Climate Change and Vegetation Dynamics at the Subarctic Alpine Treeline in Northwestern Canada

Abstract

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