Disentangling the effect of size and competition: a growth model for Aucoumea klaineana

Context Okoume (Aucoumea klaineana) is the most important timber species in Gabon, while being classified as vulnerable in the IUCN red list. Thousands of growth measurements for this species have been performed since the 1950s but, because of a lack of integrated analysis, did not bring a consistent view on its growth pattern. Aims This study aims at disentangling the effects of tree size and local competition on tree growth. Methods A growth model was fitted for okoume, using data from seven sites in Gabon and Congo. The growth model was designed to be useful for forest management, which means that it relied on variables that could be computed using forest inventory data. Results A lognormal growth model with a negative response to stand basal area was selected. A significant residual site effect on growth was found, with a slower growth in the sites near the border of the natural range of okoume than in the sites far from it. Conclusion Growth strongly responded to local stand attributes such as tree density and basal area. Growth decrease with stand age in monodominant okoume stands was correctly predicted, although tree age was not incorporated into the model.