Intrinsic water use efficiency (iWUE), defined as the ratio of photosynthesis to stomatal conductance, is a key variable in plant physiology and ecology. Yet, how rising atmospheric CO2 concentration affects iWUE at broad species and ecosystem scales is poorly understood. In a field-based study of 244 woody angiosperm species across eight biomes over the past 25 years of increasing atmospheric CO2 (~45 ppm), we show that iWUE in evergreen species has increased more rapidly than in deciduous species. Specifically, the difference in iWUE gain between evergreen and deciduous taxa diverges along a mean annual temperature gradient from tropical to boreal forests and follows similar observed trends in leaf functional traits such as leaf mass per area. Synthesis of multiple lines of evidence supports our findings. This study provides timely insights into the impact of Anthropocene climate change on forest ecosystems and will aid the development of next-generation trait-based vegetation models.

The results contradict the over-simplistic notion that global vegetation always responds with decreasing gs to elevated CO2, a finding that has important implications for predicting future vegetation feedbacks on the hydrological cycle at the regional level.

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 ∼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.

Premise of research. Understanding the relationship between field-measured operating stomatal conductance (g op ) and theoretical maximum stomatal conductance (g max ), calculated from stomatal density and geometry, provides an important framework that can be used to infer leaf-level gas exchange of historical, herbarium, and fossil plants. To date, however, investigation of the nature of the relationship between g op and theoretical g max remains limited to a small number of experiments on relatively few taxa and is virtually undefined for plants in natural ecosystems.Methodology. We used the g op measurements of 74 species and 35 families across four biomes from a published contemporary data set of field-measured leaf-level stomatal conductance in woody angiosperms and calculated the theoretical g max from the same leaves to investigate the relationship between g op and g max across multiple species and biomes and determine whether such relationships are widely conserved.Pivotal results. We observed significant relationships between g op and g max , with consistency in the g op ∶ g max ratio across biomes, growth habits (shrubs and trees), and habitats (open canopy and understory subcanopy). An overall mean g op ∶ g max ratio of 0.26 5 0.11 (mean 5 SD) was observed. The consistently observed g op ∶ g max ratio in this study strongly agrees with previous hypotheses that an ideal g op ∶ g max ratio exists.Conclusions. These results build substantially on previous studies by presenting a new reference for a consistent g op ∶g max ratio across many levels and offer great potential to enhance paleoclimate proxies and vegetation-climate models alike.All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). Fig. 3Scatterplots of theoretical maximum stomatal conductance (g max ) and stomatal density (D; A) and maximum stomatal pore area (pa max ; B) for biomes. Lines corresponding to the legend color are the fitted reduced major axis regressions. In both A and B, there are no significant differences in relationships between the boreal forest and the temperate rain forest (D: P p 0:84; pa max : P p 0:56, respectively) or between the tropical rain forest and the tropical seasonal (moist) forest (D: P p 0:11; pa max : P p 0:99, respectively).

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