Evidence for community structuring associated with the actinorhizal shrub Ceanothus americanus in tallgrass prairies in Illinois, USA

The role of the actinorhizal shrub Ceanothus americanus L. in community organisation of native grasslands was examined in three Illinois tallgrass prairie remnants. We asked can C. americanus structure tallgrass prairie at the patch scale through nitrogen fixation leading to differences in species assemblages and diversity? Expectations were that warm season (C ₄) graminoid species would be more abundant and species diversity would be greater outside Ceanothus patches than in patches associated with C. americanus where, based on results from numerous nitrogen augmentation and deposition studies, we expected greater abundance of cool season (C ₃) graminoid species and lower species diversity. Plots with and without association to C. americanus were compared for differences in floristic similarity, diversity, and C ₃ versus C ₄ graminoid species abundance. C ₃ graminoid species, including the adventive Poa pratensis L., were significantly more abundant in prairie associated with C. americanus than were C ₄ species. C ₄ species, such as Andropogon gerardi Vitman and Schizachyrium scopariumii (Michx.) Nash., were more abundant than C ₃ graminoid species in prairie patches without association to C. americanus. Means for diversity among plots associated with C. americanus were significantly lower than for other plots, nearly so for evenness and species density, but not different for sum total cover or total species richness. There were no statistically-significant differences in measures of mean total soil nitrogen at points along linear transects radiating from the base of C. americanus shrubs nor between mean soil amino-sugar-nitrogen concentrations within and outside of C. americanus patches. Nonetheless, comparative evidence supports the hypothesis that C. americanus structures composition and diversity at the local patch scale in tallgrass prairie.