Master’s Thesis Abstract by Jill Nakawatase (2003)

Spatial and temporal variability in growth-climate relationships is largely unknown for many Pacific Northwest forests, yet it is this information that is needed to predict how forests will respond to future climatic change. I studied the effects of climatic variability on 71 sites in the western (Hoh River drainage) and northeastern (Dungeness River drainage) Olympic Mountains. Study sites span a wide range of elevations (190-2000 m), are spread across two distinct climatic regimes, and are composed of multiple tree species. Basal area increment chronologies were developed for each site, and Pearson correlation analysis and factor analysis were used to quantify growth-climate relationships.

Tree growth in the Olympic Mountains responds to climatic variability differently as a function of mean climate and elevation. Low summer moisture limits growth across all elevations in the dry northeastern Olympics. Growth at low elevations in the wet western Olympics is associated with PDO and summer temperature. Heavy winter snowpack, through its effect on the length of the growing season, limits growth at high elevations in the western Olympics. In the warmer and wetter climate predicted for the Olympic Mountains, productivity at high elevations of the western region will likely increase as a result of reduced snow cover and warmer summer temperatures, except at the highest elevations. Productivity at high elevations in the northeastern region and potentially in low elevations of the western region will likely decrease due to lower summer moisture.

This study shows that growth-climate relationships on the Olympic Peninsula vary at small spatial scales. Generalizations of species response to climatic variability across altitudinal gradients and in different climatic environments, even for a single species, neglect the fine-scale variability of growth-climate interactions. Future studies must consider a variety of biogeographic settings to accurately estimate growth-climate relationships at the local to regional level.