In July of 1987, as described by USDA 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 the longest atmospheric CO2 experiment ever to be conducted anywhere in the world.
Throughout the experiment, the Phoenix global-change research team irrigated and fertilized the trees 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 cross-sectional area and trunk plus branch volume that applied equally well to both the CO2-enriched and ambient-treatment trees. Numerous subsequent wood density measurements then 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. By this means the authors developed a yearly record of 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. Then it began a gradual decline that lasted seven long years (for more details on this aspect of the experiment, see our Editorial of 5 March 2003). At the nine-year point of the study, however, the CO2-enriched/ambient-treatment wood biomass ratio finally leveled out at a value of 1.80, which value was subsequently maintained to the time of the writing of the Idso and Kimball (2001) paper and has continued essentially unaltered to the time of the writing of this summary report. Finally, fruit production began in the third year of the study, when the CO2-enriched trees produced an average of 25 fruit per tree and the ambient-treatment trees produced an average of but one fruit per tree. Thereafter, the cumulative CO2-enriched/ambient-treatment fruit biomass ratio also dropped substantially, ultimately leveling out just a little above the CO2-enriched/ambient-treatment wood biomass ratio. These findings, in the words of Idso and Kimball (2001), "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."
Giving added confidence to the above conclusions is the ancillary study of Leavitt et al. (2003), who, as described in our Editorial of 4 December 2002, evaluated the intrinsic water use efficiencies of the trees via analyses of the stable carbon isotopes of leaves that had been collected from each of them every two months throughout 1992, as well as on three occasions in 1994-95, plus wood samples that were extracted five years later from two cores that passed through the centers of each of the trees' trunks perpendicular to each other at a height of 45 cm above the ground. The ultimate finding of this endeavor was an 80% increase in intrinsic water use efficiency in response to the 75% increase in atmospheric CO2 concentration employed in the study; and since earlier work of Idso et al. (1993) had demonstrated there was very little difference in leaf stomatal conductance between the two CO2 treatments, nearly all of this water use efficiency increase had to have been the result of the CO2-induced increase in net primary productivity that led to the 80% increases in wood and fruit production.
That this result may be typical of trees in general is suggested by the facts that: (1) in a massive review of the pertinent scientific literature, Saxe et al. (1998) observed that "increasing numbers of experiments show a lack of stomatal sensitivity to CO2," especially when the data come "from long-term experiments on larger trees rooted directly in the ground," as may also be deduced from the work of Eamus (1996), and (2) the finding of Feng (1999) that for 23 sets of trees scattered throughout western North America, the average stable-carbon-isotope-derived increase in intrinsic water use efficiency (iWUE) that occurred in response to the historical increase in the air's CO2 concentration that was experienced over the period 1800-1985 yielded essentially the same value of delta iWUE/ CO2 as that derived from the sour orange tree study of Leavitt et al. In addition, we note that even greater natural CO2-induced increases in iWUE have been documented in various trees in Europe: by Bert et al. (1997) in the case of white fir, and by Hemming (1998) in the cases of beech, oak and pine.
Besides the large increase in the amount of wood and fruit biomass produced by the CO2-enriched sour orange trees, Idso et al. (2002) documented small but significant increases in the vitamin C concentration of the juice of the fruit (5 ± 1%). In addition, as described in our Editorial of 22 January 2003, Idso et al. (2001) discovered three soluble proteins in the leaves of the sour orange trees whose synthesis and mobilization are influenced by the air's CO2 concentration in ways that may facilitate the trees' large biomass response to atmospheric CO2 enrichment. Specifically, the latter researchers found that the proteins appear to function as vacuolar storage proteins, which may supply each year's first flush of new foliage with the large amounts of nitrogen needed to sustain the ultra-enhanced spring branch growth of the CO2-enriched trees, which is four to six times more rapid than that of the trees growing in ambient air (Idso et al., 2000) and likely provides the initial impetus for the reduced but still large (80%) growth response of the trees that is maintained throughout the remainder of the year.
In conclusion, we note that the long-term Phoenix CO2 enrichment experiment on sour orange trees has produced results over a 15-year time span that are consistent with what has been learned from trees that have participated in the "natural experiment" provided by the increase in the air's CO2 content produced by the Industrial Revolution. We also note that the findings of both approaches to ferreting out the consequences of this real-world global change are extremely positive.
References
Eamus, D. 1996. Responses of field grown trees to CO2 enrichment. Commonwealth Forestry Review 75: 39-47.
Feng, X. 1999. Trends in intrinsic water-use efficiency of natural trees for the past 100-200 years: A response to atmospheric CO2 concentration. Geochimica et Cosmochimica Acta 63: 1891-1903.
Idso, C.D., Idso, S.B., Kimball, B.A., Park, H.-S., Hoober, J.K. and Balling Jr., R.C. 2000. Ultra-enhanced spring branch growth in CO2-enriched trees: Can it alter the phase of the atmosphere's seasonal CO2 cycle? Environmental and Experimental Botany 43: 91-100.
Idso, K.E., Hoober, J.K., Idso, S.B., Wall, G.W. and Kimball, B.A. 2001. Atmospheric CO2 enrichment influences the synthesis and mobilization of putative vacuolar storage proteins in sour orange tree leaves. Environmental and Experimental Botany 48: 199-211.
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.
Idso, S.B., Kimball, B.A., Akin, D.E. and Kridler, J. 1993. A general relationship between CO2-induced reductions in stomatal conductance and concomitant increases in foliage temperature. Environmental and Experimental Botany 33: 443-446.
Idso, S.B., Kimball, B.A., Shaw, P.E., Widmer, W., Vanderslice, J.T., Higgs, D.J., Montanari, A. and Clark, W.D. 2002. The effect of elevated atmospheric CO2 on the vitamin C concentration of (sour) orange juice. Agriculture, Ecosystems and Environment 90: 1-7.
Leavitt, S.W., Idso, S.B., Kimball, B.A., Burns, J.M., Sinha, A. and Stott, L. 2003. The effect of long-term atmospheric CO2 enrichment on the intrinsic water-use efficiency of sour orange trees. Chemosphere 50: 217-222.
Saxe, H., Ellsworth, D.S. and Heath, J. 1998. Tree and forest functioning in an enriched CO2 atmosphere. New Phytologist 139: 395-436.