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exchange of carbon dioxide (NEE) and irradiance (as photosynthetic photon flux density or PPFD), using published and unpublished data that have been collected during midgrowing season for carbon balance studies at seven peatlands in North America and Europe.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-corr elation tower and clear, static chamber methods, which gave very simil ar results. Data were analyzed by site, as aggregated data sets by pea tland type (bog, poor fen, rich fen, and all fens) and as a single agg regated data set for all peatlands. In all cases, a fit with a rectang ular hyperbola (NEE = alpha PPFD P-max/(alpha PPFD + P-max) + R) bette r described the NEE-PPFD relationship than did a linear fit (NEE = bet a PPFD + R). Poor and rich fens generally had similar NEE-PPFD relatio nships, while bogs had lower respiration rates (R = -2.0 mu mol m(-2)s (-1) for bogs and -2.7 mu mol m(-2)s(-1) for fens) and lower NEE at mo derate and high light levels (P-max = 5.2 mu mol m(-2)s(-1) for bogs a nd 10.8 mu mol m(-2)s(-1) for fens). As a single class, northern peatl ands had much smaller ecosystem respiration (R = -2.4 mu mol m(-2)s(-1 )) and NEE rates (alpha = 0.020 and P-max = 9.2 mu mol m(-2)s(-1)) tha n the upland ecosystems (closed canopy forest, grassland, and cropland ) summarized by Ruimy et al. [1995]. Despite this low productivity, no rthern peatland soil carbon pools are generally 5-50 times larger than upland ecosystems because of slow rates of decomposition caused by li tter quality and anaerobic, cold soils.