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Real-World Increase in Air's CO2 Content Has Accelerated Growth of Natural Aspen Stands

Reference
Cole, C.T., Anderson, J.E., Lindroth, R.L. and Waller, D.M. 2010. Rising concentrations of atmospheric CO2 have increased growth in natural stands of quaking aspen (Populus tremuloides). Global Change Biology 16: 2186-2197.
In a paper published in Global Change Biology, Cole et al. (2010) write that quaking aspen (Populus tremuloides Michx.) is a dominant forest type in north-temperate, montane, and boreal regions of North America," stating that it is, in fact, "the most widely distributed tree species on the continent," while noting that aspen -- and related poplars -- are "quintessential foundation species (Ellison et al., 2005), shaping the structure and function of the communities and ecosystems in which they occur (Whitham et al., 2006; Schweitzer et al., 2008; Madritch et al., 2009)." This being the case, they felt it important to attempt to determine how this keystone species may have responded to the increase in atmospheric CO2 concentration that has occurred over the past several decades, especially within the context of the climatic changes that occurred concurrently.

To accomplish this goal, the four researchers collected branches from 919 trees after their leaves had dropped in the fall, obtaining samples that represented 189 genets or clones (five trees per clone) at eleven sites distributed throughout three regions of Wisconsin (USA). The sampled trees ranged from 5 to 76 years of age and came from second-growth unmanaged forests south of the areas defoliated by forest tent caterpillars in 1980-1982, 1989-1990 and 2001-2002. In addition, they recorded trunk diameter at breast height for each sampled tree, which parameter, in their words, "is very highly correlated with total biomass in aspen," citing the work of Bond-Lamberty et al. (2002).

So what did the Minnesota and Wisconsin scientists learn? First of all, they determined that "age-specific ring width increased over time," and that "the greatest increase occurred for relatively young trees, so that young trees grew faster in recent years than did young trees several decades ago." During the past half-century, for example, they found that the growth of trees 11-20 years old rose by 60%. In addition, they observed that "rising CO2 causes ring width to increase at all moisture levels, apparently resulting from improved water use efficiency," so that "the overall increase results from historical increases in both CO2 and water availability." And when they separated out the impacts of the two factors, they found that "the effect of rising CO2 had been to increase ring width by about 53%," as a result of "a 19.2% increase in ambient CO2 levels during the growing season, from 315.8 ppm in 1958 (when CO2 records began) to 376.4 ppm in 2003."

This is a truly remarkable finding; and Cole et al. comment that "the magnitude of the growth increase uncovered by this analysis raises the question of how much other major forest species have responded to the joint effects of long-term changes in CO2 and precipitation." In this regard, other tree species may well have experienced similar growth stimulations, particularly in light of the analysis of Tans (2009), who demonstrated that earth's land surfaces were a net source of CO2 to the atmosphere until about 1940 -- primarily due to the felling of forests and the plowing of grasslands to make way for expanded agricultural activities -- but who found that from 1940 onward, the terrestrial biosphere had become, in the mean, an increasingly greater sink for CO2, and that it had done so even in the face of massive global deforestation, for which it apparently more than compensated.

In conclusion, the combined findings of the two stellar studies of Tans and Cole et al. clearly testify to the phenomenal ability of the ongoing rise in the air's CO2 content to literally transform the face of the earth, as the planet's forests get a tremendous new lease on life, courtesy of mankind's mining and burning of coal, gas and oil.

Additional References
Bond-Lamberty, B., Wang, C. and Gower, S.T. 2002. Aboveground and belowground biomass and sapwood area allometric equations for six boreal tree species of northern Manitoba. Canadian Journal of Forest Research 32: 1441-1450.

Ellison, A.M., Bank, M.S., Clinton, B.D., Colburn, E.A., Elliott, K., Ford, C.R., Foster, D.R., Kloeppel, B.D., Knoepp, J.D., Lovett, G.M., Mohan, J., Orwig, D.A., Rodenhouse, N.L., Sobczak, W.V., Stinson, K.A., Stone, J.K., Swan, C.M., Thompson, J., Holle, B.V. and Webster, JR. 2005. Loss of foundation species: consequences for the structure and dynamics of forested ecosystems. Frontiers in Ecology and the Environment 3: 479-486.

Madritch, M.D., Greene, S.G. and Lindroth, R.L. 2009. Genetic mosaics of ecosystem functioning across aspen-dominated landscapes. Oecologia 160: 119-127.

Schweitzer, J.A., Madritch, M.D., Bailey, J.K., LeRoy, C.J., Fischer, D.G., Rehill, B.J., Lindroth, R.L., Hagerman, A.E., Wooley, S.C., Hart, S.C. and Whitham, T.G. 2008. The genetic basis of condensed tannins and their role in nutrient regulation in a Populus model system. Ecosystems 11: 1005-1020.

Tans, P. 2009. An accounting of the observed increase in oceanic and atmospheric CO2 and an outlook for the future. Oceanography 22: 26-35.

Whitham, T.G., Bailey, J.K. and Schweitzer, J.A. 2006. A framework for community and ecosystem genetics from genes to ecosystems. Nature Reviews Genetics 7: 510-523.

Archived 16 November 2010