Seasonal patterns and controls on net ecosystem CO2 exchange in a boreal peatland complex

We measured seasonal patterns of net ecosystem exchange (NEE) of CO2 in a d iverse peatland complex underlain by discontinuous permafrost in northern M anitoba, Canada, as part of the Boreal Ecosystems Atmosphere Study (BOREAS) . Study sites spanned the full range of peatland trophic and moisture gradi ents found in boreal environments from bog (pH 3.9) to rich fen (pH 7.2). D uring midseason (July-August, 1996), highest fates of NEE and respiration f ollowed the trophic sequence of bog (5.4 to -3.9 mu mol CO2 m(-2) s(-1)) < poor fen (6.3 to -6.5 mu mol CO2 m(-2) s(-1)) < intermediate fen (10.5 to - 7.8 mu mol CO2 m(-2) s(-1)) < rich fen (14.9 to -8.7 mu mol CO2 m(-2) s(-1) ). The sequence changed during spring (May-June) and fall (September-Octobe r) when ericaceous shrub (e.g., Chamaedaphne calyculata) bogs and sedge (Ca rex spp.) communities in poor to intermediate fens had higher maximum CO2 f ixation rates than deciduous shrub-dominated (Salix spp. and Betula spp.) r ich fens. Timing of snowmelt and differential rates of peat surface thaw in microtopographic hummocks and hollows controlled the onset of carbon uptak e in spring. Maximum photosynthesis and respiration were closely correlated throughout the growing season with a ratio of approximately 1/3 ecosystem respiration to maximum carbon uptake at all sites across the trophic gradie nt. Soil temperatures above the water table and timing of surface thaw and Freeze-up in the spring and fall were more important to net CO2 exchange th an deep soil warming. This close coupling of maximum CO2 uptake and respira tion to easily measurable variables, such as trophic status, peat temperatu re, and water table, will improve models of wetland carbon exchange. Althou gh trophic status, aboveground net primary productivity, and surface temper atures were more important than water level in predicting respiration on a daily basis, the mean position of the water table was a good predictor (r(2 ) = 0.63) of mean respiration rates across the range of plant community and moisture gradients. Q(10) values ranged from 3.0 to 4.1 from bog to rich f en, but when normalized by above ground vascular plant biomass, the Q(10) f or all sites was 3.3.