Woody species recruitment under monospecific plantations of pioneer trees - facilitation or inhibition?

Abstract

The successional model of forest restoration is based on the facilitation process, in which the establishment of pioneer tree species favors the late successional tree species. We tested the hypothesis of facilitation through a case study, comparing tree species diversity in the understory of two Neotropical native tree species plantations, Trema micrantha (L.) Blüme (Cannabaceae) and Schinus molle L. (Anacardiaceae). Results obtained under the plantations were compared with an adjacent area under spontaneous secondary succession, analyzing the ligneous plant diversity, soil physic-chemistry and shading. Additionally, the seed rain under the two plantations was analyzed. The area of spontaneous secondary succession and the area with T. micrantha had the highest ligneous plant diversity. The understory of T. micrantha plantation had the lowest light intensity and higher late successional species abundance. In addition, T. micrantha had higher plant diversity than S. molle, which could be explained at least partially by higher diversity in the seed rain. Higher litterfall and mineral content of leaves and twigs of T. micrantha did not coincide with higher topsoil mineral content under that species. Instead, soil under S. molle had higher level of P than the soil under T. micrantha. Data suggest that a high diversity of ligneous plants could be thrived by the spontaneous succession. On the other hand, T. micrantha had promoted a higher richness and abundance of late successional species, which could be related to a more pronounced shade effect, in agreement with the facilitation conception. The lowest diversity and density of ligneous plant species under S. molle characterizes an inhibition effect. T. micrantha could be included in restoration plans assembled with other species or combined with areas without intervention, whenever the spontaneous regeneration is possible.