Using modern plant trait relationships between observed and theoretical maximum stomatal conductance and vein density to examine patterns of plant macroevolution

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.