In Kansas, Fahnestock and Knapp examined two abundant tallgrass prairie forb species, Ambrosia psilostachya and Vernonia baldwinii, commonly found intact in patches where the grasses have been selectively grazed by bison. Bison herbivory indirectly enhanced water availability and productivity of forbs growing in grazed patches. Incident light at forb mid-canopy height was 53% greater on grazed patches than on ungrazed sites at midseason and soil temperatures were always warmer, perhaps enabling forbs to initiate growth earlier in the spring. Relative to individuals in ungrazed areas, the end of season biomass of A. psilostachya was 40% greater and reproductive biomass head count of V. baldwinii was 45% and 40% greater, respectively, in plants in grazed patches. Selective grass herbivory by bison increases light and water availability in grazed patches during periods of water limitation, which indirectly enhances the carbon gain of intact forbs in these patches, leading to increased biomass and potential fitness.
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