Floristic Homogenization and Differentiation under Deer Grazing in a Subalpine Zone in Central Japan

Changes of vegetation due to deer grazing may result in floristic homogenization or differentiation. The relationships between the changes caused by grazing are still being debated. Vascular plants and grazing by Cervus nippon were recorded in 2010 and 2018 in Betula forests and tall grassland communities in a subalpine zone, central Japan. Species composition differed significantly between vegetation types in both censuses. The α-diversity had declined significantly in both vegetation types. Metrics related to species replacement made a greater contribution to β-diversity than those related to nestedness in both vegetation types. A decrease of β-diversity based on incidence data from 2010 to 2018 in the Betula forests indicated that floristic homogenization had occurred; an increase of β-diversity based on abundance data from 2010 to 2018 in the grasslands indicated that floristic differentiation had occurred. Changes of α-diversity in both vegetation types and of β-diversity in the grasslands were significantly correlated with grazing intensity. These results showed that deterministic processes, which originally differed between the Betula forests and grasslands, would be the dominant causes of floristic changes under deer-grazing pressure