Understanding the drivers of stomatal behavior is critical for modeling terrestrial carbon cycle and water balance. The unified stomatal optimization (USO) model provides a mechanistic linkage between stomatal conductance (gs) and photosynthesis (A), with its slope parameter (g1) inversely related to intrinsic water use efficiency (iWUE), providing a key proxy to characterize the differences in iWUE and stomatal behavior. While many studies have identified multiple environmental factors influencing g1, the potential role of evolutionary history in shaping g1 remains incompletely understood. Leaf organic matter 13C discriminations (Δ13C) can be applied to estimate g1 over timescales from days to whole growing season. However, most applications assume that mesophyll conductance (gm)-a critical parameter in the Δ13C model-is infinite, due to limited information. Here, we incorporated new insight of gm to allow for more realistic parameterization of this variable, and subsequently to enable improved estimation of g1 based on a global bulk leaf Δ13C dataset comprising 2215 observations of 1521 species that span major biomes. Our analysis revealed a significant phylogenetic signal in g1 values, which differed among phylogenetic groups. Through a Bayesian phylogenetic linear mixed model, we found that species and phylogeny together explained 36.63 % of g1 variance, a contribution comparable to that of the environmental factors (44.59 %). Our findings uncovered for the first time that environmental factors, species-level and phylogenetic effects jointly shape g1 variability, thereby contributing to a more comprehensive understanding of optimal stomatal behavior in the context of global environmental change.
Keywords: Phylogeny; Plant functional type (PFT); Stable carbon isotope; Stomatal optimization; Stomatal sensitivity; Water-use efficiency.
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