Distributional patterns and co^-occurrence of Chrysosplenium species in watersheds in the upper Watarase river basin

Abstract

We studied the distribution and habitat of Chrysosplenium species in the upper Watarase river basin. Sørensen’s index was used to evaluate distributional overlaps among the species on two nested watershed scales; branch -1 2-scale （10⁰-10² km²） and second^-order watershed scale （10 -10 km²）. Seven Chrysosplenium species were distributed in the survey area. Among the seven species, two species （C. grayanum and C. kamtschaticum） were only sparsely distributed. The distribution of C. grayanum was biased towards lower elevations and that of C. kamtschaticum was biased towards higher elevations in the study area. The other five species （C. album var. stamineum, C. pilosum var. sphaerospermum, C. echinus, C. macrostemon, C. flagelliferum） were abundant in the study area. They were distributed in similar habitats; that is, habitats with similar temperature, stability of the ground surface （as indicated by altitude and slope, distance to the nearest valley, river bed slope of the nearest valley, and drainage density）. The distributions of these five species overlapped on both scales. These results indicated that C. grayanum and C. kamtschaticum had different ecological breadth in the upper Watarase basin. Chrysosplenium album var. stamineum, C. pilosum var. sphaerospermum, C. echinus, C. macrostemon, and C. flagelliferum may have similar ecological breadth in terms of climatic conditions and stability of the ground surface in this study region and on the studied scales. These five species showed similar trends of co^-occurrence on two nested scales in the upper Watarase river basin, which observed on a forest or micro-4-3 topographical scale（ 10⁴-10³ km²） in previous studies. In this study, we confirmed this phenomenon was maintained up to the branch scales.