Riparian Vegetation Classification using the Dynamic Riparian Vegetation Model

Rivers in mountainous islands and peninsulas are steep and filled with gravels, compared to large rivers flowing in wide plains of continents, where only fine sediment is transported. Landscape of riparian vegetation differs between these two types of rivers. In steep rivers, gravel sediment fields spread along the river and surrounded by relatively young trees, whereas, in mild sloped rivers, old trees colonize up to the shoreline. Particularly, gravel deposited layer, low in moisture and nutrients, suppresses recolonization of vegetation after a flood. Dynamic Riparian Vegetation Model (DRIPVEM), was used to simulate the differences in vegetations and the re-colonization period of riparian vegetation after the flush. Observed recolonization period of both herb and tree vegetation was successfully reproduced by the model both for gravelly bedded and sandy bedded rivers.

The general trend of vegetation coverage was obtained by simulating for 50 years, as a function of the deposition area at a flood time. After 50 years, the gravelly rivers produced vegetation coverage which is highly influenced by the deposition area fraction at flood time, particularly for low flood levels. However, it highly depended on the flood level with increasing flood intensity as the erosion becomes dominant. In sandy rivers, on the other hand, flood intensity affects the vegetation coverage rather than the deposition area fraction.

The low concentration of nitrogen of the deposited sediment is also a detrimental factor of the vegetation coverage. Vegetation coverage decreases with the nitrogen concentration of the deposited sediment in gravelly channels, as gravel deposition suppresses the re-colonization of vegetation; however, for sandy channels, vegetation coverage increases with increasing deposition area fraction as the higher nitrogen concentration accelerates the vegetation recovery.

These results indicate that channels with mild sloped and far from gravel sediment sources, transporting only sandy sediments, the vegetation recovery is not suppressed after a flood, and thickly forestated until shoreline. In the steep channels with gravel sediment sources nearby, gravel deposition at flood time, suppress the recovery of vegetation after a flood, then gravel field spreads towards the riparian zone.