Fact or Fiction

CO2 Science:

In an enlightening analysis recently published in the Journal of Integrative Plant Biology, Phillips et al. (2008) note there is “a long held view,” as they describe it, that “old trees exhibit little potential for growth.” Hence, they say “it may seem reasonable to conclude that old trees are not responsive to increased CO2,” as many climate alarmists do indeed claim. They go on, however, to demonstrate that this view is likely far from the truth.

The three researchers begin their analysis of the subject by stating that “hydraulic constraints in tall trees,” such as those of great age, “constitute a fundamental form of water limitation; indeed, one that is indistinguishable from soil water limitations,” citing the work of Koch et al. (2004) and Woodruff et al. (2004). They also report that “recent research indicates that tree size and its hydraulic correlates, rather than age per se, controls carbon gain in old trees,” as indicated by the study of Mencuccini et al. (2005). These findings imply, in their words, that “factors that alleviate internal or external resource constraints on old trees could improve physiological function and ultimately growth,” which is something elevated CO2 does quite well by increasing plant water use efficiency. In fact, they list several phenomena that suggest “a fundamental potential for old growth trees to show greater photosynthesis and growth under industrial age increases in CO2 than they would under constant, pre-industrial CO2 levels.”

Drawing from their own work, for example, Phillips et al. find that “500- and 20-year-old Douglas-fir trees both [our italics] show high sensitivity of photosynthesis to atmospheric CO2,” presenting data which clearly demonstrate, as they phrase it, that “under optimal conditions there exists the potential for an approximately 30% increase in photosynthetic rate with an increase in CO2 from pre-industrial to current levels [i.e., from 280 to 385 ppm] in old trees.” And they go on to note that “the phenomenon of twentieth-century ring-width increase,” which could thus be expected to accompany the 20th-century increase in the air’s CO2 concentration, has in fact been detected in several other studies, including those of LaMarche et al., (1984), Jacoby (1986), Graybill (1987), Kienast and Luxmoore (1988), Graumlich (1991), Knapp et al. (2001), Bunn et al. (2005), and Soule and Knapp (2006), to which we would also add Graybill and Idso (1993).

In further commenting on the significance of the findings of these several studies, the three researchers write that the results of LaMarche et al. (1984) “could not be explained by temperature or precipitation variation over this time period, but were consistent with, and attributed to, the rise in atmospheric CO2,” which was also the case with the results of Graybill and Idso (1993). Although these data, in their words, “appear to represent compelling circumstantial evidence for carbon fertilization of old growth trees,” they note that “this possibility has been discounted and climate change has instead been implicated for the observed responses in subsequent research.” And that invalid discounting — in our opinion — has led to the erroneous climate-alarmist claim that 20th-century global warming was unprecedented over the past two millennia or more. Instead, we believe that a goodly portion of the 20th-century increase in tree growth, which climate alarmists attribute solely to rising temperature, was clearly a consequence of the concomitant growth-promoting and water-use-efficiency-enhancing increase in the air’s CO2 content.

In summation, the analysis of Phillips et al. (2008) provides substantial support for the two-part thesis that (1) old-growth forests can continue to sequester carbon for multiple centuries in the face of ever-increasing atmospheric CO2 concentrations, and that (2) the global temperature history employed by the Intergovernmental Panel on Climate Change depicts an unrealistically large temperature increase over the course of the 20th century.

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