Shifts in dominance and species assemblages over two decades in alternative successions in central Amazonia

Following perturbation, different assemblages that originate under the same abiotic conditions initiate successional pathways that may continue to diverge or converge toward an eventual climax. Forest regeneration in the Central Amazon begins with alternative successional pathways associated with prior land use. In a 12-yr study of secondary forests, initially ranging between 2 and 19 yrs after abandonment, we compared species compositions through time along two pathways, abandoned clear-cuts dominated by Cecropia and abandoned pastures dominated by Vismia; prior results at these sites have not directly evaluated species composition. At all ages, the Chao-Jaccard similarity index of species composition was highest comparing pasture transects to each other, lowest comparing pastures transects to clear-cut transects, and intermediate comparing clear-cut transects to each other. Through time, clear-cut transects became less similar to each other, as did pasture transects. Changes in similarity reflected declining dominance along both pathways, but Cecropia dominance of clear-cut transects declined more rapidly than Vismia dominance of pasture transects. A rich association of species replaced Cecropia in clear-cut transects, resulting in decreased similarity among them. In pasture transects one genus, Bellucia, replaced the lost Vismia, so similarity of Vismia transects was maintained despite some turnover in dominance. Overall, even with turnover of individuals and decline of the dominant pioneers, the alternative pathways exhibited strikingly different species assemblies after two decades of succession, suggesting that the effect of land use persists well beyond initial floristic composition.