Convergence in Maximum Stomatal Conductance of C3 Woody Angiosperms in Natural Ecosystems Across Bioclimatic Zones

Stomatal conductance (g(s)) in terrestrial vegetation regulates the uptake of atmospheric carbon dioxide for photosynthesis and water loss through transpiration, closely linking the biosphere and atmosphere and influencing climate. Yet, the range and pattern of g(s) in plants from natural ecosystems across broad geographic, climatic, and taxonomic ranges remains poorly quantified. Furthermore, attempts to characterize g(s) on such scales have predominantly relied upon meta-analyses compiling data from many different studies. This approach may be inherently problematic as it combines data collected using unstandardized protocols, sometimes over decadal time spans, and from different habitat groups. Using a standardized protocol, we measured leaf-level g(s) using porometry in 218 C-3 woody angiosperm species in natural ecosystems representing seven bioclimatic zones. The resulting dataset of 4273 g(s) measurements, which we call STraits (Stomatal Traits), was used to determine patterns in maximum g(s) (g(smax)) across bioclimatic zones and whether there was similarity in the mean g(smax) of C3 woody angiosperms across ecosystem types. We also tested for differential g(smax) in two broadly defined habitat groups - open-canopy and understory-subcanopy - within and across bioclimatic zones. We found strong convergence in mean g(smax) of C3 woody angiosperms in the understory-subcanopy habitats across six bioclimatic zones, but not in open-canopy habitats. Mean g(smax) in open-canopy habitats (266 +/- 100 mmol m(-2) s(-1)) was significantly higher than in understory-subcanopy habitats (233 +/- 86 mmol m(-2) s(-1)). There was also a central tendency in the overall dataset to operate toward a g(smax) of similar to 250 mmol m(-2) s(-1). We suggest that the observed convergence in mean g(smax) of C3 woody angiosperms in the understory-subcanopy is due to a buffering of g(smax) against macroclimate effects which will lead to differential response of C3 woody angiosperm vegetation in these two habitats to future global change. Therefore, it will be important for future studies of g(smax) to categorize vegetation according to habitat group.