Consistent relationship between field-measured stomatal conductance and theoretical maximum stomatal conductance in C₃ woody angiosperms in four major biomes

Premise of research. Understanding the relationship between field-measured operating stomatal conductance (g_op) and theoretical maximum stomatal conductance (g_max), calculated from stomatal density and geometry, provides an important framework that can be used to infer leaf-level gas exchange of historical, herbar-ium, and fossil plants. To date, however, investigation of the nature of the relationship between g_op and theoretical g_max remains limited to a small number of experiments on relatively few taxa and is virtually undefined for plants in natural ecosystems. Methodology. We used the g_op measurements of 74 species and 35 families across four biomes from a published contemporary data set of field-measured leaf-level stomatal conductance in woody angiosperms and calculated the theoretical g_max from the same leaves to investigate the relationship between g_op and g_max across multiple species and biomes and determine whether such relationships are widely conserved. Pivotal results. We observed significant relationships between g_op and g_max, with consistency in the g_op ∶g_max ratio across biomes, growth habits (shrubs and trees), and habitats (open canopy and understory subcanopy). An overall mean g_op ∶g_max ratio of 0.26 ± 0.11 (mean ± SD) was observed. The consistently observed g_op ∶g_max ratio in this study strongly agrees with previous hypotheses that an ideal g_op ∶g_max ratio exists. Conclusions. These results build substantially on previous studies by presenting a new reference for a consistent g_op ∶g_max ratio across many levels and offer great potential to enhance paleoclimate proxies and vegetation-climate models alike.