Climate Impacts to Forest Ecosystem Processes:
Douglas-fir Growth in Northwestern U.S. Mountain
Landscapes and
Area Burned by Wildfire in Western U.S. EcoprovincesPh.D. Dissertation Abstract by
Jeremy S. Littell (2006)
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.