A Multiscale Study of Tree-line Dynamics in Southwestern Yukon

Résumé

... The overall aim of my research project is to shed light on how the forest-tundra ecotone in the mountainous Kluane region of southwest Yukon will respond to continued climatic change. Here the tree line is largely an alpine phenomenon. Valley forests in the region are dominated by white spruce (Picea glauca). With increasing altitude these forests begin to thin, and by 1200 m, individual trees are markedly shorter. By 1300 m, the spruce component of the vegetation is grouped into thick, low sprawling clumps (often with multiple stems) known as "krummholz" and interspersed with willow (Salix sp.) and birch (Betula glandulosa) shrubs. Only the occasional spruce remains at 1400 m. Although willow and birch shrubs are found at this elevation, tundra vegetation is equally important, and it dominates at higher elevations. This zone of transition that constitutes the tree line in the southwest Yukon plays a critical role in landscape structure and wildlife habitat .... Thus, changes in the location or pattern of the tree line could have important consequences for biodiversity and its conservation ..., as well as for subsistence and other human use of these biological resources. Predicting the impact of climate change on the tree line in southwest Yukon is therefore important for developing planning and management strategies that could help resource managers and local residents adapt to or cope with this change. ... to attain an accurate forecast, we need to know (1) how the tree line has responded to climatic change and variability in the past and (2) what factors limit tree growth at high elevations in the region and therefore control tree-line dynamics. The study I have designed to investigate these two questions comprises four linked components, each operating at a particular scale and focusing on a particular element of tree-line dynamics. ... The period of seedling growth is critical for trees at the tree line, since younger individuals are more susceptible to the damaging effects of extremely cold temperatures. ... To quantify seedling response to this warming, I am measuring annual increments of vertical, lateral, and radial growth, and in 2004, I will measure differences in rates of photosynthesis and respiration. ... The second component of my project uses dendrochronology, or tree ring analysis, to identify periods and rates of spruce and willow establishment and relates these events to past climatic change. ... In contrast to the dendrochronological investigation, which emphasizes the process of change, the third investigation is intended to identify changes in the pattern of land cover - specifically tree cover - that have occurred at the forest-tundra ecotone. ... The fourth component of this multiscale study uses spatial information technology to examine how the elevation, density, and relative abruptness of the tree line are related to broad-scale geophysical and topographic variables. ... The methods of all four project components have been used to study vegetation dynamics elsewhere, and many have been tested successfully in tree-line environments. However, their integration and application to a single region makes this study unique, and the results will yield new knowledge useful in determining how northern mountain ecosystems respond to climatic change. From a regional perspective, the data collected and relationships derived from each investigation could be used in modeling future landscape change in the southwest Yukon. In turn, such a model could be used to facilitate a prediction of how subalpine habitat change could affect land cover and wildlife habitat across the Kluane region.