Sensitivity of Southwestern US Mountain Ecosystems to Climate Variability: Interactions
Among Forest Dieback, Fire, and Erosion
* Allen, C D (craig_allen@usgs.gov) , USGS Jemez Mts Field Station, HCR 1, Box 1, No. 17,
Los Alamos, NM 87544 United States
Millions of hectares in the upland landscapes of the Southwestern United States have been
affected by forest dieback and severe fire activity since the late 1990s, a period of ongoing severe
drought and unusual warmth. Climate regulates physiological plant stress that can directly cause
vegetation mortality, and also influences associated insect outbreak dynamics. Climate also
interacts with fuel conditions to drive regional fire activity. Current and historic patterns of forest
dieback, fire activity, and erosion are described across landscape gradients in Southwestern
mountains, particularly the Jemez Mountains of New Mexico. Methods used include inventory
and dating of live and dead woody plants to assess demographic changes through time, long-term
(since 1991) measurements of ponderosa pine tree-growth at three sites with dendrometer bands,
monitoring of herbaceous vegetation along 3 km of permanent transects since 1991, aerial
photograph analyses of insect outbreaks and forest dieback and fire activity, and hydrological
measurements of runoff and erosion. Similarities and differences in vegetation dieback and
regional fire activity patterns between the current drought and the 1950s (when regional drought
last affected the Southwest) are explained by changes in climatic and vegetation conditions. The
current climate-induced vegetation dieback and pulse of regional fire activity have strong
feedbacks with various key ecosystem processes, including water budgets and soil erosion. For
example, severe drought and fire both markedly reduce the surface cover of live plants and dead
plant materials ("litter"), triggering nonlinear increases in erosion rates once the connectivity of
bare soil patches exceeds critical threshold values, particularly during high-intensity summer
rainfall events that characterize the Southwestern summer "monsoon". These observations
highlight the magnitude, rapidity, and complexity of climate-induced disturbance processes, and
provide an analog for potential nonlinear impacts of climate change to mountain ecosystems.