Biotic interaction strategies explain the invasive success of Nassella trichotoma

Abstract Understanding how plant species combine competition and resistance mechanisms under herbivory is central to explaining invasion dynamics in grazed ecosystems. This study aimed to assess the roles of competition and tolerance in the invasion success of Nassella trichotoma (serrated tussock) in the native grasslands of the Ventania mountain system (Buenos Aires, Argentina). Field and glasshouse experiments were conducted to evaluate (1) the competitive performance of N. trichotoma under different intensities and selectivities of defoliation, and (2) its tolerance to defoliation compared with the native, palatable grass Chascolytrum subaristatum (syn. Briza subaristata ). Apparent competition was assessed by selectively clipping palatable neighbouring vegetation to simulate grazing-driven release of N. trichotoma. Defoliation intensity and selectivity were manipulated by hand clipping to simulate grazing, while basal area was measured as a proxy for tillering dynamics.Results showed that N. trichotoma maintained biomass production comparable to the palatable species even under high defoliation intensity, indicating substantial tolerance. However, selective defoliation did not enhance its apparent competitive advantage, suggesting that grazing selectivity may influence invasion processes primarily during early recruitment stages rather than later life stages. The species’ high tiller density and fibrous tissue content likely contribute to its compensatory growth and resistance to herbivory.These findings demonstrate that N. trichotoma integrates avoidance and tolerance traits that promote persistence under grazing pressure. Effective management in the Ventania system must therefore move beyond simple adjustments in grazing intensity by integrating native-range ecological processes with insights from invaded systems. Aligning grazing management, disturbance regimes, and restoration actions with the species’ phenological constraints, particularly its early autumn recruitment window, can help limit population expansion and reduce the risk of long-term ecosystem transformation.