PNNL

The Science 

Coastal forests are often exposed to flooding, but processes driving forest impacts to soils and trees remain unclear, especially in the early stages before visible damage occurs. This study addressed this knowledge gap by simulating hurricane-scale flooding of freshwater and brackish water in an experimental forest in Maryland. Researchers monitored changes in soil and tree responses using real-time sensor networks and intensive sampling campaigns. They found that soil conditions changed quickly in both treatment plots; oxygen levels dropped, creating low-oxygen and chemically reduced environments that persisted for several days and altered belowground carbon cycling. However, the trees did not show consistent signs of stress above ground. The clearest signals of stress were found in tulip poplars exposed to brackish water, which had lower water flow after flooding and an early end to their growing season. 

The Impact 

This study presents the most comprehensive dataset of real-time changes in soil and tree function in response to hurricane-level flooding in an experimental coastal forest to date. This experiment is unique in its ability to isolate the effects of flooding and salinity using controlled events and specific treatment plots. Findings suggest that belowground changes may impact coastal forest health before any signs of stress aboveground. This work could help scientists detect forest vulnerability sooner than what was previously possible and understand the conditions that lead to coastal forest mortality. The work also provides benchmarks that can be used to improve large-scale models of coastal ecosystems, enabling better predictions of how forests respond to short-term flooding. Fields like plant physiology, soil science, ecosystem ecology, and Earth system modeling can use these insights to better study vegetation responses to environmental stressors.

Summary 

Flooding events in low-lying coastal forests are becoming more frequent, yet the early-stage impacts on these ecosystems remain poorly understood. In this study, researchers used a large-scale field experiment called TEMPEST to simulate two consecutive hurricane-scale flooding events in a coastal forest, with separate treatment plots receiving freshwater and brackish estuarine water. The team tracked real-time changes in soil conditions and tree function across two treatment plots and one control plot. They found that soil oxygen levels dropped rapidly during flooding, and hypoxic (low-oxygen) conditions persisted longer at greater soil depths—lasting up to three days. Redox conditions, which indicate how chemically oxidized or reduced the soil is, also shifted and took longer to recover than oxygen. Soil carbon dioxide emissions declined during flooding, likely due to reduced oxygen availability for respiration. Despite clear belowground changes, most trees showed minimal aboveground signs of stress. Leaf gas exchange responses were inconsistent and only tulip poplar trees in a treatment plot flooded with brackish water showed reduced water movement and early leaf senescence. While there was little observed effect of flooding on tree function, the belowground changes observed are consistent with the types of conditions ecological theory predicts cause tree death. The results suggest that repeated short-term flooding initiates clear changes in the belowground environment that may lead to long-term forest decline, particularly when saltwater is involved. 

Contact 

Daniel Stover, Environmental System Science Program, daniel.stover@science.doe.gov 

Vanessa L. Bailey, COMPASS-FME principal investigator, Pacific Northwest National Laboratory, vanessa.bailey@pnnl.gov

Peter Regier, corresponding author, Pacific Northwest National Laboratory, Peter.regier@pnnl.gov

Funding 

This research was supported by the Coastal Observations, Mechanisms, and Predictions Across Systems and Scales, Field, Measurements, and Experiments multi-institutional project. COMPASS-FME is supported by the Department of Energy (DOE), Office of Science, Biological and Environmental Research program as part of the Environmental System Science program, and by the Smithsonian Environmental Research Center. The Pacific Northwest National Laboratory is operated for DOE by Battelle Memorial Institute.

Related Links

COMPASS-FME Project Website