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This article is part of the Fall 2021 issue of the Earth Science Matters Newsletter.

The tidal freshwater zone is located along rivers at the boundary between upland watersheds and coastal estuaries. Tidal freshwater wetlands in this zone, typically comprised of wetland trees, shrubs, and grass and herbaceous vegetation, provide valued ecosystem services that benefit society, improve water quality, support robust habitats, and provide space for carbon sequestration. However, the position of the tidal freshwater forest and marsh in the coastal-river landscape make them sensitive to changes that occur in both the upstream watersheds (freshwater and nutrient inputs) and oceans (sea-level rise and salinity increases), which could threaten the services they provide. To better understand how these ecosystems responds to change, USGS research scientists measured vegetation responses to water, nutrient, and salinity levels along two coastal rivers in the Chesapeake Bay Mid-Atlantic.

The goal of the study was to determine the effects of watershed inputs of freshwater and nutrients and estuarine water level and salinity on both the current condition and historical rates of the annual growth of wetland vegetation over the past several decades. Sampling included measurement of vegetation characteristics and annual growth of trees reconstructed from tree-ring analyses at multiple wetland sites along both the Mattaponi and Pamunkey rivers in Virginia. Field sites spanned from nontidal wetland, to tidal freshwater forested wetlands just downstream of the limit of tidal influence, and through the tidal freshwater zone to the edge of the brackish estuary.

tidal freshwater forest transitioning to brackish marsh
Tidal freshwater forest transitioning to brackish marsh along the Pamunkey River, Virginia, as low-level salinization kills trees (forming ‘ghost forest’) that are replaced with marsh plants.  

The vegetation of tidal freshwater wetlands clearly changed moving downstream along both rivers, as has been found elsewhere along other coastal systems studied. Vegetation shifted from forest to herbaceous marsh as salinities increased, and trees thinned out and died leading to the creation of ‘ghost forests’ with their standing dead trees vividly showing the impact of sea-level rise on coastal forests. At upstream river sites, annual tree growth increased as freshwater inputs from the watershed increased, while tree growth further downstream benefited from higher estuarine water levels.

However, annual tree growth records over the past three decades indicate that tree growth of the living trees increased in years with somewhat greater salinity. This counterintuitive finding is likely due to competitive release of the surviving trees as neighboring plants were stressed or dying and using fewer resources. As the forest around the surviving trees dies and converts to brackish marsh, more space, sunlight, and potential soil nutrients become available, temporarily increasing growth in the remaining trees.

In other words, surviving trees have some resiliency to low-level salinization that can slow down the formation of ‘ghost forests’ in the coastal zone. This likely stabilizes the provision of important wetland ecosystem services as wetlands undergo transition from forest to marsh. However, it is likely that this temporary boost to surviving trees will be eventually overwhelmed by steadily increasing salinity associated with sea-level rise that ultimately kills trees and spreads ghost forests.

The paper, “Watershed and estuarine controls both influence plant community and tree growth changes in tidal freshwater forested wetlands along two U.S. mid-Atlantic rivers” was recently published in the journal Forests.

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