Restoration of drained peatlands in southern Finland: initial effects on vegetation change and CO2 balance.
Components of the CO2 balance for a drained minerotrophic fen and a drained ombrotrophic bog in Finland were measured for permanent plots using static chamber techniques for 1 year before and 2 years after a rewetting treatment in part of both sites. During the same period, changes in ground and bottom layer vegetation composition were monitored. The sites had originally been drained for forestry but proved unprofitable and are now being restored. After the treatment, the water table rose, the average increase being 25 cm for the fen site and 20 cm for the bog site. In the untreated areas the average water table remained at the pretreatment level. There was a clear change in vegetation composition in the rewetted area of the fen site where the cover of cottongrass Eriophorum vaginatum increased markedly. The change in vegetation composition seemed to be dependent on the nutrient status of the sites, being faster at the more nutrient-rich fen site. The rates of CO2 efflux from the soil surface decreased on those rewetted plots where all vegetation had been removed. In 1996, the CO2 efflux rates from the soil surface of the untreated plots were approx. twice as high as from the rewetted plots with a high water table. The change in water table levels and vegetation composition affected the seasonal (mid-May to end of September) CO2-C balances. For the rewetted area of the fen site, the CO2-C balance varied from 162 to 283 g m-2, being greatest in a plot with dense E. vaginatum cover and a high water table. For the rewetted area of the bog site, the CO2-C balance varied from 54 to 101 g m-2, being greatest in a hollow-level plot with a high water table and mire vegetation. For the untreated areas, the CO2-C balance was close to zero (3 g m-2 at the bog site) or negative (-183 g m-2 at the fen site), when carbon fixation by the tree stand was omitted. The results are encouraging from the practical point of view as restoration of both fen and bog sites initiated vegetation succession and CO2-C balance development towards those of pristine mires.