Physiological responses of a black spruce forest to weather

We used eddy covariance to measure the net exchange of CO2 between the atmosphere and a black spruce (Picea mariana) forest in Manitoba for 16,500 hours from March 16, 1994 to October 31, 1996. We then partitioned net exchange into gross photosynthesis and respiration by estimating daytime respiration as a function of temperature, and used these data to define the physiological responses of the forest to weather. The annual rates of gross production and respiration by the forest were both around 8 t C ha(-1) yr(-1). Both photosynthetic and respiratory response were reduced in winter, recovered with warming in spring, and varied little in summer. Respiration in mid summer increased with air temperature (T-air) at a Q(10) of around 2 to a rate of 2-8 mu mol m(-2) s(-1) at 15 degrees C. Gross photosynthesis at high light (photon flux density (PPFD) greater than 600 mu mol m(-2) s(-1)) was negligible at T-air < 0 degrees C, increased linearly with T-air from 0 degrees to 14 degrees C, and was relatively insensitive to T-air > 14 degrees C. Gross CO2 uptake at T-air > 14 degrees C increased with increasing light at an ecosystem-level quantum yield of 0.05 mol CO2 mol(-1) photons before saturating at an uptake rate of 8-18 mu mol m(-2) s(-1) at PPFDs greater than 500-700 mu mol m(-2) s(-1). Photosynthesis in summer did not appear limited by high evaporative demand or soil water depletion.