Multiscale analysis reveals pattern-process interactions at subarctic alpine treelines

Background/Question/Methods

Temperature-related variables are the most influential agents governing treelines and their dynamics at coarse-scales of analysis. However, other variables, including soils, precipitation, species biology, and a host of biotic interactions, are equally important to consider, especially in light of varied responses to current warming trends. A scale-based approach for examining treeline dynamics helps understand the relative importance of these different variables. To illustrate this, I highlight results from an ensemble of studies conducted at multiple scales in southwest Yukon. Collectively, these studies suggest a distinct top-down/bottom-up interaction at alpine treeline where terrain-induced gradients of solar radiation result in fundamental differences in plant-scale biological processes which, in turn, structure vegetation pattern at the landscape scale. 

Results/Conclusions

Varied insolation creates differences in snow depth and timing of melt, soil temperature, and permafrost on opposing slopes that result in distinct physiological differences in white spruce (Picea glauca), the dominant treeline conifer. Measurement of young individuals indicated that secondary growth and lateral primary growth was significantly greater on south-facing slopes than on north-facing slopes. Photosynthetic efficiency was reduced in individuals on south-facing slopes, while over-winter damage and mortality was significantly greater. Population-level processes also differed. Dendroecology and repeat photography indicated that treeline advanced on south-facing slopes during the 20th century, but that range expansion was limited on north-facing slopes. These process-related differences appear to be the mechanism for differences in treeline pattern at the landscape scale, including a higher treeline elevation and greater clustering of individuals on south-facing slopes. These results can be used to inform theory on the functional causation of treeline, rationalize differential treeline dynamics observed worldwide, and better inform predictions of future treeline dynamics.