Abstract
The Amazon region harbors the most important tropical forest in the world. However, its biodiversity is seriously threatened due to land-use change. Here, we evaluated changes in tree species diversity and composition after shifting cultivation in the northern Amazon forest, Amazon state, Venezuela, through a chronosequence approach. We selected three sites over a 12 km(2) extension of an old-growth forest matrix with secondary forest patches of different stand ages. A total 45 plots (each having 20 x 50 m = 1000 m(2)) were established. At each site, woody plant diversity and the composition of trees having diameter 5 cm were assessed in four secondary forests (5, 10, 15, and 20 years old stands after shifting cultivation) and in one old-growth forest ( > 100 years old), and three plots were established in each forest type. Species richness and dissimilarity pairwise beta diversity metric were calculated for paired plots among different forest types. We analyzed differences in diversity among the four successional stages and the old-growth forest using individual-based approach. Additionally, multivariate analyses were performed to examine differences among the sampled forest areas in terms of species composition along soil gradient. Species richness showed consistently increasing pattern along the succession to old-growth forest. Species richness in the old-growth forest was up to three times higher than in forests at early successional stages. Richness recovery rate in the 20-years old secondary forest two decades after the abandonment of shifting cultivation was on average equal to 70% of the species richness in the old-growth forest. In contrast, the recovery of species composition reached an average 25% in relation to the old-growth forest during the same period. Our results show that the effect of stand age and environmental drivers (i.e., soil properties) determine species diversity along succession. The environmental heterogeneity between successional stages can be analyzed by the differences in floristic composition and beta diversity observed among the analyzed plots. For that reason, we presume that beta diversity is the major determinant of species richness in secondary forests. The proposed approach contributes to the sustainable management of forest communities because it allows estimating the woody species diversity recovering after shifting cultivation disturbance across successional stages.