Mechanisms explaining the dominance of invasive exotic grasses in old-field grasslands
Exotic species invasions, one of global change drivers threatening biodiversity and ecosystem stability, is promoted by disturbances like agriculture. The successional reassembly of plant communities in old fields with cultivation legacy frequently leads to the establishment of persistent communities dominated by fast-growing, exotic plant species. Such systems show little recovery towards the historical vegetation state, and may represent alternative stable states (or “novel ecosystems”) recalcitrant to restoration. The objective of this study was to assess the mechanisms responsible for the persistence of introduced exotic species on abandoned agricultural fields in the Inland Pampa of Argentina. We sowed exotic and native grass species, alone and in mixtures, in a field manipulative experiment on remnant grasslands and old fields, under different nutrient availability. We assessed plant cover of sown and spontaneous species during two years, and plant biomass at the end of the experiment.
Plant cover was similar in both grasslands, but old fields were represented by sown species, whereas the remnant grasslands were dominated by spontaneous species. Among sown species, both exotic and native grasses established better in post agricultural grasslands than in native remnants (ANOVA Site effect: P=0.03; Species and site x species=NS) suggesting that the site effect is more important than the hypothesized feedback mechanism between plants and soil. In mixture assemblages, exotic species overcompete natives (ANOVA Site effect= NS; Assemblage; P=0.04: site x assemblage=NS) and in general, native species performed better alone than in mixtures, whereas exotic species did not show any difference. The competitive advantage of exotic species over natives was promoted by nitrogen fertilization, and was particularly evident in native remnants grasslands. The establishment of competitive exotic species and changes in soil conditions may explain the persistence of exotic grasses in post agricultural grasslands. Our results suggest that exotic dominated old fields and native grasslands may represent alternative stable states, which may have implications for the restoration of native grasslands after agriculture abandonment in the region.