Reduced herbivore pressure under rainy ENSO conditions could facilitate dryland reforestation

Most semi-arid and arid regions around the world have lost a major part of their original vegetation. Restoration of once shrublands and woodlands is often challenged by low seedling establishment success due to water stress and herbivory. In some regions, increased rainfall during El Niño Southern Oscillation (ENSO) events can significantly stimulate plant recruitment. However, recruitment seems to be strongly modulated by herbivore pressure. Also, seedling establishment in arid ecosystems can be facilitated by adult trees and shrubs whose canopies contribute to improve seedling water relations. We performed a field experiment to test the role of small mammalian herbivores and shade availability during simulated rainy ENSO conditions on the growth and survival of Prosopis chilensis saplings in semi-arid Chile. When mammalian herbivores were absent, sapling survival increased from 25% under simulated strong ENSO rain to 60% under very strong rainy events. This positive effect of increased water availability was over-ruled by strong herbivory of mainly introduced European rabbits (Oryctolagus cuniculus) and hares (Lepus europaeus). Most saplings died when herbivores were present. Shade availability did not significantly improve plant growth and survival. Our results support the hypothesis that rainy years such as those associated to ENSO events could improve the possibilities for successful tree reforestation in semi-arid ecosystems when combined with a reduction in herbivore pressure.