title: Increases in atmospheric CO2 have little influence on transpiration of a temperate forest canopy.
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-air CO2 enrichment (FACE; + 200 μmol mol(-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 1.8 × 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 . We advocate long-term experiments of duration sufficient for slow responses to manifest, and modifying models predicting forest water, energy and carbon cycles accordingly.
authors.affiliation: Nicholas School of the Environment and Earth Sciences, Duke University, Durham, NC, 27708, USA; Department of Forest Ecology and Management, Swedish University of Agricultural Sciences (SLU), SE-901 83, Umeå, Sweden.
authors.forename: Pantana
authors.initials: P
authors.lastname: Tor-ngern
authors.forename: Ram
authors.initials: R
authors.lastname: Oren
authors.forename: Eric J
authors.initials: EJ
authors.lastname: Ward
authors.forename: Sari
authors.initials: S
authors.lastname: Palmroth
authors.forename: Heather R
authors.initials: HR
authors.lastname: McCarthy
authors.forename: Jean-Christophe
authors.initials: JC
authors.lastname: Domec
mesh_terms: D001272:Atmosphere / Q000737:chemistry; D002245:Carbon Dioxide / Q000378:metabolism*; D065928:Forests; D008954:Models, Biological; D054046:Plant Stomata / Q000378:metabolism; D018526:Plant Transpiration
chemical_list: D002245:Carbon Dioxide
keywords: Liquidambar styraciflua; Pinus taeda; canopy stomatal conductance; elevated CO2; free-air CO2 enrichment (FACE); transpiration