Homogenization of understory vegetation by an overabundance of deer (Cervus nippon) in a temperate forest in central Japan

Deer browsing is a form of selective disturbance that homogenizes plant community composition and structure. In mountainous regions, disturbance intensity, together with topographical factors, affect plant communities. However, studies on the multiple effects of deer browsing and topographical factors on plant communities are limited. Here, we investigated the effects of deer exclusion on understory vegetation across the upper and lower slopes of a temperate deciduous forest in a mountainous area of Japan. We established six pairs of exclosure and control quadrats, with three pairs at each topographical position. We assessed changes in vegetation and plant structure (coverage and height), species composition, numbers of flowering/fruiting species, and damage to the plant species caused by deer browsing between 2009–2016. Vegetation coverage and the number of flowering/fruiting plant species increased in the exclosure quadrats during the study period, indicating that browsing prevents herb-layer species from reaching the flowering and fruiting stages in this site. Moreover, dissimilarity in species composition between the exclosure and control quadrats increased on the upper slope. However, the dissimilarity between upper and lower slopes in the control quadrats tended to decrease. Large herbs, which mainly include forbs and perennial species, were characteristically abundant on the lower slopes and decreased in the control quadrats on the lower slope. Our results indicate that the exclusion of deer had different effects on the understory vegetation of upper and lower slopes, suggesting that deer overbrowsing weakens the correlation between plant functional traits and topographic factors, resulting in homogenized plant communities.