COS 263-1 - Heterogeneity-Based Management Reduces the Impacts of an Invasive Grass on Native Plant Diversity

Abstract: Exotic plants invade and alter rangelands throughout the world. Understanding the drivers behind invasions and their impacts on native plant communities is critical to controlling and mitigating the impacts of invasive species. Rangelands in the northern Great Plains are being converted into novel ecosystems by invasive plants, especially Kentucky bluegrass (Poa pratensis; hereafter, “bluegrass”). Bluegrass invades rangelands and forms a dense litter layer. This dense litter accumulation leads to the formation of thatch, a unique pseudo-soil layer, that suppresses native species by altering water and nutrient cycling. Bluegrass litter and thatch are thought to be the primary drivers behind bluegrass invasion. To understand and quantify the impacts of bluegrass on rangeland plant communities, we examined how bluegrass directly influenced native plant diversity through competition and indirectly through litter and thatch accumulation using structural equation modeling (SEM). We sampled native plant diversity, bluegrass abundance, and thatch and litter accumulation in 12 pastures managed with either season-long continuous grazing, patch-burn grazing, or a heterogeneity-based rotational grazing strategy. Patch-burn pastures create structural heterogeneity by burning a quarter of each pasture every spring, which alters cattle grazing behavior and causes them to preferentially graze recently burned areas, whereas heterogeneity-based rotational pastures create structural heterogeneity by creating different paddocks within a pasture that are grazed at different intensities using interior fences.

Using SEM, we found that bluegrass negatively impacted native plant diversity in pastures managed with season-long continuous grazing. The total negative impact of bluegrass on native diversity was approximately 8.22 (SE ± 1.58) times stronger in season-long continuous pastures than patch-burn pastures and 42.46 (SE ± 2.80) times stronger than heterogeneity-based rotational grazing. Additionally, in season long continuous grazing pastures, bluegrass thatch accounted for 58.78%, 89.23%, and 99.12% of the negative impact of bluegrass on native forb, legume, and grass diversity respectively. However, bluegrass thatch did not negatively impact native plant diversity in patch-burn or heterogeneity-based rotational pastures. Furthermore, in patch-burn pastures native plant diversity was only negatively impacted through direct competition with bluegrass. These findings demonstrate that under conventional management, bluegrass has a strong negative impact on native plant diversity through the creation of thatch. However, the negative impacts of novel thatch accumulation and bluegrass as a whole can be reduced through the use of management that creates landscape-level heterogeneity. This suggests that future management of bluegrass invaded rangelands should favor practices that create landscape-level heterogeneity while also reducing bluegrass litter and thatch.