EFFECTS OF CLIMATIC VARIABILITY AND CHANGE ON FOREST ECOSYSTEMS –
EMPIRICAL EVIDENCE FROM THE WESTERN MOUNTAIN INITIATIVE NETWORK
PETERSON, DAVE (1); FAGRE, DAN (2); MCKENZIE, DON (1); ALLEN, CRAIG (3); BARON,
JILL (4); STEPHENSON, NATE (5)
(1) USDA Forest Service, PNW Research Station, Seattle, WA, (2) USGS Northern Rocky
Mountain Science Center, West Glacier, MT, (3) USGS Fort Collins Science Center, Los Alamos,
NM, (4) USGS Fort Collins Science Center, Fort Collins, CO, (5) USGS California Science
Center, Three Rivers, CA
The effects of variability and directional trends in climate during the past century are measurable
in forest ecosystems at several locations in the Western Mountain Initiative network. In the
American Southwest, prolonged drought has caused dieback and reduced productivity in large
areas, with the potential for severe fires due to fuel accumulations. In the Sierra Nevada, drought
has also affected forests, with reduced tree vigor and increased mortality. Mixed conifer forests
in this region are also stressed by elevated levels of oxidant air pollution. In the Pacific
Northwest, subalpine forests have experienced rapid in-fill of subalpine meadows throughout the
region. In the Northern Rocky Mountains, subalpine forests at altitudinal treeline have also filled
in and transitioned from krummholz to a more upright growth form at some locations. In the
central Rocky Mountains, no significant changes are apparent, although high-elevation forests
are subject to elevated levels of nitrogen deposition. There appears to be a significant trend of
increased tree growth throughout the West, focused primarily on high-elevation forest and
secondarily on Pacific coastal forest – perhaps a signal of carbon dioxide fertilization. Potential
signals of the effects of climatic change must be differentiated from effects of the Pacific Decadal
Oscillation, which is an important multi-decadal influence on biophysical variability in Western
mountain ecosystems.