TREE GROWTH IN CARBON-DIOXIDE ENRICHED AIR AND ITS IMPLICATIONS FOR GLOBAL CARBON CYCLING AND MAXIMUM LEVELS OF ATMOSPHERIC CO2

In the longest carbon dioxide enrichment experiment ever conducted, we ll-watered and adequately fertilized sour orange tree seedlings were p lanted directly into the ground at Phoenix, Arizona, in July 1987 and continuously exposed, from mid-November of that year, to either ambien t air or air enriched, with an extra 300 ppmv of CO2 in clear-plastic- wall open-top enclosures. Only 18 months later, the CO2-enriched trees had grown 2.8 times larger than the ambient-treated trees; and they h ave maintained that productivity differential to the present day. This tremendous growth advantage is due to two major factors: a CO2-induce d increase in daytime net photosynthesis and a CO2-induced reduction i n nighttime dark respiration. Measurements of these physiological proc esses in another experiment have shown three Australlian tree species to respond similarly; while an independent study of the atmosphere's s easonal CO2 cycle suggests that all earth's trees, in the mean, probab ly share this same response. A brief review of the plant science liter ature outlines how such a large growth response to atmospheric CO2 enr ichment might possibly be maintained in light of resource limitations existing in nature. Finally, it is noted that a CO2 ''fertilization ef fect'' of this magnitude should substantially slow the rate at which a nthropogenic carbon dioxide would otherwise accumulate in the atmosphe re, possibly putting an acceptable upper limit on the level to which t he CO2 content of the air may ultimately rise.