How do deer affect tree seedlings on a dwarf bamboo‐dominated forest floor?

A field experiment was conducted in Mt. Ôdaigahara, west‐central Japan, to reveal the effects of two herbivores, Sika deer ( Cervus nippon ) and mice ( Apodemus spp.), on the seedlings of five cohorts of three tree species, Abies homolepis , Fraxinus lanuginosa f. serrata and Fagus crenata . The forest floor of the study site was covered with dwarf bamboo, Sasa nipponica , which would also affect the seedlings. Eight combinations of three treatments were set: exclusion of deer, exclusion of mice and removal of dwarf bamboo. Deer were expected to affect the seedlings not only negatively by foraging but also positively by browsing dwarf bamboo that overshadows seedlings. The survival of these cohorts was analyzed by survival analysis and the differences in their survival between the treatments were investigated. The results of the experiment showed that: (1) exclusion of deer increased the aboveground biomass of dwarf bamboo and made it more difficult for seedlings to survive under the regenerated dwarf bamboo stand, and (2) deer negatively affected the seedlings, but they had positive, indirect effects in some cohorts by decreasing the aboveground biomass of dwarf bamboo. However, such effects were not detected in some cohorts which were affected excessively by deer or dwarf bamboo. No clear results on the effect of mice on seedlings were obtained. We conclude that a positive, indirect effect of deer on seedling survivorship would be observable when the magnitude of a positive, indirect effect caused by decreasing dwarf bamboo is larger than that of a negative, direct effect of deer.