Rewetting of drained boreal spruce swamp forests results in rapid recovery of Sphagnum production.
Peatland rewetting aims to restore biomass accumulation from peat-forming plants for climate change mitigation, biotope conservation and water purification purposes. Boreal spruce swamp forests in Europe have suffered heavily from drainage for forestry and are now a focus of restoration efforts. We measured Sphagnum height and biomass increment by allowing Sphagnum to grow through mesh nets located in nine undrained, nine drained and 18 rewetted boreal spruce swamp forests. At the moss patch level, rewetting led to a recovery of Sphagnum growth: height increment and biomass increment at the rewetted sites (5.9±0.7 cm year-1 and 147±15 g m-2 year-1, mean±SE) were similar to increment at the undrained sites (4.9±0.5 cm year-1 and 128±12 g m-2 year-1), while remnant patches of Sphagnum at the drained sites showed smaller increment (2.8±0.8 cm year-1 and 76±19 g m-2 year-1). Sphagnum in the ditches at the drained sites showed similar increment as the moss at the undrained and rewetted sites, while ditches at the rewetted sites had the greatest increment (8.7±0.7 cm year-1 and 183±16 g m-2 year-1). A higher water-table increased Sphagnum growth, and Sphagnum riparium grew more rapidly than the other species. At the ecosystem level, where information on moss cover was utilized, rewetting had increased Sphagnum production to values close to the undrained sites. At the drained sites, biomass production was 8±17 g (mean±SE) m-2 year-1, at the rewetted sites 42±15 g m-2 year-1 and at the undrained sites 66±12 g m-2 year-1. Sphagnum cover was the most important variable that determined Sphagnum production in the ecosystem. Synthesis and applications. Drained spruce swamp forests appear to be a suitable target for restoration as they respond readily to rather inexpensive rewetting. The result points out that even in boreal conditions, restoration results can be achieved in a time-scale of years rather than decades. At the ecosystem level, rewetting had resulted in a biomass gain of 34 g m-2 year-1 when compared to drained sites.