Time-lagged ozone and environmental controls on overstorey–understorey water-use dynamics in a Mediterranean forest revealed by empirical dynamic modeling

Tropospheric ozone (O 3 ) affects plant physiological processes, yet its role in regulating water-use dynamics across vertical forest layers remains poorly understood. Because stomatal regulation and whole-tree hydraulic processes operate over different temporal scales, accounting for time-lagged responses is essential for understanding stand-level water use. This study investigated how O 3 , vapor pressure deficit (VPD), and soil water content (SWC) influence tree water use and its vertical coupling in a Mediterranean forest. Two-year sap flow ( S f ) measurements from overstorey Quercus ilex and understorey layer Phillyrea latifolia trees were analyzed using empirical dynamic modeling (EDM), combining unified information-theoretic causality and S-map analyses. VPD exerted relatively short-term causal influences on tree water use, particularly stomatal conductance ( g t ), whereas SWC effects generally extended over lags of 0–4 days and varied with soil depth, ecosystem layer, and year. O 3 showed delayed causal influences on S f , with peak lags of approximately 2 days in overstorey and 4 days in understorey trees, with temporal variation in the estimated lags (0–5 and 1–4 days, respectively). O 3 effects on g t occurred at shorter time scales and differed between ecosystem layers and years: in the cooler and wetter summer of 2014, O 3 had a negative effect on overstorey g t , but a positive effect on understorey g t , whereas the overstorey effect shifted to positive in the hotter and drier summer of 2015. A significant causal influence of understorey S f on overstorey S f , but not in the reverse direction, was detected, suggesting that this interaction may arise from differences in water acquisition together with shared environmental forcing. These findings show that forest water use emerges from nonlinear and time-lagged responses to interacting atmospheric and soil drivers, highlighting the importance of considering O 3 effects together with drought stress when predicting Mediterranean forest responses to environmental change.