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 hydraulic engineering (i.e., tau*y: shear stress of a riverbed fixed over time by floods, h: height over the mean water level, d: distance from the boundary between water and land). PCC considered that multi-plant communities could inhabit in the same physical environment because PCC was composed of plant communities existing on a similar physical environment. In addition, PCC could simplifycomplex plant succession with abstraction main paths. 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 quantification method to decide a threshold for PCC in the Yoneshiro River and Chikuma River, whose river and climate characteristics differed. 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 (maximum is 1.0). In addition, plant communities were classified into two major groups in both the rivers. In the Yoneshiro River, the thresholds of two major groups were as follows; tau*y = approx. 0.006, h = approx. 2.5, d = approx. 75. 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) plant communities in the river can be organized hierarchically according to the physical environment using the results of flood analysis and field survey for river management; (2) 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 (3) the quantification method of the present study can improve the practicability of PCC without the need of a plant expert. |