Climate impacts to forest ecosystem processes: Douglas-fir growth in Northwestern United States mountain landscapes and area burned by wildfire in western United States ecoprovinces.

摘要

Climate plays an important role in the structure and function of forest ecosystems on seasonal to evolutionary time scales and on local to planetary spatial scales. The supply of water and thermal energy can both facilitate and limit the rates of important processes throughout the ecological hierarchy. In this dissertation, I demonstrate a combination of appropriate scale and gradient-based inquiry for two studies of climate impacts to ecosystem processes in the western U.S.A: the area burned by fire in western ecoprovinces and the growth of Douglas-fir (Pseudotsuga menziesii) in northwestern mountain landscapes. First, I present relationships between the area burned by fire and climate for the period 1916-2003 in the western U.S.A. I use a novel reconstruction technique to backcast late-20th century datasets and show that the area burned by wildfire in the West was significantly controlled by climate for the full period. Persistent, ecosystem-specific correlations between climate variables and area burned are grouped by vegetation type for 16 ecoprovinces across the West. For the period 1977-2003, between 33 and 87 percent (mean 64 percent) of the variability in ecoprovince area burned could be explained by a few significant climate variables. For the period 1916-2003, between 25 percent and 57 percent (mean 39 percent) of the total variability could be accounted for with climate. In both cases, precipitation variables were more important than temperature. The relationship between the mean and the variance for area burned exhibits a gamma distribution for independent data sets and different spatial scales of fire data. Second, I developed a network of Douglas-fir tree-ring chronologies from the western Olympic Peninsula in Washington to the eastern Rocky Mountain Front in Montana. Annual radial growth in 60-65% of the plots across the entire region is significantly correlated with variables describing precipitation, drought or water balance during the late summer prior to growth and the early summer the year of growth. Few plots are significantly positively correlated with cool-season temperature or negatively correlated with snowpack. Water availability is therefore more commonly limiting to Douglas-fir growth than factors influencing the length of the growing season.