Woody understory plant diversity in pure and mixed native tree plantations at La Selva Biological Station, Costa Rica

Tree plantations can be an important tool for restoration of abandoned pasturelands in the tropics. Plantations can help speed up secondary forest succession by improving soil conditions, attracting seed-dispersal agents, and providing shade necessary for understory growth. In this study, abundance and richness of understory regeneration was measured in three native tree plantations 15-16 years of age at La Selva Biological Station in the Costa Rican Caribbean lowlands. Each plantation contained tree species in pure plots, a mixture of the species, and natural regeneration plots (no trees planted). The greatest abundance of regeneration was found in the understory of pure plots of Jacaranda copaia (Aubl.) D.Don., Vochysia guatemalensis Donn.Sm., Dipteryx panamensis Benth, Vochysia ferruginea Mart., and in two mixed stands, while the lowest was found in the natural regeneration treatments with about half the values as in the plantation stands. There was a significant negative correlation between percent canopy openness and abundance of regeneration in the understory. Two distinctive clusters separated the regeneration treatments from the mixed and pure plantations at a very low Bray-Curtis similarity value. The natural regeneration treatments are separated from mixed and pure plantations in the two-dimensional ordination. The lack of difference between the understory make-up of pure and mixed plantations in abundance, species richness, and seed-dispersal syndromes of understory species suggests that planting mixed stands is not necessarily superior to planting pure stands for promoting understory diversity of woody species. While regeneration of woody species can be faster under pure- or mixed-species plantations than in open pastures, the abundance, richness and species composition depends on each plantation species, or species assemblages in case of the mixtures. (C) 2008 Elsevier B.V. All rights reserved.