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Increases in atmospheric CO 2 have little influence on transpiration of a temperate forest canopy
Author(s) -
Torngern Pantana,
Oren Ram,
Ward Eric J.,
Palmroth Sari,
McCarthy Heather R.,
Domec JeanChristophe
Publication year - 2015
Publication title -
new phytologist
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 3.742
H-Index - 244
eISSN - 1469-8137
pISSN - 0028-646X
DOI - 10.1111/nph.13148
Subject(s) - canopy , transpiration , stomatal conductance , temperate forest , leaf area index , temperate rainforest , environmental science , atmospheric sciences , temperate climate , acclimatization , atmosphere (unit) , carbon cycle , photosynthesis , botany , ecology , biology , ecosystem , meteorology , geology , physics
Summary Models of forest energy, water and carbon cycles assume decreased stomatal conductance with elevated atmospheric CO 2 concentration ([CO 2 ]) based on leaf‐scale measurements, a response not directly translatable to canopies. Where canopy–atmosphere are well‐coupled, [CO 2 ]‐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 [CO 2 ] of conifer and broadleaved trees in a temperate forest subjected to 17‐yr free‐air CO 2 enrichment (FACE; + 200 μmol mol −1 ). During the final phase of the experiment, we employed step changes of [CO 2 ] in four elevated‐[CO 2 ] plots, separating direct response to changing [CO 2 ] 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 [CO 2 ], 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 [CO 2 ] 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.