Super-abundant C₄ grasses are a mixed blessing in restored prairies

Abstract

Forbs comprise most of the plant diversity in North American tallgrass prairie and provide vital ecosystem services, but their abundance in prairie restorations is highly variable. Restoration practitioners typically sow C₄ grasses in high abundances because they are inexpensive, provide fuel for prescribed fires, can dominate reference sites, and suppress weeds that suppress sown forbs. However, C₄ grasses can also suppress sown forbs, calling this practice into question. We evaluated how C₄ grasses influence the abundance and diversity of sown forbs in 78 restored prairies across Illinois, Indiana, and Michigan. We found that the direct negative effects of C₄ grasses on sown forbs outweighed indirect positive effects that occurred as C₄ grasses suppressed nonsown species, which in turn suppressed sown forbs. This pattern was especially strong for the C₄ grass big bluestem (Andropogon gerardii). Therefore, strategies to promote big bluestem and other C₄ grasses would not promote sown forbs. Although C₄ grass cover was not strongly related to two hypothesized drivers (time since fire or site age), seeding density of C₄ grasses increased their cover. Sown forb cover also increased with forb seeding density, increased indirectly with fire (through its negative effect on nonsown species), and decreased indirectly with soil water-holding capacity (through its positive effect on nonsown species). These results highlight the complex interplay of species groups during grassland restoration and show how managers can promote sown forbs in restored prairies: increasing forb seeding density and reducing time since fire and the abundance of C₄ grasses and weeds.