with June temperature (R2 = 0.54, RMSE = 0.2108). Measures of competition had a stronger relationship with large sugar pine growth and defense when compared to measures of climate. The best model of large sugar pine mortality included growth, defense, and competition variables. Declining growth (i.e. the slope of BAI) and lower growth variability during the 10 years before mortality had the greatest association with mortality (area under ROC = 0.93). Internal validation of the top mortality model correctly classified 87.9% of dead sugar pine and 84.8% of live sugar pine. Results from this study highlight some of the trade-offs between growth and defense in response to climate and competition, and the importance of declining growth and defense leading to a higher probability of mortality.