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Increases in atmospheric CO(2) have little influence on transpiration of a temperate forest canopy
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Operated in cooperation with Brookhaven National Laboratory (K. Lewin and J. Nagy), the Duke Forest FACE project is supported by the Office of Science (BER) Terrestrial Ecosystem Sciences (TES) Program of US Department of Energy (DOE), which also partially sponsored R.O. through DE-SC0006967. Partial support for P.T. was from the Royal Thai Government, and for P.T., R.O. and S.P. from the Swedish programs Trees and Crops for the Future (TC4F; SLU) and Nitrogen and Carbon in Forests (Nicaf; the research council Formas). R.O. was also sponsored by the US Department of Agriculture through the Agriculture and Food Research Initiative (2011-67003-30222), and S.P. by DOE-BER, TES (DE-SC-0006700-11-ER65189). J-C.D. acknowledges support from the French Research Agency (ANR-project MACACC).. International audience. Models of forest energy, water and carbon cycles assume decreased stomatal conductance with elevated atmospheric CO2 concentration ([CO2]) based on leaf-scale measurements, a response not directly translatable to canopies. Where canopy-atmosphere are well-coupled, [CO2]-induced structural changes, such as increasing leaf-area index (L-D), may cause, or compensate for, reduced mean canopy stomatal conductance (G(S)), keeping transpiration (E-C) and, hence, runoff unaltered. We investigated G(S) responses to increasing [CO2] of conifer and broadleaved trees in a temperate forest subjected to 17-yr free-air CO2 enrichment (FACE; +200molmol(-1)). During the final phase of the experiment, we employed step changes of [CO2] in four elevated-[CO2] plots, separating direct response to changing [CO2] in the leaf-internal air-space from indirect effects of slow changes via leaf hydraulic adjustments and canopy development. Short-term manipulations caused no direct response up to 1.8xambient [CO2], suggesting that the observed long-term 21% reduction of G(S) was an indirect effect of decreased leaf hydraulic conductance and increased leaf shading. Thus, E-C was unaffected by [CO2] because 19% higher canopy L-D nullified the effect of leaf hydraulic acclimation on G(S). We advocate long-term experiments of duration sufficient for slow responses to manifest, and modifying models predicting forest water, energy and carbon cycles accordingly.
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