Stomatal conductance and not stomatal density determines the long-term reduction in leaf transpiration of poplar in elevated CO2

Abstract

Using a free-air CO2 enrichment (FACE) experiment, poplar trees (Populus x euramericana clone I214) were exposed to either ambient or elevated [CO2] from planting, for a 5-year period during canopy development, closure, coppice and re-growth. In each year, measurements were taken of stomatal density (SD, number mm(-2)) and stomatal index (SI, the proportion of epidermal cells forming stomata). In year 5, measurements were also taken of leaf stomatal conductance (gs, micromol m(-2) s(-1)), photosynthetic CO2 fixation (A, mmol m(-2) s(-1)), instantaneous water-use efficiency (A/E) and the ratio of intercellular to atmospheric CO2 (Ci:Ca). Elevated [CO2] caused reductions in SI in the first year, and in SD in the first 2 years, when the canopy was largely open. In following years, when the canopy had closed, elevated [CO2] had no detectable effects on stomatal numbers or index. In contrast, even after 5 years of exposure to elevated [CO2], gs was reduced, A/E was stimulated, and Ci:Ca was reduced relative to ambient [CO2]. These outcomes from the long-term realistic field conditions of this forest FACE experiment suggest that stomatal numbers (SD and SI) had no role in determining the improved instantaneous leaf-level efficiency of water use under elevated [CO2]. We propose that altered cuticular development during canopy closure may partially explain the changing response of stomata to elevated [CO2], although the mechanism for this remains obscure.

The Simulation of the Opposing Fluxes of Latent Heat and CO2 over Various Land-Use Types: Coupling a Gas Exchange Model to a Mesoscale Atmospheric Model

“…These results agree well with other studies (e.g. Tricker et al 2005;Garcia-Amorena et al 2006). Of course, the response of the stomatal conductance to elevated atmospheric CO 2 is complex and is determined by many environmental (Konrad et al 2008) and plant internal factors (e.g.…”

“…Thus, our results are consistent with the findings of e.g. Tricker et al (2005) and Garcia-Amorena et al (2006).…”

“…It is well established that plants regulate their transpirational water loss in relation to carbon gain by a reduction in stomatal conductance in the presence of elevated CO 2 concentrations (Tricker et al 2005;Garcia-Amorena et al 2006). We now investigate whether the coupled model is able to reproduce this behaviour.…”

Responses of Legume Versus Nonlegume Tropical Tree Seedlings to Elevated CO2 Concentration

“…SD showed a mean reduction of only 6% in response to elevated [CO 2 ], and SI showed a similar mean reduction of only 5%. These results are similar to observations in temperate species, where growth at [CO 2 ] above ambient did not lead to significant reductions in SD or SI (Estiarte et al, 1994;Reid et al, 2003;Marchi et al, 2004;Tricker et al, 2005;Ainsworth and Rogers, 2007).…”

Stomatal Function across Temporal and Spatial Scales: Deep-Time Trends, Land-Atmosphere Coupling and Global Models

“…At developmental time scales, there are strong species-specific differences in sensitivity, including cases of apparent insensitivity (Reid et al, 2003;Tricker et al, 2005;Haworth et al, 2015), as well as nonlinearities in the response over both developmental and evolutionary time scales . This has been especially problematic in attempts to use the change in D in leaf fossils, alone, as a proxy for change in global mean c a (Royer, 2014).…”