Impacts of a non-native grass invasion decline with succession

Background/Question/Methods

Biological invasions are known to inhibit native biodiversity and alter habitats and ecosystem processes. However, the majority of invasion impact research has focused on comparing species and characteristics in invaded and nearby invasion-free habitats or evaluating invasion impacts over relatively brief time periods. As such, little is known about how the impacts of invasions might change over successional time frames. To evaluate the longer-term effects of plant invasions, we established plots with communities of native trees and commonly associated herbaceous species and then experimentally invaded half of all plots with Microstegium vimineum (stiltgrass), one of the most widespread and problematic invasive plants in eastern deciduous forests. After five years, we divided the plots into quadrats and applied prescribed fire to one quadrat per plot in the spring of two subsequent years. Over the course of eight years, we quantified the relative dominance of Microstegium, the diversity, composition, and abundance of native herbaceous species, and the recruitment and performance of trees, thereby providing a long-term measure of the impacts of an aggressive invasive plant.   

Results/Conclusions

Initially, Microstegium dominated invaded plots, constituting more than 65% of average plot biomass during the first three years of the experiment. The abundant invasion resulted in 46-64% less native herbaceous species biomass, nearly 40% lower diversity, and significantly less natural tree recruitment in invaded than control plots during the early years of the experiment. However, the invasion began to rapidly decline four years into the experiment and after eight years, Microstegium represented less than 1% of plot biomass, regardless of the fire treatment. As the invasion waned, native herbaceous species recovered such that there was no difference in native biomass during years seven and eight. Across invasion treatments, fire significantly reduced tree density and increased light availability, thereby promoting native herbaceous performance, particularly forbs. The effects of fire and invasion were synergistic and persistent in that tree density was 64% lower in invaded subplots treated with fire at the end of the experiment. The pronounced initial impacts of the invasion followed by decline of the invasion and recovery of native species demonstrate that although the effects of plant invasions can be dramatic, they may be overwhelmed by ecological succession over the long term.