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, a widespread understorey species from West Africa to the African rift (Uganda).Methods Using 179 herbarium samples collected during two periods (1900-60 and 2016-21), we measured the specific leaf area (SLA), stomatal size (S), stomatal pore size (SPS), stomatal density (SD) and maximum diffusive stomatal conductance to CO2 (gcmax). Stable carbon and oxygen isotope ratios (delta 13C, delta 18O) 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. delta 13C ranged from -34.84 to -24.11 parts per thousand, and delta 18O from +26.96 to +34.16 parts per thousand. Over the past century, SLA and S increased, whereas SPS, SD, gcmax, delta 13C 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 understorey species.