Pieris japonica (Ericaceae) Dominance and Increased Organic Horizon Inhibit Plant Establishment in Forests Altered by Deer Grazing

The prevalence of unpalatable plants to deer increases in forests after browsing. Such a shrub, Pieris japonica (Ericaceae), has become the dominant understory vegetation in the Kyushu Mountain region of southern Japan. This monodominance has led to a shaded understory and a build‐up of humus layers, with a remarkable scarcity of plantlets beneath the P. japonica populations. To investigate the mechanisms by which P. japonica dominance suppresses plant establishment, we quantified the plant numbers and survival in an area dominated by this species and conducted germination tests to identify the factors hindering understory germination and initial growth. We observed a limited number of plantlets and recruitment in areas dominated by Pieris populations compared to the adjacent areas, while survival rates remained consistent across sites. Germination tests using humus collected from under the Pieris populations revealed a significantly reduced radicle length compared to the controls. This finding suggests that the modified organic horizon caused by the dominance of Pieris is a major factor inhibiting root growth and, ultimately, limiting plant establishment. These results offer insights for managing Pieris populations.