A comparison of 12 tree species of Amazonian caatinga using growth rates in gaps and understorey, and allometric relationships

1, The relative growth rate of saplings of 12 species from an oligotrophic lowland rain forest were measured in treefall gaps and understorey. Mean relative height growth (R-H) within treefall gaps was found to be slowest for tall-tree species with branched saplings, intermediate for subcanopy trees and fastest for tall-tree species with unbranched saplings. Most species had similar R-H within the understorey. R-H values were not related to leaf mass per unit area (LMA) or foliar N concentrations. 2, Allometric relationships between the total leaf area (TLA) and height were dependent upon light conditions; in general saplings of a given height had a greater TLA in treefall gaps than in understorey. The species with the largest estimated TLA values in gaps tended to have the greatest R-H values in gaps; no such trend emerged in the understorey. The values of the allometric coefficients were not related to foliar properties. 3. The relationship between stem diameter and height was only weakly dependent on light conditions and the relationship between the growth rates in these dimensions was also weak. The lack of plasticity may reflect the fact that the height-diameter relationship has little bearing on a sapling's tolerance of shade. 4. One way of accommodating the dependence of allometry upon irradiance is to add R-H as a covariate. We derive a relationship between growth rates from this resource-dependent allometric equation and show that it reasonably describes measurements taken in the caatinga forest.