Relationship between hydrogeological process and vegetation change in Kushiro Mire

The Kushiro Mire (the largest mire in Japan) has been changed by conversion to urban or agricultural uses since 1884. Owing to the channelization to smoothly drain runoff and protect farmlands from floods, runoff containing nutrients from farmland and sediments from short-cut channels have flowed directly into the mire and deposited flood-borne sediment. Consequently, alder (Alnus japonica (Thunb.) Steud.) has propagated widely around the mire after channelization, owing mainly to the lowering of the groundwater level and the increased nutrient input, resulting in the gradual shrinking of the mire. The objective of this study is to evaluate the relationship between hydrogeological process and the alder invasion. The author expanded the NICE (NIES Integrated Catchment-based Eco-hydrology) model to simulate the water/heat budget, the mass transport, and vegetation succession processes iteratively. The simulation reproduced well the spatial distribution of elevation aggradations by the sediment deposits from the channelized rivers into the mire. Furthermore, the simulation of channelized rivers showed that the recharge rate of the mire decreases greatly. This indicates that channelization also causes an increase of sedimentation/nutrient load and flooding in the downstream area around the mire. The NICE reproduced excellently the invasion of alder in the mire over the last 30 years, which represents a dramatic advance in the understanding of the drying phenomenon associated with alder invasion. The reproducibility of these simulation results suggests that the NICE includes some of the important factors affecting vegetation succession in the mire.