SEASON-LONG MEASUREMENT OF CARBON-DIOXIDE EXCHANGE IN A BOREAL FEN

Atmospheric CO2 exchange was measured in a boreal minerotrophic patter ned fen in central Saskatchewan, Canada, using the eddy correlation te chnique. The study was conducted from mid-May to early October 1994, a s part of the Boreal Ecosystem-Atmosphere Study (BOREAS). Herbaceous v egetation was dominated by buckbean (Menyanthes trifoliata) and variou s species of sedges (Carer and Eriphorum spp). Bog birch (Betula pumil a) and willow species (Salix spp.) were dominant shrubs. Brown mosses were the predominant nonvascular vegetation. Canopy photosynthesis app roached light saturation for PAR (photosynthetically active radiation) above 1000-1200 mu mol m(-2) s(-1). High temperature (>20 degrees C) and vapor pressure deficit (>1.5 kPa) decreased photosynthesis signifi cantly. On cool days with low vapor pressure deficit, canopy photosynt hesis tended to follow incident PAR. The diurnal pattern of canopy pho tosynthesis exhibited a midmorning maximum on days with high temperatu re and vapor pressure deficit. Canopy photosynthesis reached a peak of 0.59 mg CO2 m(-2) s(-1) (midday) in early July, corresponding to the period of maximum leaf area index. Another increase in photosynthesis occurred in late August as the canopy recovered from a brief rise in w ater table that inundated some of the leaf area. The daily net CO, exc hange showed significant day-to-day variability resulting from changes in environmental conditions. The integrated value of the net ecosyste m-CO2 exchange during the measurement period (mid-May to early October ) was about 88 g C m(-2). Consistent with the high productivity and hi gh water table, this fen exhibited magnitudes of CO2 exchange larger t han other northern wetlands reported in the literature.