WATER RELATIONS AND GROWTH OF TALLGRASS PRAIRIE FORBS IN RESPONSE TO SELECTIVE GRASS HERBIVORY BY BISON

The indirect effects of selective grass herbivory by bison (Bison bison) on the xylem pressure potentials (psi) and primary production of ungrazed tallgrass prairie forbs were assessed during the 1990 growing season on the Konza Prairie Research Natural Area (KPRNA) in northeast Kansas. Responses in three forb species, Vernonia baldwinii, Ambrosia psilostachya, and Aster ericoides, were measured in grazed and adjacent ungrazed patches. Seasonal predawn and midday psi responses in these forbs were also compared with responses in Andropogon gerardii, the dominant grass in tallgrass prairie. Reductions in grass leaf area by herbivory may result in greater availability of soil water to the remaining forbs, if the forbs and grasses utilize similar soil water resources. Overall, selective herbivory of the grasses had an inconsistent effect on leaf psi in the remaining forbs. When psi were relatively high during the growing season, no significant differences in water status were detected between A. gerardii and the forbs. However, when psi were low, predawn and midday psi were higher in V. baldwinii (as much as 1.0 MPa), and midday psi were higher in A. psilostachya (0.5 MPa) compared with A. gerardii. In contrast, psi were not significantly different between A. ericoides and A. gerardii. Grazing by bison substantially increased light availability to ungrazed forbs (> 40%) in grazed relative to ungrazed areas. Compared with ungrazed areas, end-of-season aboveground biomass in grazed patches was 40% greater for individuals of A. psilostachya and 33% and 26% greater for reproductive biomass and floret numbers, respectively, in V. baldwinii. We conclude that the potential benefits of selective grass herbivory on forb water relations may be reduced or offset by other factors such as significant alterations in the microclimate of grazed patches. Nonetheless, increased productivity in grazed patches in two of the three forbs studied indicates that selective consumption of grasses by bison may benefit some forb species.