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Water-use responses of 'living fossil' conifers to CO₂ enrichment in a simulated Cretaceous polar environment

Llorens, Laura, Osborne, Colin P., Beerling, David J.
Annals of botany 2009 v.104 no.1 pp. 179-188
Metasequoia glyptostroboides, Sequoia sempervirens, Taxodium distichum, carbon, coniferous forests, conifers, gas exchange, global warming, growing season, isotopes, leaves, seasonal variation, summer, transpiration, water use efficiency
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 δ¹³C 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.