Allometric scaling of adaxial–abaxial stomatal allocation in amphistomatous plants worldwide

Amphistomatous plants distribute stomata between the adaxial and abaxial leaf surfaces, but whether this dual-surface allocation follows a predictable scaling relationship, and why species deviate from it, remains poorly understood. Here, we compiled a global dataset of adaxial and abaxial stomatal densities for 559 species across 380 sites and 63 families. We quantified their allometric relationship using phylogenetic reduced major axis regression and calculated amphistomatal scaling residuals (ASR) as signed perpendicular deviations from the fitted scaling relationship. We then examined variation in ASR among plant functional groups and across climatic and edaphic gradients. Adaxial and abaxial stomatal densities showed a significant allometric relationship after accounting for phylogenetic nonindependence, with a slope significantly > 1. ASR showed a significant phylogenetic signal but did not differ significantly between woody and herbaceous species, evergreen and deciduous woody species, or annual and perennial herbs. Individual environmental associations with ASR were generally modest, whereas the selected phylogenetic model retained multiple interaction terms, indicating that associations between ASR and environmental conditions were strongly context-dependent. By quantifying departures from adaxial-abaxial scaling, our study identifies a distinct dimension of stomatal allocation that is structured by evolutionary history and associated with multidimensional climatic and edaphic context. This framework provides a basis for testing the physiological and evolutionary mechanisms underlying variation in amphistomatous stomatal allocation.