Effects of Biotic Interactions on Climate-Growth Relationships of Douglas-Fir and Ponderosa Pine

Plant processes depend on the interplay between intrinsic characteristics (e.g., photosynthetic capacity) and external variables, such as light, temperature and water availability. However, these factors are often tightly interconnected and vary significantly among species with different life history strategies and within a species across environmental gradients. Moreover, plant-plant interactions may directly affect both intrinsic variables and local environment through direct effects on resource availability and habitat structure. Yet, despite general scientific agreement that the relative effects of abiotic stress and competition are directly linked, relatively little is known about the effects of competitive interactions on climate-growth relationships of trees. This is largely because previous research addressing the issue has relied almost exclusively on short-term studies using short-lived, herbaceous species. However, unlike most short-lived plants, trees can substantially modify their ability to tolerate stress or acquire resources as a consequence of plastic responses to external environmental conditions experienced in their lifetimes, resulting in individualistic responses to environmental change. A clearer understanding of the relationship between competition and climate-growth relationships of mature trees is critically needed in order to accurately predict forest ecosystem responses to climate change and understand how local management actions could be used to influence these responses -- arguably the most important research and management challenges of our time. To address these issues, I quantify the relative influence of competition and environmental conditions on the climate-growth relationships of two dominant conifer species, Pinus ponderosa and Psuedotsuga menziesii, across their full range of growing conditions within the Colville National Forest of eastern Washington. Specifically, I analyze tree ring records using time series analysis and mixed effects models to, (1) investigate the effect of competition on climate-growth relationships; (2) assess how these relationships change between species and across environmental gradients; and, (3) explore linkages between environmental factors and drought responses across multiple spatial scales. Findings will help improve predictions about vegetation responses to climate change, address conflicting hypotheses about the dynamic role of competition along environmental gradients and help managers better understand how manipulating stand density and structure will modify tree responses to climate change