Development of floristic diversity in 10-year-old restoration forests on a bauxite mined site in Amazonia

Abstract

Patterns of plant and animal diversity were studied in a 10-year-old native species reforestation area at a bauxite-mined site at Porto Trombetas in western Pará State, Brazil. Understorey and overstorey floristic composition and structure, understorey light conditions, forest floor development and soil properties were evaluated in a total of 38 78.5-m 2 plots located in the reforestation area at varying distances up to 640 m from the boundary with the surrounding primary forest. Wildlife surveys focusing primarily on birds and bats were also conducted to assess the role of seed-dispersing animals in regeneration of woody forest species within the plantations and colonization by primary forest species not included in the original reforestation. Regeneration density, species richness and species diversity (Shannon–Wiener index) for woody perennial species, vines, herbs and grasses were strongly correlated with the diversity of planted tree species and structural development parameters, degree of forest floor development and soil pH. The better developed closed-canopy plots (>40% crown closure) were characterized by relatively well-developed litter (O1) and humus (O2) layers, more acidic soils being typical of the surrounding primary forests, and a more diverse herb, vine and woody perennial flora with a greater representation of primary forest species being characteristic of late secondary forests. In closed-canopy plots a total of 125 tree, palm and shrub species were censused (versus 34 in the more open-canopy plots), of which 75 species are known to have been introduced by natural means from the surrounding primary forest (versus 11 species in the open-canopy plots). Among the plantation plots, there was significant colonization by primary forest woody species up to 640 m away from the primary forest edge, although both the abundance and the diversity of colonizing species declined with increasing distance into the plantations. Smaller-seeded primary forest woody species dispersed by mammals and birds represented a higher proportion of the colonizing species compared with the larger-seeded species. These data are consistent with the results of the wildlife surveys, which indicated that most animal seed dispersal is provided by bats, that the most common frugivorous bat and bird species in the plantations feed on small-seeded plant species and that birds and mammals that typically disperse larger-seeded tree species (such as toucans, trogons, tapirs, deer and primates) are still rare in the reforestation area. These results suggest that while the reforestation program has been successful in creating a favorable environment for regeneration of a native primary forest species, further management interventions, such as enrichment plantings, may be required to accelerate regeneration of large-seeded primary forest species.