1. Hydrological changes due to drainage and climate warming can have great impact on the ecosystem balance of boreal mires. The possibility of ombrotrophication, i.e. the development from fen to bog, in response to altered hydrology has not been previously tested. Here, recent changes in vegetation and surface peat are studied in an aapa mire, a typical boreal mire system dominated by fen vegetation. Drainage in the catchment from 1968 onwards led to the change from richly minerogenous to ombrogenous hydrology, thus providing a long-term ombrotrophication experiment.
2. A sequence of aerial photographs (1941, 1953, 1965, 1974, 1984, 1995, 2005) revealed a dramatic shift from fen vegetation to the nearly complete dominance of peat mosses (Sphagnum) within two decades after the catchment disturbance.
3. A distinct change from Carex peat to Sphagnum peat at the average depth of 23.3 cm (SE 0.8 cm) was found in 18 peat cores. All of the new Sphagnum peat had accumulated within the last four decades. This was verified by the relationship of age and rooting depth of 37 small pines (Pinus sylvestris) and by two pollen density profiles. The ratio Ca/Mg diminished towards the surface of peat profiles indicating change from minerogenous to ombrogenous hydrology. In accordance, extremely low pH (range 3.8–4.2) and conductivity (average 14.5 μs cm−1) were measured in the surface pore water.
4. The average total dry mass of new Sphagnum peat was 7042 g m−2 (SE 442) and the recent apparent rate of carbon accumulation was 100.6 g m−2 year−1 (SE 6.3), as calculated for a 35-year period and 50% carbon content.
5. Synthesis. Remarkable potential for vegetation change and increase of peat growth is demonstrated in boreal aapa mires. Ombrotrophication can be initiated within a few decades in response to reduced input of minerogenous water. Future changes in the hydrological cycle, as indicated by climate change models, are similar to the impact of catchment disturbance in aapa mires. Diminished total water budgets during the summer cause a decrease of minerogenous input and a draw-down of water level, both of which may promote the growth of Sphagnum over fen vegetation.