Net CO2 exchange of a subarctic mountain birch ecosystem

Net ecosystem CO2 exchange was measured over a mountain birch forest in nor thern Finland throughout the growing season. The maximal net CO2 uptake rat e of about - 0-5 mg(CO2) m(-2) s(-1) was observed at the end of July. The h ighest nocturnal respiration rates in early August were 0.2Mg(CO2)m(-2) s(- 1). The daily CO2 balances during the time of maximal photosynthesis were a bout -159(CO2) m(-2) d(-1). The mountain birch forest acted as a net sink o f CO2 from 30 June to 28 August. During that period the net CO2 balance was -448 g(CO2)m(-2). The interannual representativeness of the observed balan ces was studied using a simplified daily balance model, with daily mean glo bal radiation and air temperature as the input parameters. The year-to-year variation in the phenological development was parameterised as a function of the cumulative effective temperature sum. The daily balance model was us ed for estimating the variability in the seasonal CO2 balances due to the t iming of spring and meteorological factors. The sink term Of CO2 in 1996 wa s lower than the 15-year mean, mainly due to the relatively late emergence of the leaves.