Stomatal conductance and not stomatal density determines the long-term reduction in leaf transpiration of poplar in elevated CO sub(2)

• Published in: Oecologia 2005-05, Vol.143 (4), p.652-660
• Main Authors: Tricker, Penny J, Trewin, Harriet, Kull, Olevi, Clarkson, Graham JJ, Eensalu, Eve, Tallis, Matthew J, Colella, Alessio, Doncaster, CPatrick, Sabatti, Maurizio, Taylor, Gail
• Format: Article
• Language: English
• Subjects: Populus euramericana
• ISSN: 0029-8549; 1432-1939
• DOI: 10.1007/s00442-005-0025-4

Using a free-air CO sub(2) enrichment (FACE) experiment, poplar trees (Populus x euramericana clone I214) were exposed to either ambient or elevated [CO sub(2)] 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 super(-2)) and stomatal index (SI, the proportion of epidermal cells forming stomata). In year 5, measurements were also taken of leaf stomatal conductance (g sub(s), mu mol m super(-2) s super(-1)), photosynthetic CO sub(2) fixation (A, mmol m super(-2) s super(-1)), instantaneous water-use efficiency (A/E) and the ratio of intercellular to atmospheric CO sub(2) (C sub(i):C sub(a)). Elevated [CO sub(2)] 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 [CO sub(2)] had no detectable effects on stomatal numbers or index. In contrast, even after 5 years of exposure to elevated [CO sub(2)], g sub(s) was reduced, A/E was stimulated, and C sub(i):C sub(a) was reduced relative to ambient [CO sub(2)]. 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 [CO sub(2)]. We propose that altered cuticular development during canopy closure may partially explain the changing response of stomata to elevated [CO sub(2)], although the mechanism for this remains obscure.