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 C₃ 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 ∼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.