Hydrological influences on the distribution of vegetation and tree height of Alnus japonica (Thunb.) Steud. in the Kushiro Mire, Japan

The relationship between the hydrology and plant communities assessed by continuous measurement of water, soil environment and vegetation, mainly in an alder forest, was studied in Kushiro Mire, Hokkaido, Japan. Seasonal changes in the water level were not detected, except in the willow forest located in the Kuchoro River, which showed a drawdown of the water level in summer. The height of the alder trees correlated with the mean and lowest water levels which would supply oxygen. The peak water level also affected the height of the alder forest, suggesting that waterlogging (reduction and lack of oxygen) inhibited the height of the alder trees. An overflow was only observed during the strong low pressure, typhoon or snowmelt flooding, as a result of the high peak water level. In snowmelt season, alder did not foliation stage. This suggests that the river water may only overflow the natural levee during a such large rain event, which supports the establishment of alder forests in the inflow area. Furthermore, the height of alder trees weakly negatively correlated with the carbon/nitrogen ratio of the soil, suggesting a difference in the deposition of peat. Reed-sedge community showed a large fluctuation of the water level, and resistant to drawdown of the water level, which would inhibit the invasion of alder.