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Another Strike Against the Progressive Nitrogen Limitation Hypothesis
Kubiske, M.E., Quinn, V.S., Heilman, W.E., McDonald, E.P., Marquardt, P.E., Teclaw, R.M., Friend, A.L. and Karnosky, D.F. 2006. Interannual climatic variation mediates elevated CO2 and O3 effects on forest growth. Global Change Biology 12: 1054-1068.

The authors state that "an important question is whether CO2 fertilization ... [is] sustained over time," noting that "decreases in CO2-induced growth stimulation are widely hypothesized to be caused by accelerated exploitation of soil resources, particularly N and water, by more rapidly growing plants, and by negative feedbacks on soil N processing."

What was done
In a study designed primarily to ascertain the effects of increased atmospheric CO2 and O3 concentrations on trembling aspen (Populus tremuloides Michx.), Kubiske et al. analyzed reams of data acquired from the Aspen FACE experiment, which was initiated in 1996 on a 32-ha USDA Forest Service experimental farm in Wisconsin, USA, and in which the air's CO2 concentration was enriched to a target value of 560 ppm and O3 concentrations were elevated to 1.5 times that of ambient during daylight hours only.

What was learned
The eight researchers found that individual tree and stand growth were significantly increased by the elevated CO2 treatment but decreased by the elevated O3 treatment, while the two effects negated each other for no net change in the combined CO2 plus O3 treatment. However, they also found that "growth in elevated CO2 continued to increase each year but at a decreasing rate [our italics]," such that "the annual growth increases under elevated CO2 became smaller with each successive year." Hence, they examined several possible explanations for this phenomenon, including N limitations and water limitations. In doing so, they found that "interannual variation in soil moisture did not modify the CO2 or O3 responses," and that "N limitations on growth did not differ among treatments." Also, they report that "root-specific uptake of nitrate or ammonium was not affected by elevated CO2 or O3." What they did find, however, was that the growth response to elevated CO2 "paralleled decreasing July PPF [photosynthetic photon flux] from 2001 through 2004, and decreasing previous October temperatures from 2001 to 2003."

What it means
Kubiske et al. concluded that "a several-year trend of increasingly cloudy summers and cool autumns were responsible for the decrease in CO2 growth response," explaining that "July PPF directly influences the amount of photosynthate available for stem volume growth," and that "October temperature in the north-temperate latitudes is of major importance in the photosynthetic activity of trees before leaf senescence," the stored products of which are used "to support the determinate growth phase the following year." Hence, in another blow to the Progressive Nitrogen Limitation hypothesis, they say they "found no evidence that growth responses to CO2 declined systematically with increasing N or water limitations."

Reviewed 30 August 2006