How much do local factors matter for predicting transient ecosystem dynamics? Suggestions from permafrost formation in boreal peatlands

With rapid climate warming, ecosystems will probably exhibit complex dynamics because local factors and life history attributes of species mediate the effects of regional climate change. To assess the relative importance of local vs. regional processes on permafrost formation in boreal peatlands, I sampled for permafrost and factors affecting its formation in 38 collapse scars across a 4 degrees C mean annual temperature (MAT) gradient in the discontinuous permafrost zone of northern Manitoba, Canada. Three complimentary approaches were used to model factors important to permafrost formation at both local and regional scales. In the first analysis, a mechanistic, spatial model of permafrost formation was developed as a function of Picea mariana size and proximity. In the second approach, permafrost formation was modelled as a function of two local factors, diameter of Picea mariana trees and emergent organic matter depth, and the regional factor, mean annual temperature (MAT). Finally, published aerial photography data were used to determine whether the proportion of bogs with permafrost changes across a MAT gradient. Results show that permafrost formation in boreal permafrost peatlands is best described as a locally driven process within regional climatic constraints. At local scales of 1-2 meters, the spatial and size distributions of trees controlled the spatial distribution of permafrost. At regional scales, tree size was a significantly better predictor than emergent organic matter or MAT. These results suggest that transient models of discontinuous permafrost based only on climate may poorly predict changes in vegetation and permafrost.