Increased growth efficiency ofQuercus albatrees in a CO2‐enriched atmosphere

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Abstract
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.