Intensive rotational grazing in pastures reduces the early establishment of an invasive tree species

Grazing is one of the principal drivers triggering woody invasion in grasslands and savannas worldwide, but evidence is opposing. Cattle can promote or limit woody establishment by direct and indirect mechanisms, like consumption, trampling or relaxing grass competition. Despite its increasing relevance, it is unknown if intensively managed cattle grazing can curb woody establishment. We evaluated the effect of cattle grazing with short cycles of high stocking rate and resting periods on Gleditsia triacanthos establishment, an invasive tree in grasslands. By two field experiments in pastures, we manipulated cattle grazing (with or without) and: Exp. (1) initial age-related size of G. triacanthos (small plants: 3 mo.; or large plants: 9 mo.), and Exp. (2) neighbouring vegetation (clipped or intact). Overall, grazing had a negative effect due to tree consumption and a positive effect by relaxing grass competition on tree establishment, but the first effect outweighed the second. Grazing reduced by 25% G. triacanthos survival and by 40% its biomass. Consumption was independent of plant size and neighbouring vegetation, but large plants had more resprouting capacity than small plants. Regardless of grazing, small plants had higher mortality than large plants. Intact neighbouring vegetation did not modify survival but decreased tree biomass which may affect middle or long-term tolerance and survival. Intensively managed rotational grazing with resting periods for pasture recovery may limit woody establishment, but this management strategy should be applied immediately after tree germination. The increasing intensification of production systems requires to evaluate the impact of management on invasion processes.

Automated summary

Grazing can both promote and limit woody establishment, with evidence showing both positive and negative effects. However, the negative effect of consumption outweighs the positive effect of relaxing grass competition in the case of Gleditsia triacanthos establishment. Grazing leads to a 25% reduction in survival and a 40% reduction in biomass of G. triacanthos.