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Factors controlling the interannual variability in the carbon balance of a southern boreal black spruce forest

[1] Factors controlling the seasonal and interannual variability of net ecosystem productivity (FNEP), gross ecosystem photosynthesis (Pg), ecosystem respiration (Re) and evapotranspiration (E) of a mature boreal black spruce forest in central Saskatchewan, Canada were investigated using eight years (1999–2006) of continuous eddy covariance measurements. During 2000–2006, which included a three-year drought, the forest was a weak sink for CO2 with annual FNEP ranging from 27 to 80 g C m-2 (56 ± 21 g C m−2 a−1). The beginning of the growing season occurred when daily mean air temperature exceeded 4°C and the near surface soil temperature equaled or exceeded 0°C. The length of the growing season varied from 186 to 232 days. During the extreme drought year (2003), the smaller reduction in annual Pg than in Re resulted in highest FNEP of the record. Annual FNEP decreased slightly with increasing soil water content; however, there was evidence of increased FNEP due to high water table conditions in 2004 because of the slightly higher decrease in Re than Pg. Although bulk surface conductance (gs) decreased significantly during the dry conditions in 2003, the associated increase in D prevented a significant drop in E, which resulted in only a slight decline in evaporative fraction and almost no change in water use efficiency. Interannual variation inPg, Re and FNEP in the early growing season (April–June) and late growing season (July–September) was controlled by air temperature and soil water content, respectively. However, spring (April–May) mean air temperature was the main factor determining the interannual variation in annual FNEP. The effect of late growing season soil water content on annual Pg and Re was greater than its effect on annual FNEP. The results emphasize the need to consider soil moisture conditions as well as temperature when simulating the response of the carbon balance components of this ecosystems to climate change.