Ecosystem degradation by large herbivorous mammals becomes a serious issue worldwide. In central Japan, over-grazing of forest understory vegetation by Japanese sika deer (Cervus nippon centralis) has been pronounced since 2000’s. In 2006, we constructed a deer exclusion fence around a 13-ha catchment in a natural cool-temperate forest in Ashiu, Kyoto Prefecture, Japan. The main goal of this study was to elucidate the effects of deer overgrazing of understory vegetation on the nitrogen (N) cycling and loss by comparing the fenced watershed to its adjacent 19 ha control catchment.
In the fenced catchment, the stream water nitrate concentration was decreased year by year in response to the increase in the understory vegetation coverage. Averages of annual N loss was 2.4 kgN ha-1 in the fenced watershed and 4.9 kgN ha-1, and its difference was 2.5 kgN ha-1. Nitrogen uptake by understory vegetation was estimated to be 5.5 kgN ha-1 in the fenced, and 3.3 kgN ha-1 in the unfenced, which indicated that loss of N uptake induced by deer over-grazing can directly influence hydrological N loss. In conclusion, forest understory vegetation has a small amount of biomass, but losing this component by deer over-browsing can lead to an increase in loss of nitrate to streamwater. Our finding suggests that recovery from intense herbivore pressure can retain nitrate effectively.