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Past times in polar climes
In discussing global climate change it is easy to forget that in past eras atmospheric CO2 concentrations and temperature have been much higher than at present. During the Jurassic and Cretaceous periods, the atmospheric CO2 concentration was twice the present value, and global temperatures were high enough for forests to grow in the polar regions, certainly as far north or south as 85 degrees. Did the continuous daylight of the polar summer lead to photo-inhibition or did the higher CO2 concentrations help to counteract this? And what happened during the long darkness of the polar winter? Beerling and Osborne (University of Sheffield, pp. 329-339) investigated these problems using modern species most closely related to particular Mesozoic tree species. Interestingly, for most of the species investigated, doubling the CO2 concentration under constant light (mimicking the polar summer) had no effect on net photosynthesis but did lead to a stimulation of photosynthesis in shorter autumn days. It was concluded that in continuous light factors other than CO2 may be limiting. Furthermore, for all of the species, at either normal or elevated CO2 concentrations, photo-inhibition progressively increased in the simulated polar summer but was reversed as days shortened. So, higher CO2 concentrations did not protect against photo-inhibition, at least as judged from present-day species. Moving on to consider the effects of continuous darkness (c.f. the polar winter), it was interesting that elevated CO2 concentrations inhibited dark respiration by up to 60 %, suggesting a selection for mechanisms that prevent excessive loss of carbon when photosynthesis is impossible. Results such as these, combined with computer simulations, may help to predict changes in growth and productivity resulting from climatic change (even if it is unlikely that the current trends to warming will be dramatic enough to permit the growth of forests at very high latitudes!).
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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