By capturing tissue-level responses, physiological traits related to water status and photosynthesis are increasingly being used to explain species-level responses to environmental conditions such as shade or drought. Such mechanistic traits, able to link plant physiology directly to environmental conditions, could inform broader studies of changing community composition in an era of drastic shifts in climate or disturbance regimes. However, understanding the extent of inter/intraspecies trait variation and how such traits vary along common environmental gradients remains an open question in understanding physiological traits as species-level characteristics. To address this, we quantified gas exchange traits, turgor loss point (Ψ_TLP), isotopic discrimination, stem hydraulic conductivity (K_S), and associated functional traits in co-occurring individuals of Viburnum lentago, V. trilobum, V. acerifolium, and V. rafinesquianum, while also quantifying canopy coverage and soil conditions. All four Viburnum displayed significant trait variation in gas-exchange traits, Ψ_TLP, and heavy carbon isotope discrimination along gradients of canopy openness. Particularly, plants in shadier environments were more photosynthetically active at lower light levels, yet showed lower drought tolerance (higher Ψ_TLP) and more strict stomatal regulation than individuals grown in open gaps. While physiological traits showed trends along gradients of light availability, soil depth and texture explained little trait variation. While V. acerifolium displayed a more conservative, shade-tolerant trait syndrome, little significant difference was found in traits between the congeners, highlighting the potentially limited resolution of physiological traits to differentiate trait syndromes of closely related, co-occurring species.