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

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 (gₘₐₓ) and leaf vein density (Dᵥ) and physiological measurements including operational stomatal conductance (gₒₚ), saturated (Aₛₐₜ) and maximum (Aₘₐₓ) 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 gₒₚ, gₘₐₓ and Dᵥ that together can be used to estimate gas exchange and the photosynthetic capacities of fossils. We showed that acquisition of high gₘₐₓ 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 gₘₐₓ (> 1400 mmol m⁻² s⁻¹) would have been capable of maintaining a high Aₘₐₓ as the atmospheric CO₂ declined through the Cretaceous, whereas gymnosperms with a low gₘₐₓ would experience severe photosynthetic penalty. Expansion of the ecophysiological niche space in angiosperms, afforded by coordinated evolution of high gₘₐₓ, Dᵥ and increased plasticity in gₒₚ, adds further functional insights into the mechanisms driving angiosperm speciation.