Intensively managed poplar plantations (Populus spp. and their hybrids) have the potential to provide large quantities of renewable biomass for bioenergy. Identifying clonal varieties that are productive on water-limited, marginal agricultural land unsuitable for food production with low land costs is key to realizing that potential. Carbon isotope discrimination (Δ13C) is the current method of choice for selecting varieties able to maintain higher levels of production under moderate drought stress (drought resistance). However, other physiological and morphological traits may be effective as selection criteria for drought stress and cost less to collect and analyze. The objective of this study was to test the efficacy of Δ13C, along with alternative physiological and morphological traits, in predicting drought resistance of hybrid poplar varieties. The other traits considered were leaf mass per area (LMA), petiole diameter (PD), petiole length (PL), individual leaf area (LA), adaxial stomatal density (ADAX), abaxial stomatal density (ABAX), minor vein density (MVD) and chlorophyll content (CHLOR). We found that Δ13C was a poor indicator of drought resistance and ABAX was the only single trait that was a statistically significant predictor of varietal performance under drought stress. The best indicator of drought stress was a composite of all nine variables.