Submitted to: Geophysical Research Letters
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/5/2003
Publication Date: 10/4/2003
Citation: Reid, C.D., Maherali, H., Johnson, H.B., Smith, S.D., Wullschleger, S.D., Jackson, R.B. 2003. On the relationship between stomatal characters and atmospheric CO2. Geophysical Research Letters. 30:1983. Interpretive Summary: Rising atmospheric CO2 is the touchstone measure of global change. Plant stomatal density (stomata per mm2) and/or stomatal index (ratio of stomata to all epidermal cells) are being used as proxies for estimating CO2 concentrations of the past. This study is intended to test the veracity of this practice. The CO2/stomatal hypothesis holds that as CO2 in the air increases the number of stomata on leaves, decreases. The rational is that stomata, the small pores in plant leaves through which carbon dioxide enters for photosynthesis, are required in fewer numbers as the supply of CO2 in the air increases. We examined the stomatal properties of 15 field-grown plant species, 7 from a gradient chamber with CO2 spanning the range from the Ice Age to future mid century projections (200 ppm to 550 ppm) and 8 from paired elevated and ambient plots of FACE (Free Air CO2 Enrichment) experiments. No significant reductions in stomata with rising CO2 were observed in any of the species. Species from deserts, grasslands, and forests were represented in the sampling and included annuals, perennial forbs, grasses and trees. These results are contrary to the CO2/stomata hypothesis and the observations upon which it depends and suggests that stronger field-based studies are needed before the CO2/stomata hypothesis is incorporated into models for predicting atmospheric CO2 concentrations of the past.
Technical Abstract: Leaf stomatal characters influence the response of terrestrial evapotranspiration to climate change and are used as proxies for the reconstruction of past atmospheric [CO2]. We examined the phenotypic response of stomatal index (SI), density (SD) and aperture (AP) to rising atmospheric CO2 in 15 species after four years exposure to a field CO2 gradient (200 to 550 umol mol-1 atmospheric [CO2]) or at three Free Air CO2 Enrichment (FACE) sites. Along the CO2 gradient, SI and SD showed no evidence of a decline to increasing [CO2], while AP decreased slightly. There was no significant change in SI, SD or AP with CO2 across FACE experiments. Without evolutionary changes, SI and SD may not respond to atmospheric [CO2] in the field and are unlikely to decrease in a future high CO2 world.