We studied seedling growth responses to ambient and elevated CO2 (350 and 700 μL L−1) of three maternal families of yellow birch (Betula alleghaniensis), raised both individually and in high-density stands. Seedlings in competitive, dense stands exhibited markedly lower average CO2-induced growth enhancements than individually grown plants (16% vs. 49%). Maternal families differed in their growth responses to elevated CO2. However, differences among families were contingent upon density; families which exhibited the greatest CO2-induced growth at low density exhibited the least CO2-responsiveness at high density. These data are discussed in two separate contexts; the reliability of estimates of the CO2 fertilization potential of forest species based solely on individually grown plants, and the potential evolutionary consequences of rising CO2 on regenerating forest tree populations.
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