Little trace of floristic homogenization in peri-urban Andean secondary forests despite high anthropogenic transformation

Pervasive human impact in heavily transformed landscapes may lead disturbance-adapted species to thrive, resulting in floristic homogenization across forest stands. However, environmental heterogeneity and dispersal limitation may be antagonistic forces to homogenization, maintaining inherent floristic differentiation across sites. We evaluated the extent to which peri-urban Andean forests are undergoing floristic homogenization in both late- and early-successional stands. We considered seedling assemblages as well, as they provide key insights into forests’ future. We then quantified the relative importance of dispersal limitation and environmental filtering in determining the observed patterns of floristic similarity across the landscape. We used tree, seedling, soil and climatic data from six sites located in the high plain where Bogotá lies (Colombia). Within each site, we established six 20 × 20 m plots, three in early-successional stands and three in late-successional stands, for a total of 36 plots. To evaluate the extent of floristic homogenization, we defined tree floristic similarity among late-successional stands as a baseline, reflecting our best-guess of the original species composition that would have once occurred previous to intense anthropogenic intervention. Tree floristic turnover across the landscape was alike in late- and in early-secondary stands, a finding that does not support the homogenization scenario. Seedling species composition, in contrast, was more homogeneous among early- than among late-secondary stands, an outcome suggestive of homogenization. The relative importance of spatial and environmental drivers shifted between life stages. Distance between plots was the best predictor of tree species dissimilarity (29% of variance explained), whereas seedling compositional variation was more sensitive to changes in environmental conditions (41% of the variance explained). Relative humidity and several variables related to soil fertility showed a significant effect on floristic dissimilarity across the landscape, although significant factors were often different between life stages. Synthesis. Despite high anthropogenic transformation, we found little trace of floristic homogenization in these peri-urban landscapes. Inherent floristic differentiation, promoted both by dispersal limitation and environmental heterogeneity, suggests that all patches are important to the conservation in these Andean forests, critical centres of endemism.