Experimental exotic grass invasion impacts native trees through direct and indirect mechanisms

Background/Question/Methods

Invasions of exotic species are commonly cited as causing substantial ecological harm, but much of the discussion regarding the effects of invasions has been based on anecdotal information or correlative and observational studies. Further, while studies often document potential direct effects of invasions (e.g., altered resource availability), few studies have examined their possible indirect effects (e.g., changes in consumer behavior). Here we document the impacts of the invasive annual grass Microstegium vimineum (Japanese stiltgrass) on native trees using experimental invasions over multiple growing seasons. Microstegium was introduced to North America in the early 20th century but has only recently been recognized as a widespread invasive. It primarily invades eastern deciduous forests along roads, streams, and trails, and in forest openings and bottomland hardwood areas in more than 20 states. To experimentally test for impacts of Microstegium invasions on native tree regeneration, we created replicated early-successional forest communities containing nine species of tree seeds or seedlings plus twelve herbaceous species. We randomly selected half of the plots to be seeded with Microstegium. We monitored the survival and growth of trees over two years and quantified the damage to trees by voles (Microtus spp.). 
Results/Conclusions

Tree seedling survival was reduced 27% by Microstegium, but the size of the effect differed among tree species. Tree sapling survival was not affected by the invasion but saplings were taller and smaller in diameter in invaded plots. Unexpectedly, trees in Microstegium-invaded plots received nearly six times more vole damage than in uninvaded plots. However, differences in the growth and survival of trees were not entirely explained by vole activity, suggesting that Microstegium invasions result in both direct and indirect effects on native trees. These results indicate that Microstegium invasions may cause changes in performance among tree species and inhibit forest succession, due in part to the altered behavior of native consumers.