Stomatal function across temporal and spatial scales: Deep-time trends, land-atmosphere coupling and global models

The colonization of land by plants and their interaction with biogeochemical and atmospheric processes transformed continental climate and hydrology. Stomata, which evolved to optimize the biological economics of plant carbon uptake in exchange for water loss, play a crucial role in large scale environmental processes by maintaining a connection between deep soil water reservoirs and the atmosphere, regulating terrestrial carbon sinks and altering surface energy balance as they respond to environmental changes. Stomatal feedback control of leaf gas exchange is observed at multiple temporal and spatial scales, but accurately simulating this dynamic behavior remains challenging, particularly for extreme environmental conditions, including drought. Integration of more realistic representation of stomatal conductance and its regulation of leaf gas exchange in global models is improving global simulations of carbon, water and energy fluxes, and these simulations in turn highlight some of the limitations with current leaf-scale models. Our analysis of current leaf-scale models of stomatal conductance representing empirical-based and optimization-based approaches reveals close structural similarities that result in virtually indistinguishable simulations of leaf, canopy and global fluxes. While acknowledging these similarities, future efforts must focus on more accurate parameterization of stomatal conductance models, informed by studies at multiple temporal and spatial scales, including molecular, fossil, geological, leaf, canopy and landscape.