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Has the Historical Rise in the Air's CO2 Content Stimulated Tree Growth Over the Past Century?
Volume 9, Number 32: 9 August 2006

How has the historical increase in atmospheric CO2 concentration impacted the growth of trees in the real world, i.e., in non-contrived experimental settings?

In a study designed to broach this question, Wang et al. (2006) examined ring-width development in cohorts of young and old white spruce (Picea glauca) trees in a mixed grass-prairie ecosystem in southwestern Manitoba, Canada, where a 1997 wildfire killed most of the older trees growing in high-density spruce islands, but where younger trees slightly removed from the islands escaped the ravages of the flames. "Within each of a total of 24 burned islands," in the words of the three researchers, "the largest dominant tree (dead) was cut down and a disc was then sampled from the stump height," while "adjacent to each sampled island, a smaller, younger tree (live) was also cut down, and a disc was sampled from the stump height."

After removing size-, age- and climate-related trends in radial growth from the ring-width histories of the trees, Wang et al. plotted the residuals as functions of time for the 30-year periods for which both the old and young trees would have been approximately the same age: 1900-1929 for the old trees and 1970-1999 for the young trees. During the first of these periods, the atmosphere's CO2 concentration averaged 299 ppm; during the second it averaged 346 ppm. Also, the mean rate-of-rise of the atmosphere's CO2 concentration was 0.37 ppm/year for first period and 1.43 ppm/year for the second.

The results of this exercise revealed that in comparison to the 1900-1929 period, the slope of the linear regression describing the rate-of-growth of the ring-width residuals for the 1970-1999 period (when the air's CO2 concentration was 15% greater and its rate-of-rise was 285% greater) was more than twice that of the linear regression describing the rate-of-growth of the ring-width residuals for the 1900-1929 period. As the researchers describe it, these results show that "at the same developmental stage, a greater growth response occurred in the late period when atmospheric CO2 concentration and the rate of atmospheric CO2 increase were both relatively high," and they say that "these results are consistent with expectations for CO2-fertilization effects." In fact, they say that "the response of the studied young trees can be taken as strong circumstantial evidence for the atmospheric CO2-fertilization effect."

Another thing that Wang et al. learned was that "postdrought growth response was much stronger for young trees (1970-1999) compared with old trees at the same development stage (1900-1929)," and they add that "higher atmospheric CO2 concentration in the period from 1970-1999 may have helped white spruce recover from severe drought." In a similar vein, they also determined that young trees showed a weaker relationship to precipitation than did old trees, noting that "more CO2 would lead to greater water-use efficiency, which may be dampening the precipitation signal in young trees."

In summary, Wang et al.'s unique study provides an exciting real-world example of the tremendous benefits the historical rise in the air's CO2 content has likely conferred on nearly all of earth's plants, and especially its long-lived woody species.

Sherwood, Keith and Craig Idso

Wang, G.G., Chhin, S. and Bauerle, W.L. 2006. Effect of natural atmospheric CO2 fertilization suggested by open-grown white spruce in a dry environment. Global Change Biology 12: 601-610.