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Effects of Atmospheric CO2 Enrichment on the Growth and Water Use Efficiencies of Sour Orange and Other Trees
Volume 5, Number 49: 4 December 2002

The longest atmospheric CO2 enrichment experiment ever to be conducted anywhere in the world was begun in November of 1987 - and is still ongoing - at the U.S. Water Conservation Laboratory in Phoenix, Arizona. There, eight well-watered and fertilized sour orange trees that are rooted in the ground and growing out-of-doors have been enclosed in pairs within four clear-plastic-wall open-top chambers ever since they were tiny seedlings. Two of these chambers have been continuously maintained at the local atmospheric CO2 concentration of approximately 400 ppm, while two of them have been maintained at a concentration approximately 300 ppm above that value for all but about two weeks of each year.

Many scientific journal articles have been written on various aspects of this unique long-term experiment; and we here review the results of one of the most recent of those studies, which in its online version carries a publication date of 2003. Hence, we are essentially bringing you a report of tomorrow's atmospheric CO2 enrichment news today.

This newest study of Leavitt et al. (2003) is especially important, as it reports the results of a multifaceted investigation of a phenomenon that has never before been assessed in this long-term experiment: the effects of a 75% increase in the air's CO2 content on the efficiency with which well-watered and fertilized sour orange trees utilize water. It is based, as the authors note, "on the conceptual framework developed by Farquhar et al. (1982), who defined intrinsic water-use efficiency (iWUE) as the ratio of the photosynthetic uptake of CO2 through leaf stomata to the simultaneous transpirational loss of water vapor through the same [stomatal] openings."

This ratio may be experimentally evaluated by measuring stable-carbon isotopes of various plant tissues and the air to which those tissues were exposed during their development. In this particular study, the plant materials that were utilized were leaves that had been collected every two months throughout 1992 and on three occasions in 1994-95, plus wood samples that were extracted five years later from north-south- and east-west-oriented wood cores that passed through the centers of each of the eight trees' trunks at a height of 45 cm above the ground.

The grand average result of these measurements, evaluated within the context described by Farquhar et al., was, as the authors report, "an 80% increase in [water use efficiency] in response to the [75%] increase in atmospheric CO2 concentration employed in the study."

This result is particularly interesting for a number of different reasons. First, it suggests that for a doubling of the air's CO2 content, there would likely be more than a doubling of the trees' water use efficiency. Second, in the words of the authors, "this increase in sour orange tree iWUE is identical to the long-term CO2-induced increase in the trees' production of wood and fruit biomass," as documented by Idso and Kimball (2001); and this observation suggests that a doubling of the air's CO2 content should produce more than a doubling of the trees' total productivity, which further suggests that land planted to sour orange trees, and perhaps many other tree species, will see its potential for carbon sequestration grow dramatically in a CO2-enriched world of the future.

A third important fact noted by the authors is that the CO2-induced increase in sour orange tree water use efficiency is also identical "to the increase in the mean iWUE reported for 23 groups of naturally occurring trees scattered across western North America that was caused by the historical rise in the air's CO2 content that occurred between 1800 and 1985," as documented by Feng (1999), who further notes that these iWUE trends in naturally occurring trees "are largely caused by the anthropogenic increase of the atmospheric CO2 concentration," concluding that this phenomenon "would have caused natural trees in arid environments to grow more rapidly, acting as a carbon sink for anthropogenic CO2." In addition, Leavitt et al. point out that "even greater water-use efficiency responses have been observed in European tree-ring studies," citing the work of Bert et al. (1997) with white fir and Hemming (1998) with beech, oak and pine trees.

What do these observations portend for the decades ahead? In addressing this subject at the conclusion of their paper, Leavitt et al. say "the ongoing rise in the air's CO2 content could continue to do the same for earth's trees in the future." And what is that? It is to dramatically increase their productivity and the efficiency with which they utilize water to achieve vastly enhanced growth rates.

How general could we expect this phenomenon to be? In a comprehensive review of the scientific literature pertaining to this subject, Saxe et al. (1998) determined that "close to a doubling" of the air's CO2 concentration leads to an approximate 50% increase in the biomass production of angiosperm trees and a 130% increase in the biomass production of coniferous species. With sour orange trees projected to experience just slightly more than a 100% increase in wood and fruit production in response to a doubling of the air's CO2 concentration, it is clear that the results of the Phoenix study fall well within the mid-range results typical of most other trees that have been similarly studied.

In light of these many empirical observations, we can confidently expect the growth rates of earth's trees to increase dramatically as the air's CO2 content continues to climb; and this phenomenon, in turn, should enable them to sequester increasingly greater amounts of carbon. In addition, as the planet's trees become ever more efficient at utilizing water, we can expect to see them rapidly expand into areas that are currently too dry to support their growth and reproduction. This phenomenon will also increase the magnitude of carbon sequestration by earth's trees. Hence, as time progresses, the planet's trees, if not destroyed by mankind's cutting and burning them, will provide an ever-increasing brake upon the rate of rise of the air's CO2 content.

Sherwood, Keith and Craig Idso

References
Bert, D., Leavitt, S.W. and Dupouey, J.-L. 1997. Variations in wood 13C and water-use efficiency of Abies alba during the last century. Ecology 78: 1588-1595.

Farquhar, G.D., O'Leary, M.H. and Baxter, J.A. 1982. On the relationship between carbon isotope discrimination and intercellular carbon dioxide concentration in leaves. Australian Journal of Plant Physiology 9: 121-137.

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.

Hemming, D.L. 1998. Stable Isotopes in Tree Rings: Biosensors of Climate and Atmospheric Carbon-Dioxide Variations. Ph.D. Dissertation. University of Cambridge, Cambridge, UK.

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.

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.