CARBON-DIOXIDE EXCHANGE IN A PEATLAND ECOSYSTEM

Micrometeorological measurements of carbon dioxide exchange were made in an open peatland in north central Minnesota during two growing seas ons (1991 and 1992). The vegetation at the site was dominated by Sphag num papillosum, Scheuchzeria palustris, and Chamaedaphne calyculata. T he objective of the study was to examine the diurnal and seasonal vari ations in canopy photosynthesis (P) and develop information on the net ecosystem CO2 exchange. The two seasons provided contrasting microcli matic conditions: as compared with 1991, the 1992 season was significa ntly wetter and cooler. Canopy photosynthesis was sensitive to changes in light, temperature, and moisture stress (as indicated by water tab le depth and atmospheric vapor pressure deficit). Under moderate condi tions (temperature 18-28 degrees C, vapor pressure deficit 0.7-1.5 kPa , and water table near the surface) during the peak growth period, mid day (averaged between 1000-1400 hours) P values ranged from 0.15 to 0. 24 mg m(-2) s(-1). Under high-temperature (30 degrees-34 degrees C) an d moisture stress (water table 0.16-0.23 m below the surface and vapor pressure deficit 2.2-3.0 kPa) conditions, midday P was reduced to abo ut 0.03-0.06 mg m(-2) s(-1). There was a high degree of consistency in the values of P under similar conditions in the two seasons. Seasonal ly integrated values of the daily net ecosystem CO2 exchange indicated that the study site was a source of atmospheric CO2, releasing about 71 g C m(-2) over a 145-day period (May-October) in 1991. Over a simil ar period in 1992, however, this ecosystem was a sink for atmospheric CO2 with a net accumulation of about 32 g C m(-2). These results are c onsistent with previous investigations on CO2 exchange in other northe rn wetland sites during wet and dry periods.