The topography and tree species compositions significantly differ between the ridge and valley habitats of the Kenting karst forest, southern Taiwan. The objective of this research was to explore how tree species inhabiting the ridge habitat have adapted to the drought environment in terms of physiological traits and leaf morphology. Drypetes littoralis and Aglaia formosana, 2 species inhabiting the ridge, were investigated for this purpose. As a comparison, Pisonia umbellifera and Melanolepis multiglandulosa, 2 species inhabiting the valley, as well as Diospyros maritima which is distributed in both habitats, were also studied. Results showed that ridge species had a significantly higher leaf mass per area than did valley species. The predawn leaf water potential (PWP) during the dry season was significantly lower than that during the rainy season for each of the 5 species. The net photosynthetic rate (Pn) during the dry season was also significantly lower than that during the rainy season for 4 species, with the exception of P. umbellifera. During the dry season, the PWP of ridge species Dry. littoralis and A. formosana decreased to -3.40 and -3.69 MPa, respectively, indicating that these 2 species were suffering severe water stress, while the valley species were experiencing only mild water stress. Yet, these 2 ridge species still maintained positive Pn values at about 46 and 25%, respectively, of the Pn during the rainy season. Individuals of Dio. maritima growing in the ridge habitat showed a slightly lower PWP than that of the same species growing in the valley habitat (-1.88 vs. -1.29 MPa) during the dry season, but maintained the Pn at 62% of the Pn during the rainy season. To have their Pn values decline to 0, the PWPs of ridge species Dry. littoralis, A. formosana, and Dio. maritima would need to be as low as -5.89, -4.84, and -5.03 MPa, respectively. The results indicated that these 3 ridge species possessed a high physiological tolerance ability and could adapt to the stressful conditions of strong winds and water deficits in the ridge habitat during the dry season. Individuals of Dio. maritima growing in the ridge habitat showed both physiological and morphological acclimation to the drought environment.