In this paper we briefly review current knowledge concerning the possible consequences of a changing climate for terrestrial ecosystems. Forests and grasslands of western North America figure prominently in the discussion, but our intent is to consider general principles rather than specific communities. Nevertheless, these principles are applicable to the fate and management of old-growth forests. First, we review the current climate-change scenarios and their implications for plant growth and physiology. Next we consider the severity and frequency of disturbances such as fire, insect outbreaks, and windstorms. Then we turn to the possible net effect of climate change on ecosystems, an effect that emerges from numerous interactions and feedback processes, and that is greatly affected by the speed with which species migrate. Finally, we discuss some approaches for mitigating possible impacts and for easing transitions from one community type to another.