| Summary: | It is expected that anthropogenic increases in atmospheric levels of CO and other greenhouse gases will have a substantial impact on climate in the next 100 years. Knowledge of the response of high latitude vegetation to past climate variation is useful for understanding the possible response of such vegetation to potential future anthropogenic climate changes. The objectives of this thesis were to investigate climate change, treeline dynamics and vegetation-climate relationships at the subarctic alpine treeline in northwestern Canada on a variety of spatial and temporal scales. In order to address these objectives, three hypotheses were tested: 1) Postglacial treeline change in the Mackenzie Mountains, N.W.T. was driven by changes in the seasonal and latitudinal distribution of solar radiation; 2) Establishment and mortality patterns of trees at treeline are episodic, controlled by climate variations; and 3) The position of the treeline in the Mackenzie Mountains is in equilibrium with current climatic conditions. The first hypothesis was tested using the palynological analyses of cores from three lakes in the tundra, forest-tundra and open forest of the central Mackenzie Mountains. Although there was no evidence for higher treeline in this region at any time during the Holocene, the data suggest that Picca populations in the forest-tundra were greater than present between about 8000 and 5000 yr BP, and have since declined steadily. These results are consistent with predicted changes in summer insolation based on the Milankovitch theory. The second two hypotheses were addressed using tree-ring analyses of white spruce at a number of sites in the alpine treeline zone of northwestern Canada. Dendroecological analyses of climate-growth relationships indicated that the response of trees to climate at these sites varied with tree age, which violates a basic assumption of standard dendroclimatic research. Age dependent modelling was therefore used to produce a 350 year record of summer temperatures in northwestern Canada from five sites in the N.W.T. and Yukon. Comparison of this record with white spruce recruitment/survival and mortality patterns indicated that the patterns are episodic, and controlled primarily by climatic variations. A warming trend during the last 150 years has resulted in increases in forest-tundra density, although there is evidence for only minor increases in treeline. The establishment of white spruce seedlings at sites within the upper forest-tundra, including several treeline sites, indicates that the treeline is in equilibrium with current climatic conditions. These results indicate that 1) climate-growth relationships are complex, and the simplifying assumptions made in order to reconstruct climatic records from radial growth records may in some cases be invalid; 2) in this region the response of white spruce populations to climate change on a variety of timescales has been manifested primarily as an increase in forest-tundra density, with little change in treeline altitude; and 3) seedlings are currently being produced within forest-tundra white spruce populations, and thus a rapid response to further climatic amelioration may be possible. Doctor of Philosophy (PhD)
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