Population structures and spatial patterns of two unpalatable Arisaema species (Araceae) with and without clonal reproduction in a riparian forest intensively grazed by Sika deer

General decline of understory cover can result from increased abundance of and foraging pressure by deer. But population size and degree of aggregation can increase for unpalatable understory plants that escape foraging pressure. Clonal reproduction can enable unpalatable plant species to increase their population sizes while trending toward spatially aggregated distributions. However, the details of the relationship between clonal reproduction in unpalatable plants and their dynamics under intensive deer herbivory are not clear. We compared the population structures and spatial patterns of two coexisting unpalatable plant species, Arisaema ovale (with clonal reproduction) and A. peninsulae (without clonal reproduction) in a riparian forest intensively grazed by Sika deer, and examined the null hypothesis that the extent of spatial aggregation and local population size would not differ between the clonal and non-clonal Arisaema species. In a 0.36-ha plot, A. ovale had a larger population size (1087 individuals) with a higher abundance ratio of small plants (p < 0.01) than A. peninsulae (84 individuals). Analyses of spatial point processes showed that both populations were spatially aggregated (p < 0.05). The spatial aggregation of A. peninsulae, however, became weaker than that of A. ovale, when we excluded one dense patch originating from irregular seed dispersion. These results, excluding the aggregated distribution observed in A. peninsulae, suggested a substantial contribution of clonal reproduction to the expansion of the local A. ovale population following intensive grazing by Sika deer.