Postulated Feedbacks of Deciduous Forest Phenology on Seasonal Climate Patterns in the Western Canadian Interior

A large portion of the western Canadian interior exhibits a distinctive seasonal pattern in long-term mean surface temperatures characterized by anomalously warmer conditions in spring and autumn than would be expected from a sinusoidal model. The anomaly is greatest over the southern boreal forest of western Canada, where trembling aspen (Populus tremuloidesMichx.)—a deciduous, broad-leaved species—is an important component. In this region, mean temperatures are 2°–3°C warmer in April and October but nearly 2°C cooler in June and July, relative to a best-fitting sinusoidal function. Another feature of the climate in this region is that average precipitation is low (15–30 mm month−1) from October to April but increases sharply during the summer growing season (50–100 mm month−1from June to August). Eddy correlation and sap flow measurements in a boreal aspen forest indicate profound seasonal changes in transpiration and energy partitioning associated with the deciduous nature of the forest canopy. Latent heat (water vapor) flux reaches a maximum during the summer period when leaves are present, while sensible heat flux is highest in early spring when the forest is leafless. Thus, it is postulated that feedbacks of leaf phenology of aspen forests, which occupy a large area of the western Canadian interior, may contribute significantly to the distinctive seasonal patterns of mean temperature and precipitation that occur in this region.