Changing Climates, Changing Forests: A Western North American Perspective

Abstract

The Earth’s mean surface air temperature has warmed by ~1C over the last 100 years and is projected to increase at a faster rate in the future, accompanied by changes in precipitation patterns and increases in the occurrence of extreme weather events. In western North America, projected increases in mean annual temperatures range from ~1−3.5C by the 2050s, and while projected changes in precipitation patterns are more complex to model, more frequent and severe droughts are expected in many areas. For long-lived tree species, because of their relatively slow rates of migration, climate change will likely result in a mismatch between the climate that trees are currently adapted to and the climate that trees will experience in the future. Individual trees or populations exposed to climate conditions outside their climatic niches will be maladapted, resulting in compromised productivity and increased vulnerability to disturbance, specifically insects and pathogens. In western North America, as elsewhere, several recent assessments have concluded forests are being affected by climate change, and will become increasingly vulnerable to mortality as a result of the direct and indirect effects of climate change. Droughts associated with higher temperatures may accelerate levels of tree mortality, for example, as elevated temperatures increase metabolic rates without increasing photosynthesis rates, thus compromising a tree’s ability to create defenses against insects and pathogens. As a result, distributions of the climatic niches of some tree species in western North America are predicted to change by up to 200% during this century based on bioclimate envelope modeling. We discuss the science of climate change, the implications of predicted climatic changes to forest ecosystems in western North America, and the essential roles of forest managers and researchers in addressing climate change.