An ecosystem-scale flooding experiment to disentangle mechanisms of coastal forest resilience and vulnerability to extreme flooding events

Climate change, increasing storms, and sea level rise are increasingly affecting coastal forest ecophysiology and mortality, leading to widespread ‘ghost forests’ and marsh incursion. However, it is difficult to predict the rapid changes observed at these terrestrial-aquatic interfaces as the complex interplay of hydrological, ecological, biogeochemical, and physiological responses driving ecosystem stress and change is not well understood. We describe TEMPEST, a unique manipulative experiment to simulate extreme freshwater and estuarine-water disturbance events over multiple years in 2000 m² plots. This experiment was implemented in a deciduous coastal US forest with no known prior exposure to seawater. A dense network of environmental, soil, and tree sensors captured the cascading effects of each of the three annual flood treatments—300 m³ or 15 cm water per day per plot—with sensor data streaming in real time to project scientists and then openly available for community analysis.

The first TEMPEST event in 2022 significantly but temporarily impacted the system’s hydrology, with more subtle influences on biogeochemical, soil gas flux, and vegetation components. Pedological changes and vegetation stress built rapidly in subsequent years, however, and by years two and three sap flow rates in three deciduous tree species were disproportionately and negatively affected in the saltwater plot: growing season sap flux of tulip poplars was 25% lower than in the control plot, with the trees exhibiting canopy loss. Maple and beech were also negatively affected but to a lesser extent. The novel TEMPEST experiment provides insight into how the impacts of storm surges accumulate in upland coastal ecosystems’ soils and vegetation, explores the relative influence of flooding and salinity on the magnitude of change, and will be a crucial reference to improve our models of understudied coastal ecosystems.