The effects of disturbance and community diversity on the success of an exotic forb in a Virginia grassland

Background/Question/Methods

Community biodiversity and disturbances have been suggested to contribute to species invasions. In many systems the availability of excess nutrients allows exotic species to establish and thrive. The tall-grass prairie ecosystem is dominated by warm-season grasses, which have high nutrient-use efficiency. We studied one such restored meadow in the Shenandoah Valley of Virginia that has been invaded by Galium verum, an exotic herbaceous plant with lower nutrient-use efficiency. We hypothesized that physiological differences would allow G. verum to dominate at high nutrient levels, while native grasses would thrive at lower concentrations. This meadow comprises replicate 0.6 hectare assemblages of one, three or six species of native warm-season grasses. Within each species assemblage we performed four nutrient manipulations: 1) depleted nitrogen (excess carbon introduction), 2) ambient nitrogen, 3) elevated nitrogen (fertilizer addition) and 4) fire (early-season burn). We analyzed plant-available soil nitrogen (NO₃^- or NH₄⁺), total biomass of grasses and G. verum, and tissue carbon and nitrogen content.

Results/Conclusions

We found no effect of native grass diversity on the abundance of exotic G. verum, nor did grass diversity significantly alter soil NO₃^- or NH₄⁺. Warm-season grass biomass and total biomass (grass plus all forbs) did not vary significantly among plots differing in warm-season grass diversity. Elevated nitrogen treatments produced significantly higher G. verum biomass and stem density than depleted or ambient nitrogen treatments. The fire treatment significantly reduced G. verum biomass relative to all other treatments. Nutrient treatments did not significantly alter warm-season grass biomass. As predicted, soil plant-available nitrogen drove the pattern of G. verum abundance, but early season fire inhibited G. verum despite the nutrient release afterwards. These results suggest that tallgrass prairies may become more susceptible to invasion by herbaceous species with increased nitrogen deposition. This one season study suggests that fire may retard G. verum invasion, but a longer term study is needed to evaluate whether this negative effect is short lived.