Increases in atmospheric CO2 have little influence on transpiration of a temperate forest canopy. Issue 2 (27th October 2014)

• Record Type: Journal Article
• Authors: Tor‐ngern, Pantana; Oren, Ram; Ward, Eric J.; Palmroth, Sari; McCarthy, Heather R.; Domec, Jean‐Christophe
• Is Part Of: New phytologist. Volume 205:Issue 2(2015)
• Page Start: 518
• Page End: 525
• Publication Date: 2014-10-27
• Subjects: Botany -- Periodicals; 580
• DOI: 10.1111/nph.13148
• Languages: English
• ISSNs: 0028-646X

Abstract

<abstract abstract-type="main" id="nph13148-abs-0001"> <title>Summary</title> <p> <list id="nph13148-list-0001" list-type="bullet"> <list-item> <p>Models of forest energy, water and carbon cycles assume decreased stomatal conductance with elevated atmospheric CO<sub>2</sub> concentration ([CO<sub>2</sub>]) based on leaf‐scale measurements, a response not directly translatable to canopies. Where canopy–atmosphere are well‐coupled, [CO<sub>2</sub>]‐induced structural changes, such as increasing leaf‐area index (<italic>L</italic><sub>D</sub>), may cause, or compensate for, reduced mean canopy stomatal conductance (<italic>G</italic><sub>S</sub>), keeping transpiration (<italic>E</italic><sub>C</sub>) and, hence, runoff unaltered.</p> </list-item> <list-item> <p>We investigated <italic>G</italic><sub>S</sub> responses to increasing [CO<sub>2</sub>] of conifer and broadleaved trees in a temperate forest subjected to 17‐yr free‐air CO<sub>2</sub> enrichment (FACE; + 200 μmol mol<sup>−1</sup>). During the final phase of the experiment, we employed step changes of [CO<sub>2</sub>] in four elevated‐[CO<sub>2</sub>] plots, separating <italic>direct</italic> response to changing [CO<sub>2</sub>] in the leaf‐internal air‐space from <italic>indirect</italic> effects of slow changes via leaf hydraulic adjustments and canopy development.</p> </list-item> <list-item> <p>Short‐term manipulations caused no <italic>direct</italic> response up to 1.8 × ambient [CO<sub>2</sub>], suggesting that the observed long‐term 21% reduction of <italic>G</italic><sub>S</sub> was an <italic>indirect</italic> effect of decreased leaf hydraulic conductance and increased leaf shading. Thus, <italic>E</italic><sub><italic>C</italic></sub> was unaffected by [CO<sub>2</sub>] because 19% higher canopy <italic>L</italic><sub>D</sub> nullified the effect of leaf hydraulic acclimation on <italic>G</italic><sub>S</sub>.</p> </list-item> <list-item> <p>We advocate long‐term experiments of duration sufficient for slow responses to manifest, and modifying models predicting forest water, energy and carbon cycles accordingly.</p> </list-item> </list> </p> </abstract>