Effects of vegetation and herbivores on regeneration of two tree species in a seasonally dry forest

Tree establishment in harsh environments such as seasonally dry forests has traditionally been described as facilitated by existing shrubs and trees, which ameliorate harsh abiotic conditions; however, an alternative explanation postulates that facilitation is mediated by reduced herbivore damage, especially under shrubs. We established seeding and planting trials using two tree species (Ruprechtia apetala and Schinopsis lorentzii) in a full factorial design that included three vegetation types and two herbivore treatments at three sites (234 plots per species). Six months after seeding, seedling counts represented 0.95 and 0.47% of the sown seeds and survival of planted saplings was 81% and 46% for R. apetala and S. lorentzii, respectively. Average growth was negative due to over winter diebacks for both species. The performance of our target species was influenced by vegetation in a pattern which would suggest either mixed abiotic and herbivore-mediated facilitation or entirely herbivore-mediated facilitation. By contrast, we did not find resulting patterns suggesting a strict abiotic-mediated facilitation by shrubs and trees. We conclude that in dry mountain forests, remnant woody vegetation patches are important for the effective establishment of our study species in grazed sites, whereas effective establishment is feasible outside woody patches, in ungrazed sites.