We exposed cuttings of two poplar species, Populus cathayana Rehder and Populus przewalskii Maximowicz, from Sect. Tacamahaca Spach to two watering regimes (well-watered and water-stressed conditions) and to two nutrient regimes (with or without fertilization) in a greenhouse to determine how fertilization affects the growth, morphology and physiology of poplars under different water conditions. Under stress conditions, changes in early growth and dry matter allocation, and decrease in gas exchange and the related functions are usually observed. Moreover, the measurement of carbon isotope composition (δ 13C) provides an integrated measurement of water use efficiency. And abscisic acid (ABA) is a phytohormone which plays a prominent role in various physiological and biochemical processes related to environmental stresses. So we determine these characteristics and related parameters, and our results showed the following: (1) Fertilization promoted the growth of poplars under well-watered conditions, while under water-stressed conditions its effect on growth was negative. (2) Fertilization increased δ 13C, total N concentration, chlorophyll a/b and intrinsic efficiency of photosystem II (Fv/Fm) but decreased relative water content of leaves, stomatal conductance, transpiration rate and C/N ratio under both well-watered and water-stressed conditions. (3) Fertilization appeared to increase net photosynthesis rate and decrease ABA content under well-watered conditions, while it decreased net photosynthesis rate and increased ABA content under water-stressed conditions. Moreover, compared to P. cathayana, collected from a lower altitude region, P. przewalskii, collected from a high-altitude region, has a slower growth rate and stronger adaptability to drought stress, which perhaps resulted from its chronic adaptability to the low water availability of high-altitude region; but to the nutrient stress, there was no difference between the two species.