Birth and life of tree aggregates in tropical forest: hypotheses on population dynamics of an aggregated shade-tolerant species




Several studies have described aggregated spatial patterns in tropical tree species. This study investigates aggregate dynamics of Vouacapoua americana (Aublet), a climax species whose spatial pattern is not simply related to light and soil conditions or to its short seed dispersal range.


Two rain forest sites: Nouragues and Paracou, in the Guiana Shield.


We described the spatial pattern of tree locations and spatial autocorrelation of tree diameters, using statistics derived from Ripley's K. We particularly used methods to define analysis subplots according to local density or local mean diameter. We investigated relationships between spatial distributions of adults and saplings.


At both sites, populations of Vouacapoua demonstrated several nested levels of aggregation. Tree diameters were spatially autocorrelated, revealing the existence of clusters with similar diameters. In the largest aggregates, tree diameters declined from the centre to the edge. Regeneration was aggregated and occurred mainly at cluster edges and around rare isolated trees, and sapling densities and basal area of adults were negatively correlated. We show that long-distance dispersal events are rare.


Environmental factors and seed dispersal only explain part of the observed spatial patterns. We provide two main hypotheses about Vouacapoua population dynamics. First, the lack of regeneration in aggregate centres results in the ageing of existing aggregates. We suggest that this lack of recruitment close to mature trees is due to a Janzen–Connell effect. However, aggregates can continue to grow along colonization fronts. Second, long-distance dispersal events allow the formation of new clusters and play a crucial role in the colonization process.