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=A/g _s ) 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), gas-exchange, iWUE and leaf δ ¹³ C as a proxy for long-term WUE. Leaf δ ¹³ C correlated positively with iWUE for both juvenile and mature trees. Across species, GCL showed a negative and SD a positive effect on iWUE and leaf δ ¹³ C of both juvenile and mature trees. Within species, however, only GCL was significantly associated with iWUE and leaf δ ¹³ C. Pioneer species ( Populus , Prunus , Betula ) showed a significantly lower leaf δ ¹³ C than climax forest species ( Fagus , Quercus , Tilia ), but the differentiation was not clear for iWUE. We conclude that GCL and SD can be considered as functional morphological traits impacting the iWUE and leaf δ ¹³ C of trees, highlighting their potential for rapid phenotyping approaches in ecological studies.