AOBPreview published online on May 15, 2009
Annals of Botany, doi:10.1093/aob/mcp108
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Water-use responses of ‘living fossil’ conifers to CO2 enrichment in a simulated Cretaceous polar environment
Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK
* For correspondence. Present address: Department of Environmental Sciences, Faculty of Sciences, University of Girona, Campus Montilivi, E-17071, Girona, Spain. E-mail laura.llorens{at}udg.edu
Received: 12 December 2008 Returned for revision: 3 March 2009 Accepted: 31 March 2009
Background and Aims: During the Mesozoic, the polar regions supported coniferous forests that experienced warm climates, a CO2-rich atmosphere and extreme seasonal variations in daylight. How the interaction between the last two factors might have influenced water use of these conifers was investigated. An experimental approach was used to test the following hypotheses: (1) the expected beneficial effects of elevated [CO2] on water-use efficiency (WUE) are reduced or lost during the 24-h light of the high-latitude summer; and (2) elevated [CO2] reduces plant water use over the growing season.
Methods: Measurements of leaf and whole-plant gas exchange, and leaf-stable carbon isotope composition were made on one evergreen (Sequoia sempervirens) and two deciduous (Metasequoia glyptostroboides and Taxodium distichum) ‘living fossil’ coniferous species after 3 years' growth in controlled-environment simulated Cretaceous Arctic (69°N) conditions at either ambient (400 µmol mol–1) or elevated (800 µmol mol–1) [CO2].
Key Results: Stimulation of whole-plant WUE (WUEP) by CO2 enrichment was maintained over the growing season for the three studied species but this pattern was not reflected in patterns of WUE inferred from leaf-scale gas exchange measurements (iWUEL) and 
13C of foliage (tWUEL). This response was driven largely by increased rates of carbon uptake, because there was no overall CO2 effect on daily whole-plant transpiration or whole-plant water loss integrated over the study period. Seasonal patterns of tWUEL differed from those measured for iWUEL. The results suggest caution against over simplistic interpretations of WUEP based on leaf isotopic composition.
Conclusions: The data suggest that the efficiency of whole-tree water use may be improved by CO2 enrichment in a simulated high-latitude environment, but that transpiration is relatively insensitive to atmospheric CO2 in the living fossil species investigated.
Key words: Water-use efficiency, elevated CO2, living fossil plants, conifers, paleoecology, ancient polar forests, stable carbon isotopes, stomatal conductance, canopy transpiration