Volume 14, Number 34: 24 August 2011
In an important paper recently published in New Phytologist, Prentice et al. (2011) write that "whatever their causes, CO2 concentration changes during well-documented periods of recent earth history provide opportunities to investigate the effects of CO2 on the biosphere," and they say that the most comprehensive palaeodata describing the nature of earth's biosphere during an especially cold period are those pertaining to the last glacial maximum (LGM) of 20-26.5 ka (thousand years ago), citing the study of Clark et al. (2009). And comparing those data with simulations of global gross primary production at that time, as calculated by a dynamic global vegetation model driven by ice-core-derived atmospheric CO2 concentrations and LGM climate simulations provided by the Palaeoclimate Modeling Intercomparison Project, they find that modeled global gross primary production was reduced by 27-36% relative to that characteristic of the pre-industrial Holocene (PIH).
In discussing the modeled reduction in forest area at the LGM relative to that of the PIH, which is largely responsible for this reduction in global vegetative productivity, the three researchers state that it is "broadly consistent with pollen records." And they additionally note that "models forced by LGM climate alone produce an insufficient reduction of total land biosphere carbon storage, compared with the evidence from the marine δ13C record," citing the work of Prentice and Harrison (2009). Thus, they conclude that "it is necessary to invoke ecophysiological CO2 effects in order to account for the reduction in the tropical forest area at [the] LGM as shown by pollen data," which effects are the aerial fertilization and water-use-efficiency-promoting effects of atmospheric CO2 enrichment working in reverse, as the air's CO2 content declines when moving backwards in time from the PIH to the LGM.
These observations and their implications come to bear on what Prentice et al. call the current "disputed phenomena" of "woody thickening (the observed increase in woody plant cover in savannas across the global tropics and subtropics) and the terrestrial uptake of anthropogenic CO2 (the net land uptake of CO2, which continues despite tropical deforestation releasing CO2)." We and many others have long attributed these historical and ongoing phenomena to the aerial-fertilization and water-use-efficiency-enhancing effects of atmospheric CO2 enrichment, as the air's CO2 content has risen from pre-industrial times to the present, while others have argued otherwise, invoking what they call the progressive nitrogen limitation hypothesis. However, numerous scientific papers based on real-world data have demonstrated this hypothesis to be seriously flawed, as may be seen by perusing the many items we have archived on our website under the heading of Nitrogen (Progressive Limitation Hypothesis) in our Subject Index. In addition, Prentice et al. write that the evidence from the LGM "is inconsistent with the view espoused, for example, by Korner (2006) that the ecosystem-level response of carbon uptake and storage to CO2 concentration on long time scales must be negligible because of the constraint provided by the stoichiometry of plant biomass."
In concluding their report, Prentice et al. state that "the agreement between palaeodata and model results for the LGM is consistent with the hypothesis that the ecophysiological effects of CO2 influence tree-grass competition and vegetation productivity, and suggests that these effects are also at work today." And, we would note, literally thousands of experimental observations archived in our website's Plant Growth Database suggest that these phenomena will continue to promote the productivity of the planet's vegetation -- and especially that of its woody plants -- for decades and centuries to come.
Sherwood, Keith and Craig Idso
References
Clark, P.U., Dyke, A.S., Shakun, J.D., Carlson, A.E., Clark, J., Wohlfarth, B., Mitrovica, J.X., Hostetler, S.W. and McCabe, A.M. 2009. The last glacial maximum. Science 325: 710-714.
Korner, C. 2006. Plant CO2 responses: an issue of definition, time and resource supply. New Phytologist 172: 393-411.
Prentice, I.C. and Harrison, S.P. 2009. Ecosystem effects of CO2 concentration: evidence from past climates. Climates of the Past 5: 297-307.
Prentice, I.C., Harrison, S.P. and Bartlein, P.J. 2011. Global vegetation and terrestrial carbon cycle changes after the last ice age. New Phytologist 189: 988-998.