Abstract :
[en] Background – Understanding plant responses to environmental change is essential for both tropical forest conservation and agricultural resilience. In the Congo Basin, climate change, through rising temperatures, altered rainfall, and increased vapor pressure deficits, may pose growing challenges for understory species whose physiological limits are poorly documented. Coffea canephora, native to Central African forests and a key species for global coffee production, is increasingly exposed to these pressures. Despite its ecological and genetic importance for Robusta coffee breeding, its long-term functional responses and adaptive capacity remain poorly understood. This thesis addresses these gaps by integrating research on historical herbarium specimens, recent field data, and altitudinal gradient experiments to assess variation in functional and physiological traits of C. canephora across time, space, and environmental conditions.
Methods – The study followed an integrated research approach. First, long-term phenotypic changes were quantified by comparing herbarium specimens collected before 1960 with those collected recently (2019-2022) in the Yangambi Biosphere Reserve. Leaf functional traits were measured using standardized protocols. Second, leaf functional traits, carbon (δ13C) and oxygen (δ18O) isotopes, and intrinsic water use efficiency (iWUE) of C. canephora were analyzed using herbarium specimens (old and recent) collected throughout the Democratic Republic of Congo (DRC) from 1900 to 2022. Third, multi-location trial was set up along an altitudinal gradient with three genetically distinct varieties of C. canephora. The plants were grown at three altitudes (450 to 1,700 m a.s.l.) representing contrasting environments. Growth and morphological traits were monitored for one year. Leaf functional traits were measured at the end of the trial.
Results – Comparisons between historical and recent samples from Yangambi revealed significant long-term shifts in both leaf morphology and stomatal traits. Recent specimens exhibited larger leaves and higher specific leaf area (SLA), indicating lower leaf construction costs consistent with adaptation to modern climatic conditions characterized by elevated temperatures and potentially higher atmospheric evaporative demand. Stomatal pore length decreased in recent samples, suggesting a compensatory adjustment in gas-exchange regulation under warmer and drier conditions. These patterns were robust across understory species, including C. canephora, and indicate that understory plants have already undergone phenotypic adjustments over the last century.
The more extensive analysis of old and recent specimens of Coffea canephora confirmed substantial spatial and temporal variation in functional traits and iWUE of C. canephora. Stomatal density, guard cell dimensions, and maximum diffusive conductance differed significantly across regions, reflecting diverse local climates. Foliar δ13C and intrinsic water-use efficiency also varied widely, suggesting large differences in long-term carbon acquisition and water-use strategies among populations. Trait correlations revealed consistent trade-offs between carbon assimilation and water conservation, highlighting coordinated ecological responses to environmental gradients. These findings demonstrate that C. canephora populations exhibit marked functional differentiation shaped by climatic and geographic variation.
In the multi-location trial, altitude exerted a strong influence on both morphological and physiological traits. Final plant height, number of leaves, and distance between nodes decreased with increasing altitude, reflecting constraints imposed by cooler temperatures on elongation growth. In contrast, plant stem diameter, leaf area, and aboveground biomass (AGB) were higher at the high-altitude site (1,700 m), indicating a shift toward thicker stems and larger leaves under high-altitude conditions. Relative growth rate in leaf area (RGRA) increased with altitude, demonstrating enhanced carbon gain efficiency in cooler environments. Stomatal density declined at higher elevations, while pore length increased, suggesting compensatory regulatory changes in gas exchange. Substantial phenotypic variation among genetic varieties confirmed the presence of meaningful intra-specific diversity that can support future breeding efforts.
Conclusion –This thesis demonstrates that C. canephora exhibits strong phenotypic responses to environmental variation and that considerable phenotypic changes have occurred over the past century. Historical herbarium analyses reveal significant trait shifts consistent with a response to changing climate conditions throughout the twentieth century. Contemporary spatial analyses highlight extensive functional differentiation across the species’ native range, reflecting both environmental heterogeneity and population-level trait variability. Findings from the multi-location trial confirm pronounced short-term plasticity in growth, leaf morphology, and stomatal regulation along an altitudinal gradient, with distinct genotype origin-specific responses. These findings have direct implications for the conservation of wild coffee genetic resources and for the development of climate-resilient coffee production systems in Central Africa. Safeguarding the species’ genetic and ecological diversity will be essential to ensure its persistence in the face of ongoing climate change and to support future breeding efforts aimed at securing sustainable coffee production.
