The Conservation Reserve Program (CRP) is a federally funded program designed to convert former cropland to perennial systems to reduce erosion, improve water quality, and enhance wildlife habitat and biodiversity. Though much of the 2.1 million acres of CRP in Kansas has been replanted with prairie species, CRP plant communities have been observed to be less species rich and more structurally homogeneous than remnant prairies. This lack of heterogeneity may be due to the absence of a dominant herbivore such as bison or cattle. However, few studies have examined CRP plant communities across large spatial scales or tested the effects of cattle grazing on these systems. We collected vegetation data from 108 CRP fields across the longitudinal precipitation gradient (450-1100 mm annually) in Kansas on sites spanning short-, mixed-, and tall-grass habitats. Nearly half the fields were grazed by cattle during the 2017-8 growing seasons. Plant communities were established using either lower (CP2) or higher (CP25) forb diversity in seeding mixtures. We hypothesized that plant community diversity would increase with average annual precipitation, grazing would increase community as well as structural diversity, and CP25 fields planted with additional forbs would be more diverse than CP2 fields.
Results/Conclusions
Across all fields, we found 226 plant species representing 10% of Kansas plant diversity. Species richness was positively correlated with annual precipitation, resulting in a two-fold increase in richness across the gradient. Eastern and western Kansas plant communities differed from one another with the central region being intermediate. These changes in species composition and abundance were correlated with average annual precipitation and soil pH, organic matter, and potassium. Additionally, the ratio of forb to grass cover increased with precipitation and at a greater rate in the higher diversity planting (CP25 compared with CP2), suggesting greater establishment success of forb species in the eastern region. Grazing resulted in multivariate differences in the plant communities in 2018, but not 2017. These differences were subtle and did not correspond to an increase in non-natives, a shift in floristic quality, or a consistent change in dominant grass abundance. Grazing also created greater heterogeneity in the physical structure of the vegetation across the precipitation gradient, which may be important for wildlife habitat. These results suggest that grazing can enhance plant structural heterogeneity in CRP grasslands while causing minimal change in plant community composition, at least over two years of moderate grazing.