Plant Biodiversity Homogenization across the Chronosequence in Highly Fragmented Landscapes in the Colombian Andean–Amazonian Transition

Abstract

Succession in abandoned pastures in the tropics can progress along different pathways, and the changes in biodiversity on local and landscape scales, as well as in species turnover (β diversity), are still unclear. This study assessed the role of secondary forests as a plant biodiversity reservoir or as evidence of a pattern of biodiversity homogenization in a chronosequence of abandoned pastures in two highly fragmented landscapes (hills and mountains) in the Colombian Amazon. In each landscape, the plant community composition, growth habits, species richness accumulation, indicator species identification, composition dissimilarity, and influence of above- and below-ground environmental filters were evaluated in four successional stages: (i) degraded pastures (DP) (<3 years old), (ii) early forests (EF) (10–20 years old), (iii) intermediate forests (IF) (25–40 years old), and (iv) old-growth forests (OF) (>90 years old). A total of 918 species, 78 families, and 11,284 individuals were recorded. The most representative growth habits were trees and shrubs, while palms and lianas were minimal. The species accumulated rapidly in DP and EF, contrary to that observed in IF and OF; thus, DP and EF achieve inventory completeness faster than IF and OF. It was necessary to include more plots to obtain inventory completeness in IF and OF. OF had a high α diversity with similar species composition throughout (low β diversity) and high homogeneity, suggesting biotic homogenization. On the other hand, IF presented the highest species differentiation (high β diversity) and a higher divergence in species composition than OF. The spatial distance and environmental dissimilarity had the most important role in determining species composition. Finally, our results show divergence in the plant composition across the chronosequence, where DP was opposite from OF in hills. However, in mountains, DP followed the EF and IF categories. The deviation in the species composition in EF and IF suggests an exchange of species in intermediate forest ages.