Pinus halepensis invasion in mountain pampean grassland: effects of feral horses grazing on seedling establishment.

We evaluated the mechanisms that determine the increasing abundance of Pinus halepensis in mountain pampean grasslands in Argentina that is associated with the continuous presence of feral horses. We hypothesized that direct and indirect effects of feral horse grazing on grassland may affect the establishment of P. halepensis. We conducted a field experiment to evaluate this hypothesis, studying the response of seedling emergence, survival and growth to herbaceous vegetation defoliation and soil disturbance in sites with contrasting grazing histories. We also evaluated the composition and structure of plant communities of each experimental site. Direct effects such as heavily defoliated perennial grasses and indirect effects such as reduced specific and functional richness, diversity and evenness enhanced the emergence, survival and early growth of the seedlings. High bare ground percentage and low grass biomass also positively affected seedling establishment. The experimental evidence demonstrates that the combination of invasional meltdown hypothesis and weakened biotic resistance are the mechanisms involved in P. halepensis invasion.