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Photosynthetic Responses of Understory Tree Seedlings to Seven Years of Elevated CO2
Volume 10, Number 20: 16 May 2007

A popular hypothesis suggests that as higher atmospheric CO2 concentrations increase plant biomass production, the need for nutrients likewise increases - especially for nitrogen (N) - and that soil nutrients (and especially N) thus become diminished more rapidly in CO2-enriched air than in ambient air, leading to a down-regulation of leaf photosynthetic rates that gradually erodes the initial stimulus provided by the extra CO2. Known as the progressive nitrogen limitation or PNL hypothesis, it is widely invoked by climate alarmists who stridently downplay the many positive effects of the ongoing rise in the air's CO2 content in their zeal to promote the supposedly catastrophically-negative impacts they predict to arise on the basis of theoretical climate-model simulations of CO2-induced global warming.

Noting that "this type of photosynthetic down-regulation may occur in ecosystems that have a low soil N availability, such as piedomont loblolly pine forests" - which just happens to be the type of forest setting in which the long-term Duke Forest FACE study is being conducted - Springer and Thomas (2007) set out to test the validity of the PNL hypothesis at that site, initially assuming it would be proven to be correct. As they describe it, they say they "hypothesized that after seven years of exposure to elevated CO2, significant photosynthetic down-regulation would be observed in these tree species," namely, saplings of red maple (Acer rubrum L.), hickory (Carya glabra Mill.), redbud (Cercis canadensis L.) and sweetgum (Liquidambar styraciflua L.).

So what did they do? And what did they find?

Noting that during the first year of the Duke Forest FACE experiment, DeLucia and Thomas (2000) had examined the photosynthetic responses of these particular saplings to the 200-ppm increase in atmospheric CO2 concentration employed in the study, Springer and Thomas say they "reexamined the photosynthetic responses of saplings of the same four understory species to determine whether the enhancement of photosynthesis observed during the first year of exposure to elevated CO2 was sustained in the seventh year of the experiment."

This work revealed, in their words, absolutely "no evidence of photosynthetic down-regulation in any species in either early or late summer." In fact, not only did their measurements not reveal any down-regulation of photosynthesis, they say they observed "a small increase [our italics] in the photosynthetic capacity of all [our italics] of the study species in response to elevated CO2," such as they say "has been demonstrated in several studies (Campbell et al., 1988; Ziska and Teramura, 1992; Idso et al., 1991)."

In summing up the situation, Springer and Thomas once again state, in the opening sentence of the concluding paragraph of their paper, that "the progressive N limitation hypothesis predicts a diminished response of plant productivity to elevated CO2 as N availability decreases because of the increased nutrient demands of greater plant biomass production (Luo et al., 2004)," and in the final sentence of that paragraph, they reiterate that "after seven years of elevated CO2 treatment in the Duke Forest FACE experiment, we see little evidence of progressive N limitation in the leaf level processes of these four species of understory trees." In fact, they find evidence of just the opposite.

When considered in the light of the many similar findings archived under the heading of Nitrogen (Progressive Limitation Hypothesis) in our Subject Index, it is becoming ever more evident that earth's plants will not be denied the huge biological benefits already being bestowed upon them by the ongoing rise in the air's CO2 concentration, the fervid protestations of the world's climate alarmists notwithstanding.

Sherwood, Keith and Craig Idso

Campbell, W.J., Allen, L.H. and Bowes, G. 1988. Effects of CO2 concentration on rubisco activity, amount, and photosynthesis in soybean leaves. Plant Physiology 88: 1310-1316.

DeLucia, E.H. and Thomas, R.B. 2000. Photosynthetic responses to CO2 enrichment of four hardwood species in a forest understory. Oecologia 122: 11-19.

Idso, S.B., Kimball, B.A. and Allen, S.G. 1991. CO2 enrichment of sour orange trees: 2.5 years into a long-term experiment. Plant, Cell and Environment 14: 351-353.

Luo, Y., Su, B., Currie, W.S. et al. 2004. Progressive nitrogen limitation of ecosystem responses to rising atmospheric carbon dioxide. BioScience 54: 731-739.

Springer, C.J. and Thomas, R.B. 2007. Photosynthetic responses of forest understory tree species to long-term exposure to elevated carbon dioxide concentration at the Duke Forest FACE experiment. Tree Physiology 27: 25-32.

Ziska, L.H. and Teramura, A.H. 1992. Intraspecific variation in the response of rice (Oryza sativa) to increased CO2 concentration - photosynthetic, biomass, and reproductive characteristics. Physiologia Plantarum 84: 269-274.