Species-Specific Stemflow Chemical Signatures Regulate Fine-Scale Ecohydrological Connectivity in Urban Trees

Stemflow is a concentrated pathway of rainfall redistribution that delivers water and dissolved constituents directly to the stem--soil interface. Although recognized as hydrologically important, its stability and ecological significance as a species-level functional expression remain insufficiently examined in tropical urban systems. This study evaluated species-specific controls on stemflow generation and solute modification among young urban trees growing under comparable site conditions in Maharagama, Sri Lanka. Stemflow volume, funneling ratio, and chemical composition were quantified across 15--16 independent rainfall events and compared with bulk rainfall chemistry. Enrichment ratios and principal component analysis were used to assess the magnitude, direction, and persistence of chemical modification. Marked and recurrent interspecific differences were observed in funneling ratio and stemflow volume, with species effects exceeding event-scale variability. Stemflow consistently enriched rainfall in major ions, particularly base cations, though the magnitude and integrated chemical structure of enrichment varied systematically among species. Multivariate ordination revealed persistent species separation in chemical space, indicating coordinated and repeatable canopy-mediated modification rather than stochastic wash-off. These findings support the interpretation of stemflow generation and chemistry as coupled hydraulic--biogeochemical traits that structure fine-scale ecohydrological connectivity. By demonstrating repeatable species-level controls under tropical urban conditions, this study highlights the role of tree identity in regulating atmospheric--soil coupling and provides a basis for integrating stemflow traits into urban ecological and hydrological models.