How will earth's trees and shrubs respond to the ongoing rise in the air's CO2 content? Many people have speculated many different things about this question, some proposing the planet's woody plants will dramatically increase their growth rates and sequester much of the CO2 released to the atmosphere by the burning of fossil fuels (Idso 1991a,b) and others claiming earth's trees and shrubs will do just the opposite and turn into carbon sources in the not too distant future (see our Editorials of 1 December 1999 and 30 May 2001). Is one of these assessments correct? Or does the truth lie somewhere between the two extremes?
In an attempt to shed some light on this question, Idso (1999) analyzed the results of 176 different atmospheric CO2 enrichment experiments that had been conducted over the years on woody plants that were grown in containers having finite rooting volumes (small pots, in layman's language), finding that almost any conceivable growth response could be obtained initially. As time progressed, however, nearly all of the responses tended rapidly towards zero, suggesting that any early CO2-induced growth enhancements observed in root-restricted woody plants were unlikely to persist beyond about five years. In contrast, the results of four studies in which trees were grown out-of-doors and rooted in the ground, so as not to experience any root restrictions, indicated that the increase in growth induced by a 300 ppm increase in atmospheric CO2 concentration was still running close to 90% after five years of treatment.
But what happens beyond that point in time? In our Editorial of 5 March 2003, we explore this question within the context of the longest atmospheric CO2 enrichment experiment ever to be conducted anywhere in the world, finding that many more years are required to reach a satisfactory conclusion. In the long-term sour orange tree study of Idso and Kimball (2001), for example, the conclusion that would have been reached at the five-year point of the experiment was totally refuted by what happened over the next five years; and the conclusion that would have been reached at the ten-year point of the study was totally refuted by what happened over the next five years.
So, after fifteen years of study is the final answer in hand? And what is it? Probably. And the answer is ... that a 75% increase in the atmosphere's CO2 concentration (an increase from a nominal base concentration of 400 ppm to an elevated concentration of approximately 700 ppm) results in an 80% increase in aboveground wood and fruit production ... in this particular experiment, where water and nutrients have always been kept at optimal levels. Clearly, more studies of more species under a variety of conditions will be required before the generality of this response can be determined. Nevertheless, what has been learned to date is extremely gratifying.
References
Idso, S.B. 1991a. The aerial fertilization effect of CO2 and its implications for global carbon cycling and maximum greenhouse warming. Bulletin of the American Meteorological Society 72: 962-965.
Idso, S.B. 1991b. Reply to comments of L.D. Danny Harvey, Bert Bolin, and P. Lehmann. Bulletin of the American Meteorological Society 72: 1910-1914.
Idso, S.B. 1999. The long-term response of trees to atmospheric CO2 enrichment. Global Change Biology 5: 493-495.
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.