A warmer climate in western North America will affect forests directly through soil moisture stress and indirectly through increased extent and severity of disturbances. Following a pathological model for individual trees, we propose the idea of stress complexes in forests, combinations of biotic and abiotic stresses that 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 multiyear drought, insects, and fire. We present examples that illustrate how stress complexes are region-specific and are magnified under a warming climate. 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, especially lodgepole pine, in the northern interior of western North America, setting up the prospect of large and intense fires. Fire and insect mortality have also reached unprecedented levels in both interior and southern Alaska, possibly precipitating extensive ecosystem 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.