Abstract
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 {}ified 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.
Citation
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
