Linking stomatal size and density to water use efficiency and leaf carbon isotope ratio in juvenile and mature trees

Water‐use efficiency (WUE) is affected by multiple leaf traits, including stomatal morphology. However, the impact of stomatal morphology on WUE across different ontogenetic stages of tree species is not well‐documented. Here, we investigated the relationship between stomatal morphology, intrinsic water‐use efficiency (iWUE) and leaf carbon isotope ratio (δ ¹³ C). We sampled 190 individuals, including juvenile and mature trees belonging to 18 temperate broadleaved tree species and 9 genera. We measured guard cell length (GCL), stomatal density (SD), specific leaf area (SLA), iWUE and bulk leaf δ ¹³ C as a proxy for long‐term WUE. Leaf δ ¹³ C correlated positively with iWUE across species in both juvenile and mature trees, while GCL showed a negative and SD a positive effect on iWUE and leaf δ ¹³ C. Within species, however, only GCL was significantly associated with iWUE and leaf δ ¹³ C. SLA had a minor negative influence on iWUE and leaf δ ¹³ C, but this effect was inconsistent between juvenile and mature trees. We conclude that GCL and SD can be considered functional morphological traits related to the iWUE and leaf δ ¹³ C of trees, highlighting their potential for rapid phenotyping approaches in ecological studies.