2022 Volume 39 Issue 1 Pages 31-41
The Ishikari Peatland, located in Central Hokkaido, was the largest mire complex in Japan with an area of 55000 ha, but over 99% of it had disappeared by the 1960s due to agricultural development. The 2.3 ha study site is one of the remaining wetlands of the Ishikari Peatland. This wetland is drained by surrounding and internal ditches, but many wetland plant species are still alive there. To conserve the wetland, irrigation from the surrounding agricultural pipeline has been carried out from May to August for more than 13 years. In this study, groundwater levels and vegetation were recorded for the year before （pre-irrigation year）, and the first and the 13th year after the introduction of irrigation to evaluate its effects. Groundwater levels at any sites in the first and the 13th year were about 30 cm higher than those in the pre-irrigation year during the irrigation months. Groundwater levels after the implementation of irrigation at depression sites were higher than the ground surface throughout the irrigation months, while at other sites, groundwater levels were more than 20 cm below the ground surface. After 13 years from the implementation of irrigation, wetland species such as Phragmites australis became dominant and alien species present in the pre-irrigation year greatly reduced at the depression sites. Outside the depressions, Betula platyphylla var. japonica in tree layers decreased drastically in some sites, but alien species and Pteridium aquilinum subsp. japonicum, which is known to invade drier wetlands, increased in many sites. At the depression sites, irrigation caused a sufficient rise in groundwater levels that resulted in maintenance and restoration of wetland vegetation, while other sites received less effect. The results suggested that topography, groundwater level and vegetation in the wetland were closely related.
