| In our previous study, we proposed a new landscape index, “plant community cluster (PCC),” which is a spatial unit for vegetation management. PCC was integrated from plant communities using hierarchical cluster analysis based on the physical environment, which was calculated with river engineering (e.g., tau*y: shear stress of a riverbed fixed over time by floods). PCC was composed of plant communities existing on a similar physical environment and could simplify complex plant succession. These results suggested that vegetation management could be enhanced by PCC; however, the threshold for clustering plant communities had to be established by an experienced analyst. Therefore, in the present study, we discussed the qualification method to decide a threshold for PCC in the Yoneshiro River and Chikuma River. We observed the inhabiting range of physical environment by each plant community and evaluated its influence on PCC (e.g., the overlap degree of the inhabiting range) in response to threshold change. We found that plant communities were properly clustered when the dissimilarity between physical environments was approximately 0.1 to 0.3. In addition, plant communities were classified into two major groups in both the rivers. It was assumed that the two major groups were located the low water channel and the high water bed. In the Yoneshiro River, when tau*y was over 0.014, bare ground and grassland were mostly covered, whereas when tau*y was >0.038, only bare ground was predominantly covered.These results indicate that: (1) the optimal spatial unit of vegetation in the view point of river engineering can be provided when the threshold of clustering was appropriately determined’ and (2) the quantification method of the present study can improve the practicability of PCC without the need of a plant expert. |
