Consumer control of grassland plant production

The effect of herbivory on grassland whole-plant production is poorly understood. Herbivores can increase grassland aboveground productivity, and laboratory experiments suggest that herbivory should reduce grass root growth. However, few field studies have directly measured the response of grassland root production to herbivores. We examined the effect of native migratory ungulates on grassland primary production by comparing aboveground (NAP) and belowground (NBP) production in grazed vs. ungrazed (fenced) grassland at nine variable sites in Yellowstone National Park. NBP was determined with minirhizotrons to account for root turnover. Grazers stimulated aboveground, below-ground, and whole-grassland productivity by 21%, 35%, and 32%, respectively. Root production was stimulated seven times more (217 g/m²) than shoot production (30 g/m²), indicating that the major effect of herbivores in this system was a positive feedback on root growth. Results refute the prevailing notion that grassland herbivory leads to a reduction in root productivity, and a concomitant decline in soil carbon content, and provide a potential explanation for how organic-rich soil developed in grassland that was grazed throughout millennia.