Changes and determinants of the shape of irregular trunk trees over time in a semi-deciduous forest in Central Africa

A better understanding of the determinants behind changes in the shape of irregular trunk trees over time is needed to predict likely changes in carbon stocks and biodiversity when environmental conditions change.  To investigate this question, we studied the changes and determinants of irregular trunk shape in 6 tree species (72 trees) from a semi-deciduous forest in Central Africa. The change in trunk shape of each tree over time was assessed from the Deficit Basal Area Index (DeBA) using photogrammetric point cloud data and conventional measurements over a 7-year period. DeBA quantifies trunk irregularity, with values ranging from zero for a perfectly circular cross-section to one for highly irregular shapes.  By comparing DeBA values for 2014 and 2021, we have tracked the evolution of trunk shape. A reduction in DeBA indicates a trend towards a more irregular shape. For all species, significant differences were observed in trunk shape between 2014 and 2021, with a mean change of 0.55±0.16, suggesting that these trees are evolving towards a more irregular shape. Among the species, Triplochiton scleroxylon showed a lower mean variation, indicating an evolution towards a more irregular shape. Notably, trees developing highly irregular shapes over time were generally large canopy trees with expansive crowns. Species with very irregular shapes tend to be large trees with wide, deep crowns, low wood density, deciduous trees, trees that don't require light and are dispersed by the wind. These results provide insight into the life-cycle strategy of tropical tree species.