Global warming and stress complexes in forests of western North America

A warmer climate in western North America will likely affect forests directly through soil moisture stress and indirectly through increased extent and severity of disturbances. We propose that stress complexes, combinations of biotic and abiotic stresses, compromise the vigor and ultimate sustainability of forest ecosystems. Across western North America, increased water deficit will accelerate the normal stress complex experienced in forests, which typically involves some combination of multi-year drought, insects, and fire. Four examples suggest how stress complexes are region-specific. Symptoms of prolonged drought and insects are currently manifested in extensive dieback of pine species in the pinyon-juniper forest of the American Southwest, an area where only a few tree species can survive. Air pollution and high stand densities from fire exclusion have compromised mixed-conifer forests of the Sierra Nevada. Bark beetles are proliferating and killing millions of hectares of dry forest in the northern interior of western North America, setting up the prospect of large and intense fires. Fire and insect mortality have also exceeded previously recorded levels in both interior and south-central Alaska, possibly precipitating extensive ecosystem changes, while extensive permafrost degradation is causing other changes. Increases in fire disturbance superimposed on forests with increased stress from drought and insects may have significant effects on growth, regeneration, long-term distribution and abundance of forest species, and short- and long-term carbon sequestration. The effects of stress complexes will be magnified given a warming climate.