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Seventeen Years of Enriching Sour Orange Trees with CO2
Volume 10, Number 44: 31 October 2007

Between November of 1987 and January of 2005, the longest CO2-enrichment study of a long-lived woody plant species ever to be conducted to the latter point in time was carried out at Phoenix, Arizona (USA). It was initiated with the out-of-doors planting of eight sour orange (Citrus aurantium L.) seedlings, and with the initial project researchers surrounding pairs of the seedlings with clear-plastic-wall open-top chambers, within which four of the trees were exposed to a continual bottom-to-top flow of ambient air, while the other four trees were exposed to a similar 24-hour 7-day-per-week upward flow of air enriched with an extra 300 ppm of CO2. Also, and throughout the entire course of the experiment, Kimball et al. (2007) - who report the final productivity results of the 17-year study - state that "the trees were fertilized and flood irrigated similar to practice in commercial orchards so as to maintain ample nutrients and soil moisture."

So what was learned from the historic 17-year experiment?

In terms of total biomass production, which was the primary focus of the summary report, Kimball et al. state that the CO2-enriched to ambient ratio of annual wood plus fruit production "peaked in years 2-4 of the experiment at about 2.4," but that "following the peak, there was a decline through year 8." Thereafter, however, they found that the annually-produced-biomass ratios "were more or less at a plateau that corresponded with the value of the ratio at final harvest of 1.69." For more detail and discussion of this temporally-varying aspect of the experiment, see our Editorial of 5 March 2003.

In terms of harvestable yield, i.e., fruit production, Kimball et al. write that "the cumulative amount of biomass due to fruit production over the duration of the experiment was increased 85% due to elevated CO2," which increase "was entirely from an increase in fruit number." In addition, they report that "the vitamin C content of the fruit was increased 7% based on samples taken from the fourth through the 12th years of the experiment," citing the study of Idso et al. (2002). Consequently, not only were there a whole lot more oranges produced by the trees in the CO2-enriched chambers, a whole lot more better-quality oranges were produced.

In their concluding discussion of one of the major implications of the study, Kimball et al. say that "rather than a continual acclimation" - i.e., rather than a gradual long-term decline in the aerial fertilization effect of the extra 300 ppm of CO2 supplied to the CO2-enriched trees (which dwindling away of the effect's efficacy is periodically postulated by many climate alarmists) - "instead there was a sustained enhancement of about 70% in annual fruit and incremental wood production over the last several years of the experiment." This observation thus led them to conclude that "the effects of elevated CO2 on trees can be large and sustained for many years," as they indeed demonstrated to be the case with sour orange trees, there having been a 70% sustained increase in biomass production over the entire last decade of the study in response to the 75% increase in the air's CO2 content employed throughout the experiment.

Sherwood, Keith and Craig Idso

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.

Kimball, B.A., Idso, S.B., Johnson, S. and Rillig, M.C. 2007. Seventeen years of carbon dioxide enrichment of sour orange trees: final results. Global Change Biology 13: 2171-2183.