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Title: Increases in atmospheric CO 2 have little influence on transpiration of a temperate forest canopy

Author: Tor-ngern, Pantana; Oren, Ram; Ward, Eric J.; Palmroth, Sari; McCarthy, Heather R.; Domec, Jean-Christophe;

Date: 2015

Source: New Phytologist

Publication Series: Scientific Journal (JRNL)

Description:  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 (LD), may cause, or compensate for, reduced mean canopy stomatal conductance (GS), keeping transpiration (EC) and, hence, runoff unaltered.  We investigated GS responses to increasing [CO2] of conifer and broadleaved trees in a temperate forest subjected to 17-yr free-airCO2 enrichment (FACE; + 200 lmol mol1). 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 1.8 9 ambient [CO2], suggesting that the observed long-term 21% reduction of GS was an indirect effect of decreased leaf hydraulic conductance and increased leaf shading. Thus, EC was unaffected by [CO2] because 19% higher canopy LD nullified the effect of leaf hydraulic acclimation on GS.  Weadvocate long-term experiments of duration sufficient for slow responses to manifest, and modifying models predicting forest water, energy and carbon cycles accordingly.

Keywords: canopy stomatal conductance, elevated CO2, free-air CO2 enrichment(FACE), Liquidambar styraciflua, Pinus taeda, transpiration.

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Tor-ngern, Pantana; Oren, Ram; Ward, Eric J.; Palmroth, Sari; McCarthy, Heather R.; Domec, Jean-Christophe 2014. Increases in atmospheric CO 2 have little influence on transpiration of a temperate forest canopy . New Phytologist. 205(2): 518-525. 8 p. 10.1111/nph.13148

 


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