Continuous harvesting of a dominant bracken alters a cool-temperate montane grassland community and increases plant diversity in Nagano, Japan

Declines in biodiversity can be caused by intense land-use or by land abandonment. Traditional plant-gathering in Japan has declined or has often been abandoned in recent decades. However, little is known about how traditional plant-gathering affects the diversity, productivity, and composition of plant communities. Traditionally, bracken (Pteridium aquilinum subsp. japonicum) served as a natural resource in Japan, and people continue to harvest it in some places. We conducted a 4-year field experiment in a cool-temperate grassland on the Sugadaira Plateau, Nagano Prefecture, comparing a continuous harvesting treatment of the dominant Pteridium with a non-harvested control. We wanted to determine whether Pteridium harvesting would alter plant species richness, total productivity, and plant community composition. Local people harvested Pteridium shoots in a 1800 m² treatment area each year from 2011 to 2014. By 2014, species richness and diversity (Shannon index) had significantly increased while Pteridium abundance (number of shoots and biomass) had decreased in the harvest treatment when compared with the control. Total productivity, measured as combined aboveground biomass of all vascular plant species, was similar between the treatment and control; however, community composition differed. Harvested plots had a higher biomass of erect-formed, invasive and native species, but lower biomass of small-formed species when compared with the control. Our results suggest that traditional plant-gathering, such as periodic Pteridium harvesting can result in increases in plant diversity while total productivity is maintained. However, long-term monitoring is recommended to detect any subsequent undesirable changes in the community, such as increases in invasive species populations.