C limatic change is likely to affect Pacific Northwest (PNW) forests in several important ways. In this paper, we address the role of climate in four forest ecosystem processes and project the effects of future climatic change on these processes. First, we analyze how climate affects Douglas-fir growth across the region to understand potential changes in future growth. In areas where Douglas-fir is not water-limited, future growth will continue to vary with interannual climate variability, but in places where Douglas-fir is water-limited, growth is likely to decline due to projected increase in summer potential evapotranspiration. Second, we use existing analyses of climatic controls on future potential tree species ranges to highlight areas where species turnover may be greatest. By the mid 21 st century, some areas of the interior Columbia Basin and eastern Cascades are likely to have climates poorly suited to pine species that are susceptible to mountain pine beetle, and if these pines are climatically stressed, they may be more vulnerable to pine beetle attack. Climatic suitability for Douglas-fir is also likely to change, with substantial decreases in climatically suitable area in the Puget Trough and the Okanogan Highlands. Third, using regression approaches, we examine the relationships between climate and the area burned by fire in the PNW and in eight Washington ecosystems and project future area burned in response to changing climate. Area burned is significantly related to both temperature and precipitation in summer, but more physiologically relevant variables, such as water balance deficit, perform as well or better in models. Regional area burned is likely to double or even triple by the end of the 2040s, although Washington ecosystems have different sensitivities to climate and thus different responses to climatic change. Fourth, we evaluate the influence of climatic change on mountain pine beetle (MPB) outbreaks by quantifying both host-tree vulnerability and pine beetle adaptive seasonality. Host-tree vulnerability is closely related to vapor pressure deficit (VPD), and future projections support the hypothesis that summer VPD will increase over a significant portion of the range of host tree species. Due to the increased host 255vulnerability, MPB populations are expected to become more viable at higher elevations leading to increased incidence of MPB outbreaks. The increased rates of disturbance by fire and mountain pine beetle are likely to be more significant agents of changes in forest structure and composition in the 21st century than species turnover or declines in productivity. This suggests that understanding future disturbance regimes is critical for successful adaptation to climate change.

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