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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 s g smax s g smax of C3 woody angiosperms across ecosystem types. We also tested for differential g s 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.

Vascular epiphytes are a major biomass component of forests across the globe and they contribute to 9% of global vascular plant diversity. To improve our understanding of the whole-plant response of epiphytes to future climate change, we investigated for the first time both individual and combined effects of elevated CO 2 (560 ppm) and light on the physiology and growth of two epiphyte species [ Tillandsia brachycaulos (CAM) and Phlebodium aureum (C3)] grown for 272 days under controlled conditions. We found that under elevated CO 2 the difference in water loss between the light (650 μmol m -2 s -1 ) and shade (130 μmol m -2 s -1 ) treatment was strongly reduced. Stomatal conductance ( g s ) decreased under elevated CO 2 , resulting in an approximate 40-45% reduction in water loss over a 24 h day/night period under high light and high CO 2 conditions. Under lower light conditions water loss was reduced by approximately 20% for the CAM bromeliad under elevated CO 2 and increased by approximately 126% for the C3 fern. Diurnal changes in leaf turgor and water loss rates correlated strong positively under ambient CO 2 (400 ppm) and high light conditions. Future predicted increases in atmospheric CO 2 are likely to alter plant water-relations in epiphytes, thus reducing the canopy cooling potential of epiphytes to future increases in temperature.