1. Recent Changes in Climate and Forest Ecosystems

In recent decades, gradual warming over the long term has been punctuated by droughts that facilitate widespread ecological disturbances. Although no single event can be attributed to climate change, it is reasonable to infer that a permanently warmer climate will escalate disturbances, causing a much faster change in ecosystem structure and function than a gradual increase in warming. Various species of pine beetles have spread across large land areas in Alaska, the western United States, and southern United States, in some cases attacking tree species that have not experienced previous outbreaks. Area burned by wildfire has been especially high during the 2000s. A reduction in the quantity and persistence of snow in mountainous regions is affecting the hydrology of forest ecosystems and downstream water supply. Interactions of multiple disturbances and stressors may result in new combinations of species and ecosystem conditions for which there is no precedent in historical or paleoecological records. Rapid shifts in climate and disturbance may strain both the resilience of forest ecosystems and the capacity of social systems and management institutions. In the future, shifting the management focus from restoring systems to building resilience will be a more viable strategy for retaining key ecological functions and ecosystem services.