Abstract: In order to elucidate the hydrophysiological mechanisms of the formation of ‘dwarf and aged’ trees, two typical  tree species Robinia pseudoacacia and Populus simonii on the Loess Plateau were chosen as the test objects to study their leaf hydraulic traits and drought resistance under effects of site conditions (gully bottom tableland and inter-gully slopeland) and tree age. For both R. pseudoacacia and P. simonii, the trees growing on the wet tableland had higher leaf maximum hydraulic conductance (K_max) and hydraulic vulnerability (P₅₀), and the K_max decreased with tree age but the P₅₀ had less difference, as compared with the trees growing on the dry slopeland. The trees growing on the tableland also had higher leaf epidermal conductance and PV curve parameters (including relative water content at turgor pressure loss point RWC_tlp, water potential at turgor pressure loss point ψ_tlp, and osmotic potential at saturated water content ψ_sat), and the leaf epidermal conductance decreased significantly with tree age whereas the PV curve parameters had definite decrease. The K_max was significantly positively correlated with ψ_tlp, and the P₅₀ had definite correlations with PV curve parameters, suggesting that there existed a tradeoff relationship between K_max and drought resistance, and P₅₀was an important drought-resistant trait for the two tree species.