Flooding with brackish water disrupts Liriodendron tulipifera sap flux in an ecosystem-scale experiment.

Coastal forests are retreating due to sea‑level rise and extreme precipitation events, yet the physiological mechanisms that precede woody plant decline remain poorly constrained. We used the ecosystem‑scale TEMPEST (Terrestrial Ecosystem Manipulation to Probe the Effects of Storm Treatments) experiment; annual, plot‑level freshwater and saltwater flooding of a mid‑Atlantic deciduous forest, to quantify short‑ and long‑term responses of sap flux in three tree species: Acer rubrum, Fagus grandifolia, and Liriodendron tulipifera. We analyzed five years of sap flux density measurements (2021-2025) using a Before-After Control-Impact (BACI) framework and linear mixed‑effects models to assess short and long-term changes in tree hydraulic function. Across the study period, species differed markedly in their responses to repeated flooding. A. rubrum showed no significant change in sap flux density under any treatment, while F. grandifolia exhibited plot‑specific increases or decreases consistent with relief from drought in the freshwater treatment plot. L. tulipifera sap flux density increased over time in all plots, but increases were dampened in the freshwater plot and strongly suppressed under saltwater flooding. Soil electrical conductivity rose progressively in the saltwater plot and emerged as a key predictor of both immediate and cumulative reductions in L. tulipifera sap flux density. As L. tulipifera constitutes a large share of canopy basal area in this coastal forest, continued saltwater intrusion is poised to accelerate mortality and drive ecosystem state-change. Our findings illustrate how ecosystem‑scale manipulations can uncover physiological thresholds relevant to coastal forest vulnerability under future sea‑level rise.