As a renewable energy resource, woody biomass has a role to play in reducing worldwide fossil fuel consumption. Species in the genera Salix and Populus are widely used for this purpose through short rotation tree plantations under temperate latitudes. In warmer climates, common short rotation species include genera such as eucalyptus or black locust (Robinia) although this latter is also cultivated in temperate climates. The objective of our study was to compare the potential of poplar (Populus deltoides × P. nigra, clone 'Dorskamp') and black locust (Robinia pseudoacacia, provenances 'Nagybudmerii' and 'Nyirseg') for biomass production in the Northwest of France. We estimated biomass production and efficiency of water use of poplar compared to those of the two provenances of black locust, (1) at two planting densities (short rotation forestry: SRF, 1428 trees per ha vs. short rotation coppice: SRC, 7272 trees per ha), and (2) 2 and 4 years after planting. Only few effects of the initial planting density on plant growth and physiology were visible two years after planting. Stem height and circumference became higher in SRF than in SRC for both species after four years. Yield at plantation level of the first SRC rotation was almost twice as high for black locust than for poplar, while both species exhibited similar stem height or stem increase rate likely due to the development of more sylleptic branches in black locust. At the end of the second rotation, plantation yield was significantly higher for poplar due to black locust mortality. The combination of higher leaf N and greater WUE probably enhanced black locust performance under the dry conditions of our study site in 2010. During the fourth year (2012), the gap between the two species in terms of Δ 13 C, used as a surrogate of water-use efficiency, was less marked than during the first years due to the fact that poplar Δ 13 C decreased (increase in WUE), while black locust Δ 13 C slightly increased (decrease in WUE). To conclude, black locust appears as an interesting option for biomass production at sites prone to water shortage. However, at high planting density, poplar exhibited a lower mortality after harvest leading to an improved biomass production at the end of the second SRC rotation compared to black locust. The choice of species/cultivar must be adapted to the production system (SRC vs. SRF) and to the specific site conditions.