Increased growth efficiency ofQuercus albatrees in a CO₂‐enriched atmosphere

summary Forests have a prominent role in the global carbon cycle, but their response to a changing atmosphere cannot be measured directly. Experimental observations of small trees in CO 2 ‐enriched atmospheres must be interpreted carefully if they are to be relevant to the potential responses of forest trees. We grew 1 white oak ( Quercus alba L.) saplings for four complete growing seasons in open‐top chambers with different partial pressures of atmospheric CO 2 White oak saplings produced 58% more dry mass in 50 Pa CO 2 and 135% more in 65 Pa, compared with plants in ambient (35 Pa) CO 2 Although this result might suggest a substantial potential for increased carbon storage in forests, the large difference in growth rate could be attributed to a stimulation of growth very early in the experiment. There was not a sustained effect of C 2 on relative growth rate after the first year, and the increased absolute growth rate could persist only so long as leaf area could increase, a condition that would not occur indefinitely in a forest. Nevertheless, annual stem wood production per unit area (growth efficiency) was 37 %, greater in elevated CO 2 . This increase in growth efficiency, a response that is consistent across diverse studies, implies a potential increase in carbon sequestration by forests, subject to critical assumptions about forest canopy development in a CO 2 ‐enriched atmosphere