Elevation-dependent coordination between stomatal and vein densities among leaf habits and growth forms in a subtropical forest

BACKGROUND AND AIMS: Stomatal and leaf venation traits are important structural components associated with plant gas exchange and water transport, but their variation along environmental gradients may depend on plant functional strategies. We investigated whether stomatal and vein traits exhibit coordinated yet trait-specific patterns across an elevational gradient, and whether such patterns vary with leaf habit and growth form in a subtropical forest. METHODS: We examined 12 woody species distributed along a 300-1360 m elevational gradient on Lushan Mountain, China. Stomatal length (SL), stomatal density (SD), and vein density (VD) were quantified from cleared leaves, with species × elevation means used as observational units. Linear mixed-effects models were applied to evaluate associations of traits with elevation, leaf habit (evergreen vs. deciduous), and growth form (shrub vs. tree), and to test elevation-dependent variation in the SD-VD relationship. KEY RESULTS: Stomatal length and stomatal density showed no significant elevational trends, whereas vein density increased with elevation, indicating contrasting responses among leaf anatomical traits. Stomatal density differed among combinations of leaf habit and growth form, with a significant leaf habit × growth form interaction and higher SD in deciduous trees than in other groups. Across species and elevations, SD was positively associated with VD, but the strength of this relationship varied nonlinearly along the gradient, with the strongest association occurring at intermediate elevations (∼800 m) corresponding to the evergreen-deciduous mixed forest zone. CONCLUSIONS: Our findings demonstrate that elevation does not generate uniform changes across leaf anatomical traits. Instead, trait variation depends on environmental context and functional strategies, with leaf habit and growth form jointly influencing stomatal density and stomatal-vein coordination. Considering trait relationships in addition to individual traits provides new insights into how plants organize leaf structure across heterogeneous environments.