| The transition from forest to tundra, commonly called treeline, is expected to advance, particularly in northern latitudes. Treeline in two subarctic locales: the western Mackenzie Mountains, Northwest Territories, and the western Hudson Bay Lowlands southeast of Churchill, Manitoba were studied, with the objectives of determining: (i) the historical and current characteristics of the trees at altitudinal and latitudinal treelines, and; (ii) what factors contributed most to the configuration of treeline at its altitudinal and latitudinal limits. Five investigations were undertaken, each employing different methods in order to assess several variables, theorized to be key drivers of pattern and process at treeline. Tree growth was assessed by dendroclimatological techniques at the two study sites. Results demonstrated the primary role of temperature in limiting tree growth, but also identified fine-scale variability in site- and species-specific responses. Winter desiccation and viability of first-year conifer needles were assessed. Minimum epidermal conductance of needles corresponded with June temperatures during the year of needle formation, though varied little with exposure. However, significant correlations were found between viability and needles facing into the predominant wind direction. Seedlings were destructively sampled and aged within forest and at treeline. Seedling establishment was related to inter-specific competition in forest and facilitation at treeline. When seedling age structures were developed for wetland and upland environments and compared with climate, temperature appeared to be the main driver of establishment patterns. Though this influence was moderated in the uplands—most likely due to a thick, continuous lichen mat that represented an inhospitable seedbed. Dendroecology was used to reconstruct recent dynamics of Picea, Larix and Abies within the forest-tundra. Evidence of an advance of treeline position and increase in stand density during the early-mid 20th century was found. Recruitment and reconstructed summer temperatures were significantly correlated, indicating that climate was the reason for change. Collectively, these results suggest a significant potential for subarctic treeline to advance in response to future climate warming—particularly for Larix. However, the rate, pattern, and timing of advance will be contingent on heterogeneity of environmental variables at finer scales, especially seedbed, interspecific facilitation/competition, and edaphic characteristics. |
