Periodicity of tree growth and carbon storage in the Yangambi Biosphere Reserve

Abstract

Congo Basin forests are crucial for sequestrating carbon from the atmosphere, notwithstanding that this key function is critically threatened. In the context of climate change, it is essential to better understand how and when carbon is fixed within the woody tissue. Carbon uptake by trees is typically a periodic process, even for tropical forests. This justifies studying both cambial activity as well as rhythms of leaf development over the seasons. Forests of Central Africa are subjected to a variety of rainfall and temperature regimes, with varying degrees of seasonality, impacting the floristic composition that makes up these forests. Nowadays, numerous studies on phenological rhythmicity are conducted in forests with clear seasonality, focusing mainly on deciduous species. However, leaf and cambial phenology of evergreen species from tropical climates with low seasonality, such in the Congo Basin are still little studied, preventing a good understanding of relationships between climate, tree growth and carbon sequestration processes. Here, I propose an original approach aiming at characterizing cambial and foliar phenologies of 6 abundant evergreen or brevi-deciduous tree species during the onset of the rainy season in Yangambi forest: Leplaeae thompsonii, Panda oleosa, Petersianthus macrocarpus, Scorodophloeus zenkeri, Synsepalum subcordatum and Trilepisium madagascariense. The objective was also to analyse the relation between climatic variables and anatomical features of the cambial zone during the transitional season between dry and short rainy seasons from February to May 2022. In order to meet these goals, the first objective was to determine the status of cambium for the tree species by using micro-coring technique and by studying its ring anatomy. The second objective was to characterise the leaf phenology of the trees monitored using binoculars and phenocam over the study period. The last objective was to link the cambial and leaf phenologies together and to relate them to the evolution of the weather parameters collected locally. On average for the 6 species between 5 and 8 cells of 5 to 8 µm each are observed within the 11 µm wide cambial zone. Cambial activity was observed for P. oleosa, P. macrocarpus, S. zenkeri and T. madagascariense with an average of 4, 6 and 5 cells in the enlargement, thickening and in lignification phases, respectively. In addition, high variability of cambial status but also of leaf phenology exists traducing by different and asynchronous leaf and cambial phenomena between and within the different species. The phenological trends observed at the scale of the monitoring plots seem to be comparable to those of the forest. Finally, strong correlations are observed for some individuals between time exposure and rainfall with P. oleosa and S. zenkeri, and with temperatures for T. madagascariense and P. macrocarpus. But these parameters are insufficient to predict cells production. To conclude, evidence was found that a detailed study of the cambial activity offers appealing perspectives for analysis of periodic tree growth and carbon sequestration. Obtained results are innovative in the field of research associated with cambial analysis and phenology in general, providing relevant elements about cambial status for the study period. It emphasises the importance of combining cambial, phenological and climatic parameters together.