Biotic resistance in a stochastic world: Do rodents act as a filter to alien tree invasion in pampean old fields?

Biotic resistance from post-dispersal seed predators has been shown to influence invasion and spread of woody plants in grasslands and savannas. Yet, the role of rodent granivores as a barrier to woody plant invasion remains poorly studied. We examined the impact of rodent exclusion and canopy disturbance on two exotic tree seedling establishment (Gleditsia triacanthos and Robinia pseudoacacia) in an old-field grassland of the Inland Pampa, Argentina. The experiment was conducted over 4 years with substantial climatic variation, which affected both rodent abundance and seedling performance. Our results showed that, for both tree species, rainfall was the main factor affecting both seedling emergence and survival, while the effect of rodent seed predation and disturbance depended on variability in natural rodent density. The response of G. triacanthos seedling emergence after the onset of favorable water conditions was more rapid than the recovery of rodent populations. This delay provided an opportunity to escape from potential seed predation, reducing its importance as a biotic filter to woody invasion. Robinia pseudoacacia seedlings emerged during the first growing period and died because of hydric stress, while remnant seeds did not remain viable in the seed bank after 2 years of sowing. From our results, we suggest that the dynamic of tree invasion depends highly on climatic variability, which has direct effects on seed germination and survival, as well as effects mediated by rodent abundance and herbaceous competition.