Relationship between ecosystem productivity and photosynthetically active radiation for northern peatlands

We analyzed the relationship between net ecosystem exchange of carbon dioxide (NEE) and irradiance (as photosynthetic photon flux density or PPFD), using published and unpublished data that have been collected during midgrowing season for carbon balance studies at seven peatlands in North America and Europe. NEE measurements included both eddy-correlation tower and clear, static chamber methods, which gave very similar results. Data were analyzed by site, as aggregated data sets by peatland type (bog, poor fen, rich fen, and all fens) and as a single aggregated data set for all peatlands. In all cases, a fit with a rectangular hyperbola (NEE = a PPFD P_max(α PPFD + P_max) + R) better described the NEE-PPFD relationship than did a linear fit (NEE = β PPFD + R). Poor and rich fens generally had similar NEE-PPFD relationships, while bogs had lower respiration rates (R = -2.0 μmol m^-2 s^-1 for bogs and -2.7 μmol m^-2 s^-1 for fens) and lower NEE at moderate and high light levels (P_max = 5.2 μmol m^-2 s^-1 for bogs and 10.8 μmol m^-2 s^-1 for fens). As a single class, northern peatlands had much smaller ecosystem respiration (R = -2.4 μmol m^-2 s^-1) and NEE rates (α = 0.020 and P_max = 9.2 μmol m^-2 s^-1) than the upland ecosystems (closed canopy forest, grassland, and cropland) summarized by Ruimy et al. [1995]. Despite this low productivity, northern peatland soil carbon pools are generally 5-50 times larger than upland ecosystems because of slow rates of decomposition caused by litter quality and anaerobic, cold soils.