Climatic and Phenological Controls of the Carbon and Energy Balances of Three Contrasting Boreal Forest Ecosystems in Western Canada

Abstract

Seasonal and interannual variability in the carbon and energy cycles of boreal forests are controlled by the interaction of climate, ecophysiology and plant phenology. This study analyses eddy-covariance data from mature trembling aspen, black spruce and jack pine stands in western Canada. The seasonal cycles of the surface carbon and energy balances were tightly coupled to the seasonal cycle of soil temperature. The contiguous carbon-uptake period was ∼50 days longer for the black spruce and jack pine stands than the trembling aspen stand, with 30 days difference in spring and 20 days difference in autumn. The black spruce and jack pine carbon-uptake period spanned the warm season, with gross ecosystem photosynthesis beginning during spring thaw and continuing until air temperature dropped to below freezing in autumn. In contrast, the trembling aspen carbon-uptake period was determined by the timing of leaf emergence and senescence, which occurred well after spring thaw and before autumn freeze. Regression analysis identified spring temperature as the primary factor controlling annual net ecosystem production at all three sites, through its influence on the onset of the growing season. Precipitation and soil water content had significant but secondary influences on the annual carbon fluxes. The impact of spring warming on annual net ecosystem production was 2–3 times greater at the deciduous-broadleaf than the evergreen-coniferous sites, confirming the high sensitivity of boreal deciduous-broadleaf forests to spring warming. The analysis confirmed the pivotal role of phenology in the response of northern ecosystems to climate variability and change.