Effects of the expansion of vascular plants in Sphagnum-dominated bog on evapotranspiration

Plant succession triggered by drainage, which results in the expansion or invasion of vascular plants, has been reported from many peatlands. However, the effects of the vascular plant's expansion on evapotranspiration (ET), which is a key component of the water balance of ombrotophic bog, are still contradictory. To investigate the effects, ET was measured at a Sphagnum-dominated bog and an adjacent transition peatland dominated by Sasa, dwarf bamboo, in Hokkaido Island, northern Japan, using the eddy covariance technique during the four growing seasons from 2007 through 2010. Cumulative gap-filled ET during a snow-free period of 6.5 months was 362 (2008) and 374 mm (2010) at the Sphagnum site and 300 (2008) and 372 mm (2010) at the Sasa site. In the mid-growing season (late June to mid-September) with the highest leaf area index (LAI) at the Sasa site, ET was 2.14 ± 0.03 (mean ± 1 standard deviation of the four years) and 1.92 ± 0.19 mm d−1, respectively, at the Sphagnum and Sasa sites. ET was smaller at the Sasa site, except for 2010 with an unusual hot wet summer; mean air temperature and precipitation were higher than their 30-year normal values by 1.75 °C and 172 mm, respectively. At the Sphagnum site, ET was stable despite such interannual variation in meteorological conditions. However, ET increased significantly at the Sasa site in 2010 probably because of LAI increase due to the enhanced growth of Sasa plants. The ET increase at the Sasa site suggests that ET will increase at the Sasa-dominated area, if the future warming environment accompanies more precipitation.