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.

Subalpine fir (Abieslasiocarpa (Hook.) Nutt.) regeneration following fire was studied at two locations that burned in 1902 northeast of Mount Rainier, Washington. Tree establishment dates were compared with local climatic records using multiple and logistic regression to identify potential relationships between seasonal climate and annual tree establishment. The influence of microsite features on forest regeneration was also explored. Little regeneration occurred in the first 30 years after the fires, and most trees established in the 1950s, 1977, 1983, and 1989. The dominance of trees <50 cm tall at both sites indicates that trees are continuing to establish on these burns. Establishment is positively correlated with warm, dry springs with low snow accumulation, and cool, wet summers with some variation in significant monthly climate between sites. Tree establishment following fire is greater near other trees and woody debris than other microsite features; however, the microsites available and used for tree establisment may change over time as snags fall and trees establish. More than 50% of the recently established trees (<50 cm tall) are located near larger trees and shrubs, and few (<10%) trees are growing in exposed mineral soil. Older trees (≥50 cm tall) are frequently observed near woody debris. The response of subalpine fir regeneration to changes in climate will depend on the magnitude and seasonality of changes in weather, especially during spring. Tree establishment may be enhanced if there is less snow accumulation and a longer growing season. However, warmer, drier summers could result in additional moisture stress to seedlings, especially at sites with high solar radiation (south and west aspects) and well-drained soils.

Three sites with fire-generated second-growth (70–100 years old) Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) in southwestern Oregon were examined using dendroecological techniques to determine (1) temporal patterns of radial growth and (2) the effects of variation in climate on growth. Long-term patterns of radial growth vary among sites, but similar interannual variation in radial growth indicates a common response to regional climate. Growth is positively correlated with the Palmer Drought Severity Index and precipitation during summer. Furthermore, growth is positively correlated with precipitation during autumn prior to the growth year, which suggests the benefits of soil moisture recharge for subsequent stemwood production. Annual precipitation is strongly seasonal, and soil moisture stress in summer is apparently severe enough to be the dominant climatic influence on radial growth. Positive correlations of growth with most monthly temperatures reflect the benefit of warm temperatures on photosynthesis and radial growth during periods of adequate soil moisture. Although coastal Oregon is generally considered to be a high precipitation environment, conditions are clearly dry enough during summer to limit carbon gain in second-growth Douglas-fir. If future climatic conditions result in increased soil moisture stress during summer, productivity of such second-growth stands may decrease below current levels.