Functional attributes of exotic-dominated old fields and native remnant grasslands in the Inland Pampa, Argentina

Background/Question/Methods

The successional reassembly of plant communities in old fields with a strong legacy of cultivation 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 characterize the ecosystem functioning of exotic-dominated grasslands established on abandoned agricultural fields, and to compare them with native grassland remnants, in the Inland Pampa of eastern Argentina. In this way, we aimed to assess potential shifts in key ecosystem attributes that might facilitate the persistence of a degraded vegetation state dominated by introduced exotic species. During one year, we measured plant composition and biomass, aboveground net primary productivity, litter decomposition, soil respiration, and several chemical parameters in the upper soil layer, in eight remnants, native grassland fragments and in eight old-field grassland plots.

Results/Conclusions

Native remnant grasslands were dominated by C4 perennial tussock grasses, whereas old-field grasslands comprised a mix of C3 and C4 perennial exotic grasses. Total soil carbon and nitrogen pools, and the cationic exchange capacity of the upper soil layer were higher in old fields than in grassland remnants (P<0.05). Aboveground net primary productivity was 1089 ± 217 and 683 ± 105 g m^-2 year^{-1 in the native remnants and old fields, respectively (P=0.10). In contrast, litter decomposition was faster in old fields than in native remnants (P<0.05). This resulted in remnant grasslands having greater standing (live and dead) biomass and litter accumulation (P<0.05) than old-field grasslands. In addition, soil respiration was significantly higher (P<0.05) in old fields than in native grasslands, suggesting increased soil biological activity with exotic plant dominance. Our results suggest that exotic species dominance may alter ecosystem functioning at the plant-soil interface in ways that might reinforce the persistence of fast-growing, competitive exotic species in novel communities arising during post-agricultural succession.}