Forage productivity, species evenness and weed invasion in pasture communities

A long standing ecological paradigm suggests that diverse plant communities should be more resistant to weed invasion compared with plant communities that have few species. Data from three separate studies were used to test the hypothesis that increased forage species diversity reduces weed invasion in pasture communities. The first study measured weed invasion in experimentally constructed pasture communities containing 1–15 species. A second experiment conducted in the greenhouse involved sowing a common perennial weed species, curly dock (Rumex crispus), into forage mixtures with 5 or 10 different forage species and three monocultures. The third study investigated the relationship between weed abundance and forage diversity in 37 pastures surveyed across the northeastern United States. Consistent negative relationships between forage species diversity and weed abundance were found. The causal mechanisms explaining the negative relationships between forage diversity and weed invasion were difficult to delineate since diversity was often correlated with highly productive pasture communities. The results suggest that maintaining both productive pasture communities (>150 g m−2 of aboveground biomass) and an evenly distributed array of forage species should be combined to effectively reduce weed invasion. Managing pastures for increased forage species diversity could be a useful cultural control method for weeds. Increasing the diversity of forage plants also has other ecosystem benefits to pasture communities (e.g., greater primary production, stability) beyond that of weed suppression.