In outdoor common gardens, high latitude populations of deciduous tree species often display higher assimilation rates (A) than low latitude populations, but they accomplish less height. To test whether trends in A reflect adaptation to growing season length or, alternatively, are garden growth artefacts, we examined variation in height increment and ecophysiological traits in a range-wide collection of Populus balsamifera L. populations from 21 provenances, during unconstrained growth in a greenhouse. Rooted cuttings, maintained without resource limitation under 21 h photoperiod for 90 d, displayed increasing height growth, A, leaf mass per area and leaf N per area with latitude whereas stomatal conductance (gs) showed no pattern. Water-use efficiency as indicated by both gas exchange and δ13C increased with latitude, whereas photosynthetic nitrogen-use efficiency decreased. Differences in δ13C were less than expected based on A/gs, suggesting coextensive variation in internal conductance (gm). Analysis of A–Ci curves on a subset of populations showed that high latitude genotypes had greater gm than low-latitude genotypes. We conclude that higher peak rates of height growth in high latitude genotypes of balsam poplar are supported by higher A, achieved partly through higher gm, to help compensate for a shorter growing season.