Changes in sub-alpine tree distribution in western North America: a review of climatic and other causal factors

Changes in the distribution of sub-alpine tree species in western North America have been attributed to climatic change and other environmental stresses. These changes include tree-line fluctuations throughout the Holocene and recent invasion of sub-alpine meadows by forest. Most palaeoecological studies suggest that the tree-line was higher during a period of warmer climate approximately 9000 to 5000 BP and lower during the last 5000 years, with short periods of local tree-line advance. Recent advances in sub-alpine tree distribution can be compared with weather records, allowing an examination of relationships between tree advance and climate at a finer resolution. In general, recent sub-alpine forest advances in western North America, based on studies representing three climatic zones (maritime, Mediterranean and continental), have been associated with climatic periods favouring tree germination and growth, although factors such as fire and grazing by domestic livestock have had an impact in some areas. Limitations to tree establishment (e.g., winter snow accumulation, summer drought) vary in relative importance within each climate zone, as do predicted consequences of anthropogenic climatic change. Recent increases in establishment of sub-alpine trees may continue if climatic change alleviates the limitations to tree establishment important in each climatic zone. However, factors such as topography and disturbance may modify tree establishment on a local scale.