Monitoring the Diameter growth of irregular trunk trees in the Celtis forest in northern Republic of Congo

Abstract

Information on the growth of tropical trees is essential for the management of tropical forests. However, tree diameter measurements taken over irregularities result in negative diameter growth due to trunk shrinkage over time. The use of the close-range photogrammetric approach to harmonise diameter measurements of irregular trunk trees is likely to improve the diameter growth of these trees. This study uses close-range photogrammetric point cloud data and conventional measurements collected on 72 irregular trunk trees at Loundoungou to examine the diameter growth of irregular trunk trees in the Celtis forest in northern Republic of Congo. Significant differences were observed in the diameter above the irregularities and at 1.30 m from the ground between 2014 and 2021, suggesting the evolution of the trunk from 2014 to 2021. The relative change in diameter above the irregularities was 4 times greater than the diameter at 1.30 m above ground. Variations in trunk diameter growth were observed within each diameter type, with the rate of diameter growth above irregularities higher for the data set and for larger diameter trees. Diameter growth models using diameter at 1.30 m above ground were best (lowest AIC and BIC), suggesting that diameter at 1.30 m above ground is, therefore, the most appropriate predictor for irregular trunk trees. The results of this study highlighted the ability of the close-range photogrammetric approach to detect diameter growth at 1.30 m above ground, which is important for improving forest carbon balance estimates and decision-making in tropical forest management.