Bamboo took over when deer were fenced out

Overgrazing by deer populations can be devastating to plants and trees in forests. In regions where deer impact is particularly severe, fencing is sometimes employed as a conservation measure to safeguard forest and ground flora.

900 hectare fenced

In Nikko National Park, Japan, researchers examined how plants on the ground were affected 21 years after sika deer were fenced out from a 900 hectare area. They surveyed vascular plant cover in 240 plots and measured forest density on both sides of the fence in stands dominated by larch and oak (Larix kaempferi and Quercus crispula). The main findings were:

• Vascular plant species richness was found to be lower in forest areas protected by the fence.
• At the same time, sasa bamboo, often the most abundant understory component in open Japanese forests, was taller and denser inside the fence where deer had been excluded.
• In addition, the lack of natural tree regeneration inside the fence was also notable, likely due to strong bamboo competition.

The results suggest that deer exclusion has benefited sasa bamboo while inhibiting the competition of other species, thereby lowering vascular plant richness. Vascular plant communities outside the fence instead seemed to be strongly shaped by abundant sika deer. Several species found there are known to be unpalatable or grazing tolerant, and there were few tree seedlings.

”These results show that fencing to protect forest flora against overgrazing may have unintended consequences where there is a risk that competitive species can dominate understory plant communities,” says Lisa Peterson, researcher at the Swedish University of Agriculture Sciences.

Long wait for return

The outcompeted species may have to wait a long time for their chance to return. Their opportunity will come when the sasa bamboo flowers and dies, which occurs every 60 – 100 years.

Instead of fencing, improved game management to maintain moderate deer populations may be a better strategy for large scale conservation of forest flora and for promoting natural tree regeneration.

“Current deer-management efforts are clearly not enough for maintaining floral diversity and for enabling natural forest regeneration,” says Lisa Peterson, lead author of the scientific article Long-Term Deer Exclusion Releases Dwarf Bamboo, Reducing Vascular Plant Diversity.