On the relationship between stomatal characters and atmospheric CO 2

Leaf stomatal characters influence the response of terrestrial evapotranspiration to climate change and are used as proxies for the reconstruction of past atmospheric [CO 2 ]. We examined the phenotypic response of stomatal index (SI), density (SD) and aperture (AP) to rising atmospheric CO 2 in 15 species after four years exposure to a field CO 2 gradient (200 to 550 μmol mol −1 atmospheric [CO 2 ]) or at three Free Air CO 2 Enrichment (FACE) sites. Along the CO 2 gradient, SI and SD showed no evidence of a decline to increasing [CO 2 ], while AP decreased slightly. There was no significant change in SI, SD or AP with CO 2 across FACE experiments. Without evolutionary changes, SI and SD may not respond to atmospheric [CO 2 ] in the field and are unlikely to decrease in a future high CO 2 world.