With the abandonment of heavy agriculture in New England, reforestation threatens the unique habitat grasslands provide. Strategies for decreasing woody incursion remove biomass and may create a battleground for native and nonnative species. The presence of nonnative nitrogen fixers in these grasslands may facilitate nonnative plants, which have strong resource capture, at the expense of native plants. This study explores this hypothesis, observationally and experimentally, with Trifolium species. At Harvard Farm, within a long-term study to assess the effects of cow grazing and mowing on plant communities, we surveyed plant species richness and cover in paired 1x1m plots with and without Trifolium. In the greenhouse, we explored the effects of soil type and plant litter by growing Rumex acetosella, an invasive plant, and Oxalis stricta, a native wildflower, in three soil types (potting, Rumex, or non-Rumex-associated field soil) with four litter treatments (no litter, Trifolium, Rumex, or general leaf litter). Greenhouse data were then analyzed according to overall biomass difference from the initial to final mass and root:shoot ratios.
Results/Conclusions
Analysis of species richness in the paired plots yielded higher Shannon-Wiener diversity in Trifolium plots, but there were no significant effects on species richness except the addition of Trifolium itself. Percent cover of nonnatives decreased in the presence of Trifolium, but Trifolium appeared to have no significant effect on native species cover or richness. From the greenhouse results, analysis of biomass differences showed growth of both species was highest in potting soil and lowest in Rumex soil. As for the effects of litter, growth was highest with no litter and lowest with Rumex litter. Trifolium litter had a positive effect on Rumex and a negative effect on Oxalis when compared to leaf litter. Strong resource capture in invasive species is supported with root:shoot ratio results. The root:shoot ratio was significantly lower in potting soil than in field and Rumex soil for R. acetosella, which is consistent with the idea of plasticity of above-ground or below-ground biomass allocation, dependent on the availability of nutrients. Overall, unlike the field results, the greenhouse results suggest Trifolium may facilitate invasive species with strong resource capture with resulting negative effects on natives in the presence of invasive species.