Bark water vapour conductance ( g _bark ) is a rarely considered functional trait. However, for the few tree species measured to date, it appears high enough to create stem water deficits associated with mortality during droughts, when access to water is limited. I tested whether g _bark correlates with stem water deficit during drought conditions in two datasets of tropical trees: one of saplings in forest understories during an annual dry season and one of potted saplings in a shadehouse during extreme drought conditions. Among all 14 populations of eight species measured, g _bark varied more than 10-fold (0.86–12.98 mmol m ⁻² s ⁻¹ ). In the forest understories, g _bark was highly correlated with stem water deficit among four deciduous species, but not among evergreen species that likely maintained access to soil water. In the shadehouse, g _bark was positively correlated with stem water deficit and mortality among all six species. Overall, tree species with higher g _bark suffer higher stem water deficit when soil water is unavailable. Incorporating g _bark into soil–plant–atmosphere hydrodynamic models may improve projections of plant mortality under drought conditions.