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 biso n) on the xylem pressure potentials (psi) and primary production of un grazed tallgrass prairie forbs were assessed during the 1990 growing s eason 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 adjacen t ungrazed patches. Seasonal predawn and midday psi responses in these forbs were also compared with responses in Andropogon gerardii, the d ominant grass in tallgrass prairie. Reductions in grass leaf area by h erbivory may result in greater availability of soil water to the remai ning forbs, if the forbs and grasses utilize similar soil water resour ces. Overall, selective herbivory of the grasses had an inconsistent e ffect on leaf psi in the remaining forbs. When psi were relatively hig h during the growing season, no significant differences in water statu s were detected between A. gerardii and the forbs. However, when psi w ere low, predawn and midday psi were higher in V. baldwinii (as much a s 1.0 MPa), and midday psi were higher in A. psilostachya (0.5 MPa) co mpared with A. gerardii. In contrast, psi were not significantly diffe rent between A. ericoides and A. gerardii. Grazing by bison substantia lly increased light availability to ungrazed forbs (> 40%) in grazed r elative 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 a nd floret numbers, respectively, in V. baldwinii. We conclude that the potential benefits of selective grass herbivory on forb water relatio ns may be reduced or offset by other factors such as significant alter ations in the microclimate of grazed patches. Nonetheless, increased p roductivity in grazed patches in two of the three forbs studied indica tes that selective consumption of grasses by bison may benefit some fo rb species.