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 (δ¹³C). 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 δ¹³C as a proxy for long-term WUE. Leaf δ¹³C 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 δ¹³C. Within species, however, only GCL was significantly associated with iWUE and leaf δ¹³C. SLA had a minor negative influence on iWUE and leaf δ¹³C, 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 δ¹³C of trees, highlighting their potential for rapid phenotyping approaches in ecological studies.