Net ecosystem productivity and its uncertainty in a diverse boreal peatland

Net ecosystem exchange (NEE) of CO2 was measured in four peatlands along pl ant community, hydrologic, and water chemistry gradients from bog to rich f en in a diverse peatland complex near Thompson, Manitoba, as part of the Bo real Ecosystem-Atmosphere Study (BOREAS). A simple model for estimating gro wing season net ecosystem productivity (NEP) using continuous measurements of photosynthetically active radiation (PAR), and peat temperature was cons tructed with weekly chamber measurements of NEE from May to October 1996. T he model explained 79-83% of the variation in NEE across the four sites. Mo del estimation and parameter uncertainty calculations were performed using nonlinear regression analyses with a maximum likelihood objective function. The model underestimated maximum NEE and respiration during the midseason when the standard errors for each parameter were greatest. On a daily basis , uncertainty in the midday NEE simulation was higher than at night. Althou gh the magnitude of both photosynthesis and respiration rates followed the trophic gradient bog less than poor fen less than intermediate fen less tha n rich fen, NEP did not follow the same pattern. NEP in the bog and rich fe n was close to zero, while the poor and intermediate fens had higher NEP du e to a greater imbalance between uptake and release of CO2. Although all si tes had a positive growing season NEP, upper and lower 95% confidence limit s showed that the bog and rich fen were either a source or sink of CO2 to t he atmosphere, while the sedge-dominated poor and intermediate fens were ac cumulating approximately 20-100 g CO2 C m(-2) over the 5 month period in 19 96. Peatland ecosystems may switch from a net sink to a source of carbon on short timescales with small changes in soil temperature or water table pos ition. Since the difference between production and decomposition is small, it is important to understand and quantify the magnitude of uncertainty in these measurements in order to predict the effect of climatic change on pea tland carbon exchange.