Factors controlling the seasonal and interannual variability of net ecosystem productivity (FNEP), gross ecosystem photosynthesis (P-g), ecosystem respiration (R-e) 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 F-NEP 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 degrees C and the near surface soil temperature equaled or exceeded 0 degrees C. The length of the growing season varied from 186 to 232 days. During the extreme drought year (2003), the smaller reduction in annual P-g than in R-e resulted in highest F-NEP of the record. Annual F-NEP decreased slightly with increasing soil water content; however, there was evidence of increased F-NEP due to high water table conditions in 2004 because of the slightly higher decrease in R-e than P-g. Although bulk surface conductance (g(s)) 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 in P-g, R-e and F-NEP 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 F-NEP. The effect of late growing season soil water content on annual P-g and R-e was greater than its effect on annual F-NEP. 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.
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