Seedling density modifies the growth responses of yellow birch maternal families to elevated carbon dioxide.

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. [ABSTRACT FROM AUTHOR]