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ABSTRACT Sea level rise and storm surges affect coastal forests along low‐lying shorelines. Salinization and flooding kill trees and favour the encroachment of salt‐tolerant marsh vegetation. The hydrology of this ecological transition is complex and requires a multidisciplinary approach. Sea level rise (press) and storms (pulses) act on different timescales, affecting the forest vegetation in different ways. Salinization can occur either by vertical infiltration during flooding or from the aquifer driven by tides and sea level rise. Here, we detail the ecohydrological processes acting in the critical zone of retreating coastal forests. An increase in sea level has a three‐pronged effect on flooding and salinization: It raises the maximum elevation of storm surges, shifts the freshwater‐saltwater interface inland, and elevates the water table, leading to surface flooding from below. Trees can modify their root systems and local soil hydrology to better withstand salinization. Hydrological stress from intermittent storm surges inhibits tree growth, as evidenced by tree ring analysis. Tree rings also reveal a lag between the time when tree growth significantly slows and when the tree ultimately dies. Tree dieback reduces transpiration, retaining more water in the soil and creating conditions more favourable for flooding. Sedimentation from storm waters combined to organic matter decomposition can change the landscape, affecting flooding and runoff. Our results indicate that only a multidisciplinary approach can fully capture the ecohydrology of retreating forests in a period of accelerated sea level rise.
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Effect of Flooding on Water-Table Elevation and Salinity in an Abandoned Coastal Agricultural Field
Water-levels and salinity were measured in seven shallow (ca. 2 m deep) wells installed at distances proximal, medial, and distal to the source of tidal flooding between 2017 and 2019 in a warm-season grass meadow adjacent to a salt marsh. Water-table fluctuations greater than 10-cm were associated with seawater, precipitation, or a combination of the two. When the field was flooded by tides (> 0.5 m above predicted), groundwater salinity increased; when the field was flooded by precipitation (> 2.5 cm), the salinity of the groundwater decreased. The increased head gradient that accompanied the rise in the water table appeared to be sufficient to allow the freshwater from precipitation to push the salt water down and towards the marsh creek, resulting in a freshening of the groundwater that persisted until the next saltwater flooding event. Thus, the relative frequency between saltwater flooding, salinization, freshwater flooding, and flushing controlled the groundwater salinity. These findings indicate the importance of high-tide events in the process of salinization of the groundwater and the ameliorating effects of rainfall events whose magnitude is sufficient to increase groundwater elevation at least ten centimeters. Further, they contribute to a growing body of evidence in support of the interaction between fresh- and saltwater flooding events to enhance the salinity of groundwater and drive ecosystem transition from uplands to salt marshes.
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- Award ID(s):
- 1832221
- PAR ID:
- 10512339
- Publisher / Repository:
- Coastal Education and Research Foundation (CERF)
- Date Published:
- Journal Name:
- Journal of Coastal Research
- ISSN:
- 1551-5036
- Subject(s) / Keyword(s):
- extreme tides rainstorms salt-marsh migration agriculture plant community change
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.2112/JCOASTRES-D-23-00071.1
