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 m2 a1).
The beginning of the growing season occurred when daily mean air temperature exceeded
4C and the near surface soil temperature equaled or exceeded 0C. 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
in Pg, 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.
