Restored soil conditions alter competition dynamics of Andropogon gerardii: Implications for species loss during tallgrass prairie restoration
Grassland restorations often experience loss in plant species over time coinciding with increasing dominance of perennial C4 grasses. Changes in soil conditions may alter species coexistence and explain the increased dominance of C4 grasses as restoration proceeds. We hypothesized the competitiveness of a dominant grass, Andropogon gerardii, varies with soil condition influenced by restoration age. We removed intact soil cores (7.62 cm diameter x 20 cm deep) from a cultivated field (initial conditions of prairie restoration), a 4-year-old restoration, a 16-year-old restoration, and a remnant prairie (target conditions of a restoration) to conduct a competition experiment. One dominant grass was grown in each soil core with one of four subordinate forbs in each restored soil treatment (n = 16 per soil treatment, total n = 64). The focal grass was also grown in monocultures of two and as single plants (n = 16 per soil treatment) for comparison to interspecific mixtures. Relative competition intensity and relative yield index values were calculated with measurements of relative growth rate based on maximum height, net absolute tiller appearance rate, and final biomass. Responses were analyzed using mixed models followed by means comparisons using protected Fisher’s LSD.
There was partial support for soil condition changing competition dynamics of A. gerardii. Relative competition intensity mean index values, using the net absolute tiller appearance rate with a monoculture comparison (P = 0.033), indicated competition intensity is altered by restored soil age. Relative yield mean index values, using biomass with a control comparison (P = 0.028) and net absolute tiller appearance rate with a monoculture comparison (P = 0.029), demonstrated importance of competition changed with restored soil conditions. Competition indices had significantly higher index values (greater A. gerardii:forb production) in the 16-year restored soil compared to 4-year restored soil. The relative yield index using biomass with a control comparison, showed that the 16-year restored soil also had significantly more A. gerardii:forb production than remnant prairie soil, suggesting the increased competiveness of A. gerardii may facilitate greater suppression of subordinate forb species in older restorations. This change in competition dynamics is likely due to increased soil organic matter, less available nutrients, and growing biomass of soil mutualists (AMF) that favor growth of the dominant species.