Invasive grass litter controls rangeland plant community dynamics

Invasive species cause significant ecological impacts worldwide, affecting numerous ecosystems. In North America, changes to historic fire and grazing regimes have created an opportunity for the invasion of herbaceous plants, such as Kentucky bluegrass (Poa pratensis; hereafter: "bluegrass"). Bluegrass is one of the most aggressive, non-native grasses affecting rangelands across the Great Plains, with its success potentially attributed to the build-up of a novel thatch layer and accumulation of dense litter. Management of bluegrass is complicated as strategies that reduce bluegrass abundance also impact physiologically and morphologically similar native species, creating a need to understand and target the mechanisms that promote bluegrass invasion. To determine the mechanisms behind bluegrass invasion, we monitored the influence of targeted litter and thatch removal on plant community dynamics in south-central North Dakota over 50 months. We found that bluegrass invasion negatively impacted plant species richness, evenness, and occurrence, and that once thatch and litter were removed, all community dynamics metrics increased. These impacts were primarily driven by changes in invasive litter cover and depth. However, species evenness was also influenced by live bluegrass and thatch. These results are among the first to empirically tie thatch accumulation to negative impacts on plant community dynamics. However, our results suggest that litter accumulation, not thatch, is the primary, mechanism driving bluegrass invasion. Ultimately, our results suggest that management of bluegrass should focus on actions that can impede litter formation, such as the combination of fire and grazing, that allows native plants to establish and express themselves.