Increased growth efficiency of Quercus alba trees in a CO₂ -enriched atmosphere

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 CO₂ -enriched atmospheres must be interpreted carefully if they are to be relevant to the potential responses of forest trees. We grew¹ white oak (Quercus alba L.) saplings for four complete growing seasons in open-top chambers with different partial pressures of atmospheric CO₂ White oak saplings produced 58% more dry mass in 50 Pa CO₂ and 135% more in 65 Pa, compared with plants in ambient (35 Pa) CO₂ 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 C₂ 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 CO₂ . 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 CO₂ -enriched atmosphere.