A seeding experiment for testing tree-community recruitment under variable environments: Implications for forest regeneration and conservation in Mediterranean habitats

As a result of historical and ecological factors, woody-plant communities are composed of species with different biogeographic origins that presumably diverge in their ability to recruit under variable environments. The aim of this study is to evaluate the conservation implications of the differences in recruitment of six tree species of different biogeographical origin coexisting in a Mediterranean mountain forest. Under field conditions, we experimentally sowed a total of 16,650 seeds belonging to two Mediterranean (Quercus ilex and Q. pyrenaica), two Sub-Mediterranean (Acer opalus subsp. granatense and Sorbus aria), and two boreo-alpine (Pinus sylvestris and Taxus baccata) species. We selected the three most common successional scenarios where seeds are dispersed (woodland, shrubland, and open areas), and half of the seeds were irrigated simulating a rainy summer in terms of Mediterranean ecosystems. Seedling survival was strongly affected by the type of habitat as well as irrigation. The recruitment of the six species was possible only in wooded areas under wet conditions (20-60%). In shrubland, only Mediterranean and Sub-Mediterranean species recruited in typical dry years (10-50%), but irrigation increased their survival (25-65%) and allowed P. sylvestris to recruit (32%). Open areas constituted the most adverse scenario for seedling recruitment, especially for non-watered seedlings (0% for boreo-alpine species, 1-24% for the rest). Overall, the benefit of a wet summer was more critical for the recruitment of boreo-alpine species (P. sylvestris and T. baccata) than for the rest, although a rainy summer is also necessary for the recruitment of A. granatense and S. aria in shrubland habitats. Our experimental results indicate that the maintenance of current species composition in Mediterranean mountain forests is critically dependent on the periodic, scattered occurrence of wet summers. From a conservational standpoint, the predictions of less rainy summers and greater aridity in the future imply a change in the relative abundance of species in the mature forest, with Mediterranean species becoming dominant and boreo-alpine species becoming even rarer and, consequently, a serious threat for the persistence of boreo-alpine species at their southern distribution limit.