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summary The responses of yellow‐poplar ( Liriodendron tulipifera L.) seedlings to elevated levels of atmospheric CO 2 were investigated to identify attributes governing growth and physiological responses to CO 2 . Based on the pattern of leaf initiation and nutrient requirements of the species, it was predicted that (1) CO 2 , enrichment would enhance growth of yellow‐poplar seedlings both through accelerated leaf area production and through higher rates of carbon assimilation per unit leaf area; and (2) growth enhancement of yellow‐poplar by CO 2 enrichment would be reduced by nutrient limitations. The hypotheses were tested in an experiment in which yellow‐poplar plants were grown from seed for 24 weeks in controlled‐environment chambers. The experimental design comprised three atmospheric CO 2 concentrations (371, 493, and 787 cm 3 m −3 ), two levels of mineral nutrients (unfertilized or weekly additions of complete nutrient solution), and three harvests (6, 12, and 24 weeks). Plant growth rate, water use, foliar gas exchange, component dry weights, and nutrient contents were measured. Both hypotheses were rejected. Whole‐plant dry weight increased similarly with CO 2 , enrichment in plants provided with additional mineral nutrients and in unfertilized plants, although the fertilized plants grew 10‐fold larger. The increase in dry weight resulting from elevated CO 2 occurred only in root systems. Although leaves were produced continuously during the experiment, leaf area was slightly reduced in elevated CO 2 , and the whole‐plant growth response was wholly attributable to an increase in carbon assimilation per unit leaf area. Although the compensation between photosynthesis and leaf area reduced the potential growth response to CO 2 , the reduction in leaf area ratio was associated with a significant increase in water‐use efficiency. This unexpected result demonstrated the importance of feedbacks and interactions between resources in shaping the response of a plant to CO 2 .
Norby et al. (Mon,) studied this question.