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.

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Identifying appropriate indicator species for the impact of deer on forest vegetation is crucial for forest management in deer habitats and is required to be sensitive to temporal and spatial variations in deer density. Dryopteris crassirhizoma was selected as a new indicator to evaluate the response to these variations. We examined the population-level characteristics, morphological characteristics at the individual level, and grazing intensity of D. crassirhizoma at temporally different deer density sites in Hokkaido, Japan. The response of D. crassirhizoma to spatial variation in deer density was also examined within and between two regions in Hokkaido, Japan. Although the population-level characteristics and morphological characteristics did not significantly respond to short-term decreases in deer density, grazing intensity significantly decreased with decreasing deer density. The grazing intensity was also positively related to the spatial variation of deer density within both regions, but the estimated coefficient of the grazing intensity differed between regions. We concluded that D. crassirhizoma can be a useful indicator species of the impact of deer on forest vegetation. The grazing intensity of the indicator species was sensitive to temporal and spatial variations in deer density within the region.