November 1, 2010

Quaking Aspen Rejoice

Filed under: Adaptation, Plants

The fall is here again, and deciduous trees across America are putting on their annual display of fall colors. Americans are particularly fond of Quaking Aspen trees that really know how to put on a show in the fall with leaves turning spectacular tints of red and yellow in the autumn. The range of Quaking Aspen is extensive in North America including many picturesque locations in the Rockies (makes us think about John Denver). The tree appear to quake (shake, quiver) due to the unusual architecture of the leaves that makes them move a bit even in the lightest of winds. Aside from putting on a great show in the fall, Aspen wood is white and soft, but fairly strong, and has low flammability. Accordingly, it is used to make matches, packing and stuffing materials, animal bedding, and even serves as a popular material for the interior of saunas.

Two articles have appeared recently in major journals showing us that Quaking Aspen cannot wait for higher levels of atmospheric carbon dioxide (CO2) concentrations.

Figure 1. Range of the Quaking Aspen.

The first article appears in Environmental Pollution and was written by three scientists from the University of Illinois, Michigan Technological University, and the United States Department of Agriculture. Before we go further, we did wonder a bit why the authors chose this journal for their article? The article deals with aspen trees and carbon dioxide, which we do not see as a pollutant, and tropospheric ozone, which we agree is a pollutant. McGrath et al. note early in the article that “[CO2] is expected to increase 50% by 2050. Elevated [CO2] increases photosynthetic rate in C3 plants, above-ground dry matter production, yield and maximum LAI” (LAI is leaf area index). So even before the experiment is conducted, the authors were fully aware that elevated CO2 has many positive benefits for aspen.

If you visit Rhinelander, Wisconsin, you can find aspen trees growing in chambers with atmospheric CO2 being maintain at 560 ppm while others grow in chambers with ambient levels of CO2 near 370 ppm; the trees have been growing in the chambers since 1997. McGrath et al. found “Trees in elevated [CO2] plots showed a stimulation of leaf area index (36%), while trees in elevated [O3] plots had lower leaf area index (-20%). While individual leaf area was not significantly affected by elevated [CO2], the photosynthetic operating efficiency of aspen leaves was significantly improved (51%).” Once again, we find evidence that elevated CO2 causes more leaves, an increase in photosynthesis, and an overall increase in the efficiency of the photosynthetic process—CO2 is hardly a pollutant in the eyes of the quaking aspen!

Critics of the McGrath et al. article would point out that the trees were growing in highly controlled and highly managed chambers, and the results from the chambers may not apply in the forests of the real world—fair enough. However, another article has been published recently in Global Change Biology by four scientists at the University of Minnesota and the University of Wisconsin; financial support was provided by the National Science Foundation. The Cole et al. team fanned out into Wisconsin and found plenty of stands of Quaking Aspen, and they collected cores from the living trees. The trees produce one ring every year, but as seen in the figure below, the width of the rings has been increasing since 1935. Any thoughts on what may be the primary cause of the trees to producing ever-larger rings?

Figure 2. Mean ring width over time. The overall increase in ring width is evident even in the raw data, as are the effects of dry (e.g. 1988) and wet (e.g. 1993) years. The number of trees included in the data set increases with time, as does the mean age of trees in the sample (source: Cole et al., 2010).

You guessed it—the rise in atmospheric CO2 over the past 70 years! The authors write “rising CO2 causes ring width to increase at all moisture levels, apparently resulting from improved water use efficiency, but the overall increase shown in [our Figure 2 above] results from historical increases in both CO2 and water availability. For a tree of average age (22 years in this set), the effect of rising CO2 has been to increase ring width by about 53%. This change corresponds to a 19.2% increase in ambient CO2 levels during the growing season, from 315.8 µLL-1 in 1958 (when CO2 records began) to 376.4 µLL-1 in 2003, a period that spans the ages of all but 12 of the 919 trees studied.” Imagine that—CO2 increased by about 60 ppm (or µLL-1) and ring widths increase by 53%.

Other concluding remarks are interesting as well. Cole et al. note that Quaking Aspen “contributes substantially to the productivity of northern temperate forests, which has increased over the past half-century.” Also, “direct analysis presented here reveals that rising concentrations of CO2 during the past five decades have already strongly influenced growth rates of this major component of North American forests, especially under high-moisture conditions.” The bottom line comes in the abstract where they clearly state

“Aspen growth has increased an average of 53% over the past five decades, primarily in response to the 19.2% rise in ambient CO2 levels. CO2-induced growth is particularly enhanced during periods of high moisture availability. The analysis accounts for the highly nonlinear changes in growth rate with age, and is unaffected by sex or location sampled.”

Enough said – Quaking Aspen see elevated levels of atmospheric CO2 as a gift, not a threat.


Cole, C.T., J.E. Anderson, R.L. Lindroth, and D.M. Waller. 2010. Rising concentrations of atmospheric CO2 have increased growth in natural stands of quaking aspen (Populus tremuloides). Global Change Biology, 16, 2186-2197.

McGrath, J.M., D.F. Karnosky, and E.A. Ainsworth. 2010. Spring leaf flush in aspen (Populus tremuloides) clones is altered by long-term growth at elevated carbon dioxide and elevated ozone concentration. Environmental Pollution, 158, 1023-1028.

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