Effects of understory degradation induced by sika deer on soil microarthropods in beech forests: comparison between two different climatic conditions

Understory vegetation dominated by dwarf bamboo ( Sasa spp.) has been decreasing in Japan because of deer overgrazing. Declining understory vegetation may reduce soil microarthropods abundance. However, the effects of understory degradation on soil microarthropods under various climatic conditions remain poorly understood. We compared soil microarthropods, soil properties, and erosion rates (cm year −1 ) between rich understory plots ( Sasa ) and no understory plots (No Sasa ) in six beech forests in Kyushu and San-in. The Kyushu region experiences more rainfall and less snow depth than the San-in region. In Kyushu, the abundance of soil microarthropods was 40 % significantly lower in the No Sasa plot than in the Sasa plot, whereas there were no significant differences in San-in. In both regions, soil bulk density was significantly higher in the No Sasa plot, but the effects of understory degradation were larger in Kyushu. These results suggest that understory degradation leads to soil runoff and reduces habitable pore spaces for soil microarthropods. Additionally, structural equation modelling analysis indicated that soil erosion following understory degradation was influenced by climatic conditions, and the increase in bulk density induced by soil erosion reduced the abundance of soil microarthropods. This finding suggests that the effects of understory degradation on soil microarthropods via changes in bulk density vary depending on climatic conditions and are stronger in erosion-prone areas. Our results imply that prevention measures for understory vegetation are urgently needed to maintain soil biodiversity, particularly in regions with climatic conditions that are highly susceptible to soil erosion.