Elevated [CO2] and forest vegetation: more a water issue than a carbon issue?
Joseph A. M. Holtum A C and Klaus Winter BA School of Marine and Tropical Biology, James Cook University, Townsville, Qld 4811, Australia.
B Smithsonian Tropical Research Institute, PO Box 0843-03092, Balboa, Ancon, Republic of Panama.
C Corresponding author. Email: joseph.holtum@jcu.edu.au
Functional Plant Biology 37(8) 694-702 https://doi.org/10.1071/FP10001
Submitted: 2 January 2010 Accepted: 30 March 2010 Published: 26 July 2010
Abstract
Studies of responses of forest vegetation to steadily increasing atmospheric concentrations of CO2 have focussed strongly on the potential of trees to absorb extra carbon; the effects of elevated [CO2] on plant–soil water relations via decreased stomatal conductance and increased ambient temperature have received less attention, but may be significant in the long term at the ecosystem level. CO2 augmentation experiments with young trees demonstrate small increases in aboveground carbon content, but these increases tend to diminish as trees get older. By contrast, several experiments suggest continued decreases in transpiration and increased soil water content under these conditions. In tropical forests, the major cause of increases in aboveground biomass observed in the recent past is not necessarily elevated [CO2]. Undoubtedly, the potential of monitoring trees in forest dynamics plots to deduce CO2-specific alterations in forest structure and standing biomass will unfold in the decades to come. The comprehensive understanding of responses of forest vegetation to elevated [CO2] in the Anthropocene will depend upon the inclusion of detailed measurements of soil water pools and water fluxes through the soil–plant–atmosphere continuum in future tree CO2 augmentation experiments and forest dynamics plot studies.
Additional keywords: climate change, carbon sequestration, evapotranspiration, FACE (free air CO2 enrichment), water use efficiency.
Acknowledgements
Neal Smith, STRI Emeritus Scientist, persistently prodded us to write this. Support is acknowledged from the Smithsonian Tropical Research Institute (KW and JAMH), the JCU Special Studies Program (JAMH) and the Reverend Dr RG Dunn (JAMH).
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