Abstract :
[en] The Congo Basin forests, as a critical global carbon sink, face increasing threats from climate change, with temperature projections indicating a significant rise above pre-industrial levels by 2027, alongside increased variation in rainfall. Despite their significance, limited studies have focused on understanding the mechanisms of tree growth and the species-specific responses to changing climate conditions.
This thesis investigates the dynamics of secondary growth in two semi-deciduous tropical forests of the Congo Basin: the Luki and Yangambi biosphere reserves. Through three interlinked studies presented as Chapters 2, 3, and 4, the research aims to elucidate the mechanisms underlying secondary growth in this ecologically vital and unique region. To achieve this goal, the study addresses several interrelated research questions: Is there a dormant period in the seasonal tree-growth cycle? If so, what are the timing and duration of dormancy and the growing season? How do climatic factors influence the growth cycle? What anatomical marks does the growth cycle leave on the wood? These questions, spanning multiple chapters, provide a comprehensive understanding of secondary growth dynamics in the Congo Basin.
This thesis comprises five chapters. It begins with an introductory chapter (Chapter 1), which provides a general overview of the Congo Basin and tree growth, with a particular focus on the uncertainties surrounding the seasonality of secondary growth in this region.
Chapter 2 examines diel and seasonal stem growth patterns in 17 trees of 11 dominant canopy and understory species in the Mayombe forest. Highly-resolved measurements of radial stem size variations and associated weather conditions provided insights into secondary growth dynamics. Findings indicate that tree growth primarily occurs from 6 PM to 9 AM, and peaks during the rainy season (October to May). Growth cessation, observed for 1-4 months in some species, typically coincides with the dry season (June to September). A linear mixed-effects model demonstrated a positive relationship between annual radial stem growth and rainfall. These findings suggest that many tree species, including Terminalia superba, possess significant resilience to projected increases in temperature and rainfall, though species-specific growth responses vary. It is important to emphasize, however, that when using dendrometers, there remains a possibility of occasionally mistaking swelling caused by turgor pressure for irreversible growth. This highlights the need for more precise methods, such as cambial marking and micro-coring.
Chapter 3 focuses on the formation, distinctness, and periodicity of growth rings in 18 common tree species from two contrasting semi-deciduous rainforests. The presence of growth rings was confirmed across all species using the cambial marking technique, followed by micro- and macroscopic observations. Trees from the site with a well-defined dry season exhibited a higher likelihood of forming periodic growth rings compared to those from a site with less distinct rainfall seasonality. However, the distinctness of growth rings did not show significant variation across the different sites. Trees that exhibited faster growth rates were more likely to form periodic growth rings, potentially due to their greater sensitivity to seasonal environmental fluctuations, which influence the cessation and resumption of cambial activity. This study highlights the importance of understanding cambial dynamics through a more precise method such as microcore sampling and detailed phenological analysis to capture lateral meristem activity.
Chapter 4 investigates cambial phenology in relation to climatic variables, tree morphological traits, leaf phenology, and reproductive behaviors across two semi-deciduous rainforests with distinct precipitation patterns. Biweekly microcores from 30 trees (10 species) and dendrometer data from 59 trees (20 species) were analyzed to characterize cambial activity and dormancy. Results indicate complex cambial dynamics, with species-specific cycles of activity and dormancy or continuous growth. Trees in the more climate-sensitive Luki site, with a pronounced dry season, may face increased vulnerability due to climate variability. Conversely, trees from the less-seasonal Yangambi site demonstrated resilience to climatic fluctuations but remained sensitive to internal growth dynamics, particularly morphological and reproductive traits.
Chapter 5 offers a comprehensive examination of secondary growth in tropical tree species of the Congo Basin, discussing results from Chapter 2, Chapter 3, and Chapter 4. By synthesizing these distinct but interrelated dimensions, we achieve a clearer understanding of how environmental variables, tree physiology, and growth patterns converge to shape the dynamics of secondary growth in tropical rainforests.
Collectively, this thesis advances our understanding of tree growth and its drivers in the Congo Basin, emphasizing the diversity and resilience of tropical tree species to anticipated climate change. However, further research is needed to elucidate the underlying mechanisms driving cambial dynamics and their long-term implications for forest carbon sequestration and ecosystem functioning in this critical region.
