Annals of Botany 89: 329-339, 2002
© 2002 Annals of Botany Company
Physiological Ecology of Mesozoic Polar Forests in a High CO2 Environment
1Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK
* For correspondence. E-mail d.j.beerling{at}sheffield.ac.uk
Received: 2 August 2001; Returned for revision: 10 October 2001; Accepted: 30 November 2001.
Fossils show that coniferous forests extended into polar regions during the Mesozoic, a time when models and independent palaeo-CO2 indicators suggest that the atmospheric CO2 concentration was at least double that of the present day. Consequently, such polar forests would have experienced high CO2 interacting with an extreme variation in light. Here we describe an experiment investigating this plant–environment interaction for extant tree species that were important components of polar forests, and give results from the first year of treatment. Specifically, we tested the hypotheses that growth in elevated CO2 (1) stimulates photosynthesis; (2) reduces photoinhibition during the polar summer; and (3) reduces respiration of above- and below-ground plant organs. Our results indicate that CO2 fertilization generally does not affect photosynthesis under continuous daylight characteristic of the polar summer but does increase it when the period of illumination is shorter. Growth in elevated CO2 did not alter the potential for photoinhibition. CO2 enrichment significantly reduced leaf and root respiration rates by 50 and 25 %, respectively, in a range of evergreen taxa. Incorporating these observed CO2 effects into numerical simulations using a process-based model of coniferous forest growth indicates that a high palaeo-CO2 concentration would have increased the productivity of Cretaceous conifer forests in northern Alaska. This results from decreased respiratory costs that more than compensate for the absence of high CO2–high temperature interactions during the polar summer. The longer-term effects of CO2 enrichment on seasonal changes in the above- and below-ground carbon balance of trees are discussed.
Key words: Atmospheric CO2, carbohydrates, fossil plants, photosynthesis, photoinhibition.
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