In July of 1987, as described by U.S. Department of Agriculture researchers Idso and Kimball (2001) , eight 30-cm-tall sour orange tree (Citrus aurantium L.) seedlings were planted directly into the ground at the Agricultural Research Service's U.S. Water Conservation Laboratory in Phoenix, Arizona, where they were enclosed in pairs within four clear-plastic-wall open-top chambers. Then, in November of that year, the two scientists 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 pump air that was enriched with carbon dioxide to a concentration that was 300 ppm greater than that of the surrounding ambient air, which had an average CO2 concentration of 400 ppm. And thus was born one of the longest atmospheric CO2 experiments ever to be conducted anywhere in the world.
Throughout the experiment, the Phoenix global-change research team irrigated and fertilized the trees according to standard agronomic practices in the area, in order to keep them as free as possible from 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 the second and third years of the study, they also determined the total 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 crosssectional area and trunk plus branch volume that applied equally well to both the CO2-enriched and ambient-treatment trees. Then they made numerous wood density measurements that finally allowed them to calculate the total aboveground woody biomass of each tree at the midpoint of every month. In addition, 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. And by these means the two researchers developed a yearly record of total fruit biomass production to accompany their monthly record of wood biomass production.
As the experiment progressed, the CO2-enriched/ambient-treatment ratio of cumulative aboveground wood biomass rose rapidly from an initial value of unity to a value slightly in excess of 3.0 at the two-year point of the study, as shown in the figure below, which depicts the changing ratio of the aboveground wood biomass of the CO2-enriched trees to that of the ambient-treatment trees over the first two years of the study.