02518nas a2200145   4500008004100000245010200041210006900143300001300212490000800225520203800233653002002271100002102291700002002312856004002332       1993                        eng d00aWater relations and growth of tallgrass prairie forbs in response to selective herbivory by bison0 aWater relations and growth of tallgrass prairie forbs in respons  a432 -4400 v1543 aThe indirect effects of selective grass herbivory by bison (Bison bison) on the xylem pressure potentials (ψ) 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 ψ 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 ψ in the remaining forbs. When ψ were relatively high during the growing season, no significant differences in water status were detected between A. gerardii and the forbs. However, when ψ were low, predawn and midday ψ were higher in V. baldwinii (as much as 1.0 MPa), and midday ψ were higher in A. psilostachya (0.5 MPa) compared with A. gerardii. In contrast, ψ 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.10aWater relations1 aFahnestock, J.T.1 aKnapp, Alan, K.  uhttp://www.jstor.org/stable/2995662