Effects of a non‐native grass invasion decline over time

Abstract

Summary Most research on dynamics and impacts of plant invasions has evaluated patterns and effects over brief time periods (i.e. <4 years). As such, little is known about the persistence of invasions and their long‐term impacts on native species. To experimentally evaluate longer‐term effects of invasions, we established field plots with native tree and herbaceous species and then invaded half of the plots with the most widespread invasive grass in the eastern United States (Microstegium vimineum). Over 8 years, we quantified invader and native plant biomass, native plant diversity, and tree density and size. Microstegium was dominant during the first 4 years of the experiment, constituting more than 60% of herbaceous plant biomass in the plots, and native herbaceous biomass was 57% lower and diversity was 44% lower in invaded vs. control plots. However, both Microstegium and herbaceous native species declined in later years. By the end of the experiment, Microstegium was only 2% of community biomass, and there was no difference in native herbaceous biomass in invaded and control plots. We applied a fire treatment in years 6 and 7 to test if repeated disturbance is required to maintain invader dominance and to evaluate how this common management practice in eastern US forests affected invasive and native plants. Tree density was 65% lower and tree diameters were 28–51% smaller on average in fire‐treated compared with control quadrants, resulting in significantly greater light availability in fire‐treated areas. Consequently, Microstegium and native herbaceous species biomass increased significantly where fire was applied. However, only native species were persistent, and after 8 years, Microstegium was nearly absent, regardless of the fire treatment. Synthesis. The invasive grass was initially abundant and suppressed native species, but invader decline over time corresponded with succession to native herbaceous species dominance when fire was applied, and to native tree dominance without fire such that after 8 years, the initial effects of invasion were no longer apparent. Thus, our data provide some of the first experimental evidence that while the initial effects of plant invasions can be dramatic, invaders and their impacts may decline over time.