Climate drivers of Douglas-fir growth in the western United States

Abstract

Douglas-fir (Psuedotsuga menziesii) spans the entire mountain system of the western United States, successfully occupying many different climatic and ecological niches. Its occupation of many different growth environments, and its temporal persistence on the landscape, make this species an ideal representative of forest-climate interactions in western mountain ecosystems. To quantify climate-growth relationships in Douglas-fir, I developed a comprehensive network of chronologies collected across the "climate space" of the species. By sampling throughout climate space at the continental scale, I account for a large percentage of variability in growing environments for Douglas-fir. Data are summarized across six regions - Pacific Northwest, Northern Rockies, Central Rockies, Southern Rockies, California, and Southwest. Tree growth data were combined with data from the Variable Infiltration Capacity Hydrologic Model, which includes typical climate variables as well as "plant relevant" variables. Climate data include precipitation, temperature, potential evapotranspiration, actual evapotranspiration, and vapor pressure deficit. Climatic water deficit was calculated as actual evapotranspiration minus potential evapotranspiration. The relationship between tree growth and climate was analyzed at four spatial scales (plot, watershed, region, and continent) and three temporal scales (monthly, interannual, interdecadal). Results suggest that variability in growth is tightly coupled to both interannual and decadal climatic variability, with evident linkages to the El Niño Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO). Temperature exerts a top-down control on tree growth, regardless of the magnitude of precipitation. As air temperature increases, evaporative demand also increases, causing increased vapor pressure deficit and climatic water deficit (potential evapotranspiration minus actual evapotranspiration), altering the dynamics of water availability in forest ecosystems. Additional analyses focus on how large-scale climate teleconnections modify regional climate patterns that ultimately limit tree growth. Proximity to the dipole dictates the relative effect of ENSO events from region to region, and the strength and location of the dipole change when ENSO and PDO are in phase. ENSO- and PDO-related changes in regional climate result in increased variability and extremes in tree growth. Changes in tree growth occur in phase with ENSO across all regions, but the strongest response is at the extreme ends of the dipole. The complex relationship between tree growth and climate documented in this study can provide parameters for growth models, inform climate-change adaptation plans, and project future growth in Douglas-fir forests.