Ecological impacts of the invasive grass Sorghum halepense on native tallgrass prairie

Invasive plants frequently have competitive advantages over native species. These advantages have been characterized in systems in which the invading species has already become well established. Surprisingly, invader impacts on native communities currently undergoing invasion are lacking from most ecological studies. In this work we document and quantify shifting patterns in plant community structure in a native ecosystem (remnant tallgrass prairie) undergoing invasion by the invasive exotic Sorghum halepense (Johnsongrass). Further, we use manipulative field and greenhouse studies to quantify impacts of potential allelochemicals contained in whole-plant S. halepense leachates on growth of the dominant native grass, Schizachyrium scoparium (Little Bluestem), and tested the inhibitory effects of the potential soil legacy of S. halepense on the native grass in the greenhouse. Plant diversity indices revealed three distinct plant communities within the remnant prairie: a native community, a densely S. halepense invaded area, and a transitional zone between the two. Dominance of the native grass, determined by relative percent cover, significantly declined with increased S. halepense invasion via rhizomatous growth. Annual global positioning system monitoring of the S. halepense invasion front was used to quantify advancement into native prairie, documented at an average rate of 0.45 m year^-1. In the manipulative field and greenhouse studies, native S. scoparium treated with invasive S. halepense leachate had significantly less biomass and fewer inflorescences than control plants. These findings indicate the prolific clonal growth in conjunction with the plant chemistry of S. halepense play a significant role in displacement of the native grass.