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Summary(1)The research conducted, including the rationaleThe direct effect of aridity on photosynthetic and water-transport strategies is not easy to discern in global analyses because of large-scale correlations between precipitation and temperature. We analyze tree traits collected along an aridity gradient in Ghana, West Africa, that shows little temperature variation, in an attempt to disentangle thermal and hydraulic influences on plant traits.(2)MethodsPredictions derived from optimality theory of the variation of key plant traits along the gradient are tested with field measurements.(3)ResultsMost photosynthetic traits show trends consistent with optimality-theory predictions, including higher photosynthetic rates in the drier sites, and an association of higher photosynthetic rates with greater respiration rates and greater water transport. Leaf economic and hydraulic traits show less consistency with theory or global-scale pattern, especially predictions based on xylem efficiency-sfatety trafeoff. Nonetheless, the link between photosynthesis and water transport still holds: species (predominantly deciduous species found in drier sites) with both higher sapwood-to-leaf area ratio (AS/AL) and potential hydraulic conductivity (Kp), implying higher transpiration, tend to have both higher photosynthetic capacity, and lower leaf-internal CO2.(4)ConclusionsThese results indicate that aridity is an independent driver of spatial patterns of photosynthetic traits, while plants show a diversity of water-transport strategies along the aridity gradient.Plain language summaryAlong an aridity gradient in Ghana, West-Africa, we used optimality theory to explain why higher photosynthetic rates should be found at drier places and how plants arrange water transportation to support quicker photosynthesis at the drier site. We also reported surprising data-theory inconsistency for some hydraulic traits along the aridity gradient where further research is needed.