Summary Understanding the drivers of geological‐scale patterns in plant macroevolution is limited by a hesitancy to use measurable traits of fossils to infer palaeoecophysiological function.Here, scaling relationships between morphological traits including maximum theoretical stomatal conductance (g max) and leaf vein density (D v) and physiological measurements including operational stomatal conductance (g op), saturated (A sat ) and maximum (A max) assimilation rates were investigated for 18 extant taxa in order to improve understanding of angiosperm diversification in the Cretaceous.Our study demonstrated significant relationships between g op, g max and D v that together can be used to estimate gas exchange and the photosynthetic capacities of fossils. We showed that acquisition of high g max in angiosperms conferred a competitive advantage over gymnosperms by increasing the dynamic range (plasticity) of their gas exchange and expanding their ecophysiological niche space. We suggest that species with a high g max (> 1400 mmol m−2 s−1) would have been capable of maintaining a high A max as the atmospheric CO 2 declined through the Cretaceous, whereas gymnosperms with a low g max would experience severe photosynthetic penalty.Expansion of the ecophysiological niche space in angiosperms, afforded by coordinated evolution of high g max , D v and increased plasticity in g op , adds further functional insights into the mechanisms driving angiosperm speciation.

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.

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