Changing treelines : how variability in scale and approach improve our understanding

Trant, Andrew J. (2013) Changing treelines : how variability in scale and approach improve our understanding. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

With treeline position expected to expand northward, and upward, with climate change, there is increased attention towards understanding the mechanisms that control these patterns. Using a regional and broad scale approach, treeline dynamics, disturbance and response to recent climate warming were explored along an altitudinal gradient in the Mealy Mountains, Labrador (Canada), and across the circumpolar subarctic. To complement the western science approach to understanding climate change, a critique of modules taught to the Innu Guardians will be presented. At a regional scale in the Mealy Mountains, treeline dynamics over the past few centuries showed species-specific differences in when treeline was established and the relative rates of subsequent infilling and mortality. All tree species showed increased variability in recruitment correlations to climate across the treeline ecotone with black spruce (Picea mariana) showing consistently positive correlations, while larch (Larix laricina) shifted from strongly positive to negative and white spruce (Picea glauca) showing consistently negative correlations. Long-lived black spruce krummholz also showed significant recent increases in radial growth. Disturbance at treeline resulted in characteristic gap dynamics with low-levels of tree mortality attributed to porcupine (Erethizon dorsatum) herbivory and small-scale outbreaks of spruce budworm (Choristoneura fumiferana) and larch sawfly (Pristiphora erichsonii). These outbreaks occurred at lower magnitudes than observed in other parts of the boreal forest due to decreased tree density, supporting the Resource Concentration Hypothesis. Canopy gaps associated with windthrow did not have significantly different canopy structures but seedling densities of larch and black spruce, were significantly higher in exposed soil associated with windthrow disturbance. At broader spatial scales across circumpolar treeline, growth form and advancement were not significantly related to the extent of site warming but rather treeline advancement was predicted by diffuse, or 'gradual' treelines and closeness to the ocean. At the regional scale, it was expected that larch and black spruce would capitalize on warming temperatures, since these species are already exhibiting early signs of response. The frequency and magnitude of insect outbreaks is expected to increase with climate change and these data suggest that larch, and to a lesser extent, black spruce, will have enhanced regeneration dynamics. At broad spatial scales, ecological factors will continue to be significant drivers of treeline change until temperatures warm past a point where regeneration limitations, such as seed viability and establishment, are removed. Teaching and communicating climate change science illustrated the best results when taught in field conditions, while some difficulties were encountered in differentiating between short-term versus long-term change. Working with the Innu Environmental Guardians to develop a framework for detecting change in northern forests, helped to contextualize the western science approach to treeline research.

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