What especially aggravates their angst in this regard is the fact that "clear evidence in support of this 'progressive nitrogen limitation' (PNL) hypothesis has not emerged to date (see e.g. Moore et al., 2006)," plus the fact that "it is well established that elevated CO2 can increase net primary productivity, even in ecosystems where nitrogen supply is demonstrably limiting to plant growth (e.g. Lloyd and Farquhar, 1996, 2000; Nowak et al., 2004." Consequently, the two researchers set out to once again demonstrate (as has been done multiple times before) the consistent and enduring positive growth response of entire ecosystems to atmospheric CO2 enrichment over prolonged periods of time, which phenomenon is the absolute antithesis of the progressive nitrogen limitation hypothesis.

What Prentice and Harrison did was examine various aspects of the palaeorecord and see if they were either consistent or inconsistent with the PNL hypothesis. In doing so, they determined that (1) "reduced terrestrial carbon storage during glacials, indicated by the shift in stable isotope composition of dissolved inorganic carbon in the ocean, cannot be explained by climate or sea-level changes," but that it is "consistent with predictions of current process-based models that propagate known physiological CO2 effects into net primary production at the ecosystem scale," and that (2) "restricted forest cover during glacial periods, indicated by pollen assemblages dominated by non-arboreal taxa, cannot be reproduced accurately by palaeoclimate models unless CO2 effects on C3-C4 plant competition are also modeled." And as a result of these observations, the two scientists say they "do not find support for the opinion (e.g. Korner, 2000)" - which "questions the relevance of plant-physiological effects of CO2 over the long term and at the ecosystem scale" - "that other constraints [such as low soil nitrogen concentrations] effectively eliminate the ecosystem-level effects of changing CO2 concentration on carbon storage over long time scales," further concluding that "the palaeo-record also supports the attribution of increases in the woody component of tropical savannas to physiological effects of rising CO2."

These findings, as well as those of many other researchers that are documented in reviews of their work archived under the heading of Nitrogen (Progressive Limitation Hypothesis) in our Subject Index, totally suck the wind out of the sails of the recent theoretical model study of Thornton et al. (2009), which has been touted by climate alarmists as suggesting that the aerial fertilization effect of the ongoing rise in the air's CO2 content will not allow earth's vegetation to extract as much carbon from the atmosphere as real-world experiments indicate it will. Much to the contrary, the growth-promoting effect of the upward trend in the atmosphere's CO2 concentration is here to stay; and it will only increase in prowess as the air's CO2 content continues to rise.

Sherwood, Keith and Craig Idso

References
Idso, S.B. 1989. A problem for paleoclimatology? Quaternary Research 31: 433-434.

Korner, C. 2000. Biosphere responses to CO2 enrichment. Ecological Applications 10: 1590-1619.

Lloyd, J. and Farquhar, G.D. 1996. The CO2 dependence of photosynthesis, plant growth responses to elevated CO2 concentrations and their interactions with soil nutrient status. I. General principles and forest ecosystems. Functional Ecology 10: 4-32.

Lloyd, J. and Farquhar, G.D. 2000. Do slow-growing species and nutrient-stressed plants consistently respond less to elevated CO2? A clarification of some issues raised by Poorter (1998). Global Change Biology 6: 871-876.

Moore, D.J.P., Aref, S., Ho, R.M., Pippen, J.S., Hamilton, J. and DeLucia, E.H. 2006. Inter-annual variation in the response of Pinus taeda tree growth to long term Free Air Carbon dioxide Enrichment (FACE). Global Change Biology 12: 1367-1377.

Nowak, R.S., Ellsworth, D.A. and Smith, S.D. 2004. Functional responses of plants to elevated atmospheric CO2 - Do photosynthetic and productivity data from FACE experiments support early predictions? New Phytologist 162: 253-280.

Prentice, I.C. and Harrison, S.P. 2009. Ecosystem effects of CO2 concentration: evidence from past climates. Climate of the Past 5: 297-307.

Thornton, P.E., Doney, S.C., Lindsay, K., Moore, J.K., Mahowald, N., Randerson, J.T., Fung, I., Lamarque, J.-F., Feddema, J.J. and Lee, Y.-H. 2009. Carbon-nitrogen interactions regulate climate-carbon cycle feedbacks: results from an atmosphere-ocean general circulation model. Biogeosciences 6: 2120-2120.