Multiscale assessment of woody species recruitment in Mediterranean shrublands: facilitation and beyond

Aim Forest recovery in Mediterranean environments is influenced by factors such as aridity, herbivory and facilitation by shrubs, as well as by seed limitation in the case of highly fragmented forests. How these various factors interact can determine the direction of secondary succession, yet these interactions are poorly understood. We assessed the relative importance of several factors in forest species recruitment in Retama sphaerocarpa (L.) Boiss (Retama) shrublands at different spatial scales. Location Centre of the Iberian Peninsula. Methods We surveyed mid- and late successional woody species common in Holm oak forests in 29 Retama shrublands that are distributed along an environmental gradient of increasing aridity and herbivory (regional scale) and are located at various distances from forest patches of different sizes (landscape and local scale). In each Retama shrubland, we analysed the effects of microhabitat (under Retama canopy vs open gaps), aridity, presence of nurse shrubs, herbivory, based on the density of pellet droppings, and propagule pressure, measured as the ratio between the size of forest patches acting as seed sources and their distance to Retama shrubs. Results Quercus ilex, Asparagus acutifolius and Juniperus oxycedrus were the mid- and late successional woody species most recruited in Retama shrublands. Their frequency, which increased with precipitation, was higher under Retama canopy than in gaps. Differences in Q.ilex recruitment between Retama canopy or gaps increased with rainfall, suggesting a decrease in Retama facilitation effectiveness with increasing aridity. Frequency of recruited oaks increased with the total area of woodland remnants located at < 0.5km, yet propagule pressure did not modulate the positive effect of Retama and rainfall on recruitment. Forest patches at distances > 0.5km did not contribute to recruitment. Conclusions Presence of the shrub R.sphaerocarpa and dispersal-related processes at local scale are main determinants of the colonization of shrublands by late successional woody species. Preserving Retama shrublands and Q.ilex woodland remnants is therefore crucial for extensive passive restoration of Mediterranean oak forests. However, facilitation by Retama is much weaker when both aridity and herbivory are high, regardless of seed source availability.