Effects of Sika Deer (Cervus nippon) and Dwarf Bamboo (Sasa senanensis) on TrilliumPopulations in Akan National Park, Eastern Hokkaido, Japan

Abstract

The competition between Trillium spp. and other species could modify the utility of Trillium spp. as indicators of the impact of deer on forest vegetation. To evaluate whether Trillium spp. (T. camschatcense and T. tschonoskii) are appropriate indicators of the impact of sika deer (Cervus nippon) in the presence of dwarf bamboo (Sasa senanensis), which competes with Trillium populations, we examined the size class of large‐growth‐stage Trillium plants, the frequency of the presence of deer pellets and the coverage of S. senanensis in deer‐excluded sites (where deer have been eliminated since 1995) and unfenced control sites in Akan National Park, eastern Hokkaido, Japan. The result of a cumulative link mixed model (CLMM) suggests that size class at deer‐excluded sites is influenced by competitive understory species, even without deer grazing. The CLMM result for unfenced control sites suggests that C. nippon negatively influences the size class of Trillium populations; however, the negative effect weakened with increasing S. senanensis coverage. Thus, S. senanensis can function as an unpalatable neighbor that makes it difficult for C. nippon to detect Trillium plants. We conclude that the size class structure of Trillium spp. can be a useful index of C. nippon usage under sparse or medium S. senanensis understories; however, it is difficult to use these indices under dense S. senanensis understories because of the weakened negative effect of C. nippon grazing.