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Updating the World's Longest Atmospheric CO2 Enrichment Experiment
Idso, S.B. and Kimball, B.A.  2001.  CO2 enrichment of sour orange trees: 13 years and counting.  Environmental and Experimental Botany 46: 147-153.

What was done
In July of 1987, the authors planted eight 30-cm-tall sour orange tree (Citrus aurantium L.) seedlings directly into the ground at Phoenix, Arizona, USA, surrounding them in pairs within four clear-plastic-wall open-top chambers.  Then, in November of that year, they began to continuously pump ambient air through two of the chambers via perforated plastic tubes that lay upon the ground beneath the trees, while through the other two chambers they began to similarly pump air that was enriched with carbon dioxide to a concentration that was 300 ppm greater than that of the ambient air.

Throughout the experiment, the authors irrigated and fertilized the trees so as to keep them continually free of water and nutrient stresses; and they measured the circumferences of the trees' trunks at a height of 45 cm above the surface of the ground at the midpoint of every month.  At the ends of years two and three, they also determined the trunk and branch volume of each tree from trunk and branch length and diameter measurements; and from these data they developed a relationship between trunk cross-sectional area and trunk plus branch volume that applied equally well to both the CO2-enriched and ambient-treatment trees.  Numerous wood density measurements then allowed them to calculate the total aboveground biomass of each tree at the midpoint of every month.

All of the oranges produced by the trees were picked, counted and weighed each year; and a large number of the fruit were dried in ovens to determine the amount of dry matter they contained.  By this means the authors developed a yearly record of fruit biomass production to accompany their monthly record of aboveground wood biomass.

What was learned
The CO2-enriched/ambient-treatment ratio of aboveground wood biomass rose rapidly from an initial value of unity to a value in excess of 3.0 at the two-year point of the study.  Then it began a gradual decline that lasted seven years.  At the nine-year point of the experiment, however, the enriched/ambient wood biomass ratio leveled out at a value of 1.8, which was maintained to the time of the writing of the authors' paper some four years and three months later.

Fruit production began in the third year of the study, when the CO2-enriched trees each produced an average of 25 fruit and the ambient-treatment trees each produced but one fruit.  Thereafter, the yearly CO2-enriched/ambient-treatment fruit dry weight ratio dropped substantially, becoming essentially identical to the enriched/ambient wood biomass ratio at the end of year six; and for the last four years of the study, this ratio too has been essentially constant at a value of 1.8.

What it means
There is no way to truly know how trees will respond to long-term atmospheric CO2 enrichment without actually doing a long-term experiment or taking advantage of a long-term "natural experiment."  The authors have pursued the first of these paths in their ground-breaking research, tentatively concluding that their findings "are indicative of the likelihood that the CO2-enriched trees may have reached an equilibrium condition with respect to the CO2-induced enhancement of wood biomass and fruit production, and that they will not substantially depart from these steady-state responses over the remainder of their lifespan."  This finding, although still conjectural, is extremely important, as it demonstrates the ability of one tree species to maintain a high degree of growth stimulation -- an 80% increase in wood and fruit production in response to a 75% increase in the air's CO2 content -- over a substantial period of time.  We applaud the authors and the USDA's Agricultural Research Service for their commitment to this long-term effort.