PARENT SESSION
Oral Session #28: Plant-Animal Interactions. Presiding: S. Louda.
Tuesday, August 7, 2001. 8:00 AM to 12:00 PM. Hall of Ideas J.

Grazers stimulate whole grassland production in Yellowstone Park.

Frank, Douglas¹, Guido, Daniel¹, Kunz, Michelle¹, ¹

ABSTRACT- Information on factors that control terrestrial productivity is principal to understanding ecosystem processes and global chemical cycles. Yet, measures of whole ecosystem production are rare, due to a paucity of data on root productivity, which represents the largest fraction of production in most ecosystems. The role of consumers is generally considered inhibitory, primarily because of the expected negative effects of herbivores on root growth. We measured shoot and root production inside and outside permanent fences at nine diverse sites, ranging widely in elevation, soil characteristics, and seasonal use by migratory elk, bison, and pronghorn in Yellowstone National Park. Shoot production in grazed grassland was determined with movable exclosures to account for material removed by herbivores. Root production was measured with minirhizotron tubes in grazed and fenced grassland. Grazers facilitated aboveground and belowground grassland productivity. Shoot and root stimulation was positively associated with consumption by herbivores. Grazers also promoted root turnover, thus increasing the input of labile organic matter to the soil. These results refute the prevailing notion that grassland herbivory always leads to a reduction in root productivity, and indicate that grazing by migratory animals can stimulate whole ecosystem production.

KEY WORDS: herbivore, grassland, Yellowstone National Park, roots