Productivity and compensatory responses of yellow-poplar trees in elevated C0₂

INCREASED forest growth in response to globally rising CO₂ concentrations could provide an additional sink for the excess carbon added to the atmosphere from fossil fuels^1,2. The response of trees to increased CO₂, however, can be expected to be modified by the interactions of other environmental resources and stresses, higher-order ecological interactions and internal feedbacks inherent in the growth of large, perennial organisms^3,4. To test whether short-term stimulation of tree growth by elevated CO₂ can be sustained without inputs from other environmental resources, we grew yellow-poplar (Liriodendron tulipifera L.) saplings for most of three growing seasons with continuous exposure to ambient or elevated concentrations of atmospheric CO₂. Despite a sustained increase in leaf-level photosynthesis and lower rates of foliar respiration in CO₂-enriched trees, whole-plant carbon storage did not increase. The absence of a significant growth response is explained by changes in carbon allocation patterns, specifically a relative decrease in leaf production and an increase in fine root production. Although these compensatory responses reduced the potential increase in carbon storage in increased CO ₂ concentrations, they also favour the efficient use of resources over the longer term.