Sorghum halepense displaces the common prairie grass Schizachyrium scoparium: The possible role of allelopathy

One of the most successful invasive weed species in the southern U.S. is Sorghum halepense (Johnson grass). This exotic species from the Mediterranean region produces many seeds and grows clonally via extensive rhizome production. In addition, the allelopathic properties of S. halepense have been well documented. What has been lacking is evidence of negative effects of these allelochemicals in natural communities, particularly on native species undergoing invasions by this exotic grass. This study evaluated the effects of S. halepense leachate on growth of adults of the native prairie species Schizachyrium scoparium (little bluestem) in remnant tall-grass prairie in North Central Texas. In the field and greenhouse over one growing season, S. scoparium plants were treated weekly with S. halepense leachate while control plants received water. In a factorial design in the greenhouse, S. scoparium were also exposed to two soil types: native soil from a remnant prairie and invaded soil from a S. halepense infested prairie. In both the field and greenhouse, S. scoparium plants treated with leachate had significantly less biomass and fewer reproductive structures than control plants. Effects on growth of the two different soil types were less straightforward. Determining the legacy these allelochemicals may be leaving in the soil and what effects these toxins may have on growth of S. scoparium even after S. halepense is removed requires further study. Overall, these results suggest that allelopathy is contributing to the competitive success of S. halepense in the displacement of S. scoparium in a natural community by inhibiting both overall biomass and the development of reproductive structures in the native prairie grass. The potential impact this exotic allelopathic invader may have on the genetic diversity of S. scoparium requires consideration for successful restoration of currently invaded communities.