Temporal scaling of moisture and the forest-grassland boundary in western Canada

A major challenge in understanding how forests may respond to climate change is that many of the critical processes, such as precipitation patterns and disturbance regimes, are highly variable and operate over a wide range of temporal and spatial scales. Analyses of vegetation-climate relationships have been conducted as a simple scaling technique to identify the most important long-term factors controlling the boundary between boreal forest and grassland in western Canada. In an earlier study, the boundary was found to correspond closely to the zero isoline of a climatic moisture index, based on mean annual precipitation minus potential evapotranspiration (PET) by the Jensen-Haise method. In this study, similar results were obtained using PET estimates from the more process-based Priestley-Taylor equation or a simplified form of the Penman-Monteith equation, although actual PET estimates were sensitive to input parameters. It is concluded, based on this analysis and other evidence, that forest distribution in the region is controlled by chronic moisture deficits. A simple method of estimating PET is presented that is suitable for assessing long-term changes in moisture regimes from limited climate data.