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.
Mount Rainier National Park, Tahoma Tree establishment patterns vary considerably at different locations in the subal-Woods, Star Route, Ashford, pine zone of Mount Rainier National Park, with substantial temporal variation in Washington 98304, U.S.A. regeneration of subalpine fir (Abies lasiocarpa). Recruitment in subalpine meadows has been continuous on the west side of Mount Rainier since about 1930, David L. Peterson but has occurred in short, discrete periods on the east side. Variation in snowpack Cooperative Park Studies Unit, from west to east on the mountain has a substantial impact on climatic factors University of Washington, Box that limit tree establishment. Warm, dry summer climate facilitates tree establish-352100, Seattle, Washington 98195, ment on the west side where snowpacks are generally very high; cool, wet sum-U.S.A. mer climate enhances tree establishment on the east side where snowpacks are lower. Density of tree establishment is significantly greater in heath-shrub (ericaceous) vegetation than in other vegetation types. Within heath-shrub vegetation types, tree establishment is highest at lower elevations, on topographic convexities, and in plant communities dominated by Phyllodoce empetriformis. Survival of subalpine fir seedlings during the first 3 yr after germination is significantly greater in heath-shrub vegetation than other vegetation types. If the climate becomes warmer and drier during the next century, continued rapid regeneration of trees can be expected in subalpine meadows on the west side of Mount Rainier National Park. This may result in displacement of wildflower meadows that are an attraction for park visitors. A better understanding of climatic and environmental limitations on tree establishment will assist resource managers in developing sound management strategies for subalpine ecosystems.
Wetlands in the remote mountains of the western US have undergone two massive ecological “experiments” spanning the 20th century. Beginning in the late 1800s and expanding after World War II, fish and wildlife managers intentionally introduced millions of predatory trout (primarily Oncorhynchus spp) into fishless mountain ponds and lakes across the western states. These new top predators, which now occupy 95% of large mountain lakes, have limited the habitat distributions of native frogs, salamanders, and wetland invertebrates to smaller, more ephemeral ponds where trout do not survive. Now a second “experiment” – anthropogenic climate change – threatens to eliminate many of these ephemeral habitats and shorten wetland hydroperiods. Caught between climate‐induced habitat loss and predation from introduced fish, native mountain lake fauna of the western US – especially amphibians – are at risk of extirpation. Targeted fish removals, guided by models of how wetlands will change under future climate scenarios, provide innovative strategies for restoring resilience of wetland ecosystems to climate change.
Questions: Does tree establishment: (1) occur at a treeline depressed by fire, (2) cause the forest line to ascend upslope, and/or (3) alter landscape heterogeneity? (4) What abiotic and biotic local site conditions are most important in structuring establishment patterns? (5) Does the abiotic setting become more important with increasing upslope distance from the forest line? Location: Western slopes of Mount Rainier, USA. Methods: We performed classification analysis of 1970 satellite imagery and 2003 aerial photography to delineate establishment. Local site conditions were calculated from a LIDAR‐based DEM, ancillary climate data, and 1970 tree locations in a GIS. We used logistic regression on a spatially weighted landscape matrix to rank variables. Results: Considerable establishment after 1970 caused forest line elevation to increase over 150 m in specific locations. Landscape heterogeneity increased with distance from the 1970 forest line. At a broad spatial context, we found establishment was most common near existing trees (0‐50 m) and at low elevations (1250‐1350 m). Slope aspect (W, NW, N, NE, and E), slope angle (40‐60°), and other abiotic factors emerged as important predictors of establishment with increasing upslope distance from the forest line to restricted spatial extents. Conclusions: Favorable climatic conditions likely triggered widespread tree establishment. Readily available seed probably enhanced establishment rates near sexually mature trees, particularly in the less stressful environment at low elevations. The mass effect of nearly ubiquitous establishment in these areas may have obscured the importance of the abiotic setting to restricted spatial extents. Topographic variability apparently produced favorable sites that facilitated opportunistic establishment with increasing upslope distance from the forest line, thereby enabling additional trees to invade the alpine tundra.
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