Impact of over-wintering waterfowl on tuberous bulrush (Bolboschoenus planiculmis) in tidal flats

We investigated the effects of grazing by over-wintering waterfowl on tuberous bulrush (Bolboschoenus planiculmis) biomass in tidal flats. Our aim was to address the influence of the grazing pressure of different waterfowl groups during winter on tuber burial depth. An enclosure experiment was conducted to estimate the level of grazing, and individual numbers of waterfowl were monitored. Plant biomass rapidly decreased following the arrival of over-wintering waterfowl in October. Tuber biomass accounted for more than 69.2% of the total belowground biomass and was mainly used by waterfowl. Tuber biomass was highest at depths of 20–40 cm and lowest at depths >40 cm. After the arrival of migratory waterfowl, tuber biomass decreased relative to its depth range because of different shoveling abilities among ducks, geese, and swans. Ducks and geese grazed mainly on stems and tubers from surficial layers. Swans grazed deeper than geese; this correlated with biomass loss from deeper sediment layers (r: 0.78, P < 0.05). Differential grazing pressure along the sediment depth gradient had a direct influence on the asexual reproduction of the tuberous bulrush. Thus, this tuberous species may develop an equilibrium mechanism between grazing depth avoidance and energy depreciation in the subsequent year's sprouts.

► We examined the impact of waterfowl grazing on Bolboschoenus planiculmis biomass.
► Belowground biomass consisted mostly of tubers, the primary food of waterfowl.
► Variation in waterfowl grazing resulted in different grazing pressures with depth.