Spatial and century-scale temporal variation of wild Coffea canephora leaf functional traits and water use efficiency in Congo Basin forests

Abstract Background and Aims Understanding spatiotemporal variation in plant functional traits and intrinsic water use efficiency (iWUE) is essential to evaluate how plants respond to environmental change. In forests of the Congo Basin, we examined spatial and century-scale temporal trends in the morphological and physiological characteristics of the leaves of Coffea canephora Pierre ex A. Froehner, a widespread understory species from West Africa to the African rift (Uganda). Methods Using 179 herbarium samples collected during two periods (1900-1960 and 2016-2021), we measured the specific leaf area (SLA), the stomatal size (S), the stomatal pore size (SPS), the stomatal density (SD), and the maximum diffusive stomatal conductance to CO2 (gcmax). Stable carbon and oxygen isotope ratios (δ13C, δ18O) were measured from leaf cellulose to infer variation in photosynthetic activity iWUE. Key Results We found a significant spatiotemporal variation in leaf morphological and physiological traits and iWUE. δ13C ranged from -34.84‰ to -24.11‰, and δ18O from +26.96‰ to +34.16‰. Over the past century, SLA and S increased, whereas SPS, SD, gcmax, δ13C and iWUE decreased. Spatially, morphological traits appeared shaped by long-term environmental adaptation, while physiological traits responded more to short-term drivers such as atmospheric CO2 and precipitation, highlighting a functional decoupling that may limit photosynthetic performance of C. canephora under future climate change. The trait correlations showed coordinated functional trade-offs: SLA was negatively correlated with iWUE, while S, SD, and gcmax were positively associated, reflecting trade-offs between carbon gain and water conservation. Conclusions Our study underscores the value of herbarium-based multitrait approaches in reconstructing long-term plant responses and their relevance for understanding climate sensitivity in tropical understory species.