Adiantum reniforme var. sinensis (Adiantaceae) is an endangered fern endemic to the Th ree Gorges region in southwest China. To elucidate possible ecophysiological mechanisms restricting its distribution, eff ects of the availability of light (28%, 14% and 7% of open fi eld) and soil moisture (60% and 40% of fi eld capacity) on dry matter production and allocation, leaf morphology and water use effi ciency (WUE) were examined in A. reniforme var. sinensis and its widespread congener A. capillus-veneris . Both species had lower root/shoot ratio (R/S) and higher specifi c leaf area (SLA) when grown at low light. However, A. reniforme var. sinensis showed less plasticity for total leaf area (LA) and leaf area ratio (LAR) than A. capillus-veneris , and its root mass, total mass and WUE decreased as light availability decreased. Under water stress, all traits of both species except WUE were signifi cantly aff ected. However, drought stress decreased total mass of A. capillus-veneris but did not have a signifi cant eff ect on A. reniforme var. sinensis . Compared with A. capillus-veneris , A. reniforme var. sinensis had signifi cantly higher R/S but lower values for other analyzed traits. Th ese results suggest that A. reniforme var. sinensis is relatively superior in drought tolerance but inferior at low light, allowing it to persist in habitats with low soil moisture and high light availability but with few coexisting species present.