02104nas a2200193   4500008004100000245011400041210006900155300001300224490000800237520147500245653001001720653002001730653002401750653002201774653001701796100002101813700002001834856005601854       1994                        eng d00aResponses of forbs and grasses to selective grazing by bison: interactions between herbivory and water stress0 aResponses of forbs and grasses to selective grazing by bison int  a123 -1310 v1153 aTwo abundant tallgrass prairie forb species, Ambrosia psilostachya and Vernonia baldwinii, are commonly found intact in patches where the grasses have been selectively grazed by bison. Microclimatic patterns and physiological responses of these forbs were measured in grazed and ungrazed patches. These experiments demonstrated that bison herbivory indirectly enhanced water availability and productivity of forbs growing in grazed patches. This was due primarily to the reduction in transpiring grass leaf area in grazed patches and an increase in light availability. In grazed patches, incident light at forb mid-canopy height was 53% greater than ungrazed sites at midseason and soil temperatures were always warmer (e.g., 10 ?C at 5 cm), perhaps enabling forbs to initiate growth earlier in the spring. Enhanced leaf xylem pressure potential and stomatal conductance in plants in grazed areas were most evident when water availability was low (i.e., late in the growing season and over short-term dry periods characteristic of the tallgrass prairie environment). Relative to individuals in ungrazed areas, end-of-season biomass of A. psilostachya was 40% greater and reproductive biomass and head number of V. baldwinii was 45% and 40%  greater, respectively, in plants in grazed patches. A favorable growing environment maintained in grazed patches during periods of water limitation enhances carbon gain in forbs leading to increased biomass and potential fitness.10abison10aLight reduction10aSelective herbivory10atallgrass prairie10awater stress1 aFahnestock, J.T.1 aKnapp, Alan, K.  uhttp://link.springer.com/article/10.1007/BF00044867