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Plants of Today (and Even More So of Tomorrow): Free at Last!
Volume 15, Number 23: 6 June 2012

In an illuminating Commentary article in a recent issue of New Phytologist, Tissue and Lewis (2012) write that "atmospheric CO2 over the past 800,000 years has varied generally as a function of glacial periods, with minima (c. 170-200 ppm) during glacial periods and maxima (c. 280-300 ppm) during inter-glacial periods," citing Luthi et al. (2008). More specifically, they indicate that "during the Last Glacial Maximum (LGM, 18,000-20,000 years ago), atmospheric CO2 ranged from 180-200 ppm, which is approximately half the current CO2 (392 ppm), and among the lowest CO2 observed during the evolution of vascular land plants over the past 350 million years [italics and bold added]." Therefore, as the Beatles once musically lamented about temperature - "it's been a long cold lonely winter" - one could surely state the analogous about the atmosphere's long-term CO2 concentration; for as Tissue and Lewis continue ...

"Glacial plants were severely carbon limited over a very long time period, until atmospheric CO2 began rising during the glacial-interglacial transition." In fact, they indicate that "controlled environment studies with modern plants grown in glacial CO2" have shown "significant carbon limitations on plant physiology even when other resources were generally not limiting [italics added]," citing Dippery et al. (1995) and Tissue et al. (1995). So in spite of anything one could have done to enhance their productivity (other than supply them with more CO2), glacial-age plants simply could not produce the bounty that today's plants do. In fact, they were fortunate to merely survive.

On the other hand, earth's vegetation is not in "plant heaven" yet, for as Tissue and Lewis continue, "studies suggest that as CO2 rises from glacial to future levels, the limitation imposed by CO2 on growth and physiology becomes secondary to other environmental factors, such as temperature and drought." And so it does; but with more CO2 in the air, plants have been proven to be better able to deal with these stresses than they were over the prior 350 million years. In the case of rising temperatures, for example, there is abundant experimental evidence that at higher atmospheric CO2 concentrations plants actually prefer higher temperatures (see the many pertinent items we have archived under Growth Response to CO2 with Other Variables: Temperature in our Subject Index). And with respect to the stress of drought, plants are much better equipped to deal with it now-a-days in light of the increased water use efficiency that they typically exhibit in CO2-enriched air (see Growth Response to CO2 with Other Variables: Water Stress in our Subject Index).

Yes, earth's plants are gradually being freed from the environmental fetters that have for so long held them back and prevented them from exhibiting their true productivity potential, thanks to the life-giving carbon dioxide that has been emitted to the atmosphere by mankind's historic and ongoing burning of fossil fuels. Long may the trend continue!

Sherwood, Keith and Craig Idso

References
Dipery, J.K., Tissue, D.T., Thomas, R.B. and Strain, B.R. 1995. Effects of low and elevated CO2 on C3 and C4 annuals. I. Growth and biomass allocation. Oecologia 101: 13-20.

Luthi, D., Le Floch, M., Bereiter, B., Blunier, T., Barnola, J.-M., Siegenthaler, U., Raynaud, D., Jouzel, J., Fischer, H., Kawamura K. and Stocker, T.F. 2008. High-resolution carbon dioxide concentration record 650,000-800,000 years before present. Nature 453: 379-382.

Tissue, D.T., Griffin, K.L., Thomas, R.B. and Strain, B.R. 1995. Effects of low and elevated CO2 on C3 and C4 annuals. II. Photosynthesis and leaf biochemistry. Oecologia 101: 21-28.

Tissue, D.T. and Lewis, J.D. 2012. Learning from the past: how low [CO2] studies inform plant and ecosystem response to future climate change. New Phytologist 194: 4-6.