EVIDENCE FOR THE PROMOTION OF ABOVEGROUND GRASSLAND PRODUCTION BY NATIVE LARGE HERBIVORES IN YELLOWSTONE-NATIONAL-PARK

We examined the effect of native large herbivores on aboveground prima ry production of nonforested habitat in Yellowstone National Park, Wyo ming. Productivity of vegetation grazed by elk (Cervus elaphus) and bi son (Bison bison) was compared with that of ungrazed (permanently fenc ed) vegetation at four sites. Two methods were used that, we believed, would provide the most accurate measurements under the different graz ing regimes encountered in the study. Production of ungrazed vegetatio n in permanent exclosures (10 x 10 m or 15 x 15 m, 3 per site) and tha t of vegetation that was grazed only in the winter was taken as peak s tanding crop. Production of vegetation grazed during the growing seaso n was the sum of significant increments (P<0.05) in standing crop insi de temporary exclosures (1.5 x 1.5 m, 6 per site) moved every four wee ks to account for herbivory. Aboveground productivity of grazed vegeta tion was 47% higher than that of ungrazed vegetation across sites (P<0 .0003). This result could be explained by either a methodological or g razer effect. We believe it was the latter. Results from a computer si mulation showed that sequential sampling with temporary exclosures res ulted in a slight underestimation of production, suggesting that the r eported differences between treatments were conservative. We suggest t hat stimulation of aboveground production by ungulates may be, in part , due to the migratory behavior of native ungulates that track young, high quality forage as it shifts spatially across the Yellowstone ecos ystem.