Natural wetland plant foods meet energetic requirements for waterfowl and sandhill cranes (Grus canadensis) if production surpasses foraging efficiency thresholds. Although knowledge of aboveground plant production and migratory bird use are available to estimate carrying capacity on intensively managed moist-soil wetlands, data are lacking regarding production and use of underground plant foods in managed areas. In 1996 and 1997, we determined use of underground foods by mallards (Anas platyrhynchos) and sandhill cranes on moist-soil managed wetlands where mowing, discing, and sustained flooding treatments were tested to enhance chufa (Cyperus esculentus) production. Although there were no differences among treatments in use of belowground foods, within-treatment analyses showed that avian foraging reduced chufa tubers from 17–27% except in the 1996 sustained flooding treatment. Johnson grass (Sorghum halepense) rhizomes were reduced only in the 1997 mowed and sustained flooding treatments, whereas field bindweed (Convolvulus arvensis) rhizomes were consumed only in the 1997 mowed treatment. Based on use levels, a threshold level of about 500 kg/ha of underground foods should be produced before use occurs. However, use of underground foods also may have been related to the amount of aboveground seed available in each treatment. Mallards initially may have been attracted to flooded wetlands with high aboveground seed production leading to subsequent use of underground foods. By ignoring underground wetland foods, carrying capacity for migratory birds using moist-soil managed wetlands may be underestimated.
aboveground standing crop
belowground standing crop