Net assimilation rates of 10-year-old naturally regenerated bigtooth aspen (Populus grandidentata Michx.) at two sites in northern Michigan, USA, were compared to test the hypothesis that site-related differences in aboveground production depends solely on differences in rates of CO2 assimilation. On a leaf weight basis, aboveground production on the better site was over twice that on the poorer site, whereas maximum CO2 assimilation rates on the better site were only 48% higher, leading to a rejection of the hypothesis. Soil respiration rates on the two sites were comparable, suggesting that the difference between sites in aboveground production can be attributed in part to a difference in the proportion of total assimilate allocated belowground.