A steady increase in sea levels is pushing saltwater into U.S. wetlands, killing trees from Florida to as far north as New Jersey. But with sea level projected to rise by as much as six feet this century, the destruction of coastal forests is expected to become a worsening problem worldwide.

On a recent afternoon, University of Florida watershed ecologist David Kaplan and Ph.D. candidate Katie Glodzik hiked through the Withlacoochee Gulf Preserve, on the Big Bend coast of northwestern Florida. Not long ago, red cedar, live oaks, and cabbage palms grew in profusion on the raised “hammock island” forests set amid the preserve’s wetlands. But as the researchers walked through thigh-high marsh grass, the barren trunks of dead cedars were silhouetted against passing clouds. Dead snag cabbage palms stood like toothpicks snapped at the top. Other trees and shrubs, such as wax myrtle, had long been replaced by more salt-tolerant black needlerush marsh grass. Saltwater, flowing into this swampy, freshwater-dependent ecosystem as a result of rising sea levels, is turning these stands of hardwoods into “ghost forests” of dead and dying trees. “The loss of these islands changes the landscape from a mosaic to one dominated by a single habitat — salt marsh,” said Kaplan, noting that the change means reduced habitat for some species of wading and migratory birds, as well as for turtles and snakes. A similar transformation is occurring in coastal floodplains across the southeastern and mid-Atlantic regions of the United States, representing what scientists say is the leading edge of climate change in what were once largely freshwater ecosystems. From Florida’s hammock islands to North Carolina’s swamp forests, rising sea levels, often compounded by regional water management practices, continue to push saline water further inland, wiping out swampy woodlands. “Ghost forests are a dramatic expression of climate change,” says Gregory Noe, a research ecologist with the U.S. Geological Survey who has been studying the impacts of dying cypress in the iconic swamps along the Savannah River, which forms much of the border between Georgia and South Carolina.

In addition to killing off bottomland forests, saltwater intrusion is affecting wetland habitats in places along the U.S. East Coast like the Delaware Bay estuary, where encroaching sea levels are inundating the salt marsh and killing a line of forest fringing the coast — allowing a non-native, invasive strain of reed to colonize.



While many of the ecological consequences of saltwater intrusion are still unknown, important swampy habitats for threatened species, such as the endangered red-cockaded woodpecker, are in peril, according to Marcelo Ardón, assistant professor of ecosystem ecology at North Carolina State University. Fisheries for species such as drum and catfish that are dependent on healthy wetland forests are being affected, as well.



Because of the low elevation and flat or gently sloping characteristics of the southern U.S and mid-Atlantic coastal forests, these regions are among the most vulnerable globally to saltwater intrusion, experts say. But other wetland and estuarine ecosystems are threatened worldwide by increasing salinity. China’s Pearl River estuary on the South China Sea is experiencing the agricultural and ecological effects of saltwater intrusion. Drought and rising sea levels are allowing saltwater to spread as far as 50 miles into Vietnam’s Mekong Delta, affecting ecosystems and rice production.



Predicted sea level rise along Florida's Big Bend coast over the next century. Mapping and data courtesy of Climate Central and NASA. Graphic by Yale e360

The rate of global sea level rise has increased significantly over the past several decades, a trend linked to climate change. According to the U.S. National Oceanic and Atmospheric Administration, during most of the 20th century sea levels were rising at 0.6 inches per decade. Since 1992, however, sea level has been rising at 1.2 inches per decade. Experts predict, for example, that by 2100, global warming will cause sea level to rise by as much as four to six feet worldwide, including from 3.3 to 6.6 feet along Florida’s Big Bend coast .



Before bottomland hardwood forests disappear, the subtle signs of saltwater intrusion take place in the soil. Plant growth halts, trees and plants produce fewer seeds, and the seeds that do drop to the ground have a harder time germinating. “Even with relatively low levels of salinity, trees slow their growth,” Ardón says of swamp trees such as water tupelo and pond pine. Cypress trees are often the last to die, since they are among the most salt-tolerant of coastal forest trees.



Saltwater also begins breaking down peat — partially decomposed vegetation — faster, which can have drastic consequences in places like North Carolina’s Albemarle-Pamlico Peninsula, where elevation on average is only two feet or less above sea level. The loss of peat causes land to subside, enabling saltwater to move farther inland. Forest habitat can then transition to marsh and, eventually, open water.



In Florida’s 413-acre Withlacoochee Gulf Preserve, Kaplan says there has been a steady decline in the raised hammock island forests, demonstrating how sea level rise is dynamically “re-shaping the coastal landscape in this region.” The cabbage palms and cedars rooted atop the flat, limestone substrate are resilient to occasional periods of salt exposure during storms. However, recent annual rates of sea level rise have upped the duration and intensity of salt-water exposure — the two factors that trigger tree die-off, says Kaplan, an assistant professor in the University of Florida’s Department of Environmental Engineering Sciences. Kaplan and his colleagues categorize the hammock island forests of the preserve into three groups: “healthy,” “stressed,” and “remnant” (or relic) stands. The forests, says Kaplan, are increasingly grouped in the latter two categories.



On my recent visit to the Withlacoochee preserve, I pointed to a forested hammock island that appeared to be healthy. But Kaplan prompted me to take a closer look; barren cedar branches and an understory choked with marsh elder and other salt marsh shrubs became visible within the stand. “I would group it in ‘stressed,’” he says.



Along the Savannah River, the coastal floodplain swamp is beginning to lose a footrace with rising seas. As mineral sediment combines with dead plants and roots to form peat, wetland forest soils build annually at the rate of about tenth of an inch, says Noe, the research ecologist with the USGS. This is a natural accretion that typically takes place in some southern swamps. But the accretion is not fast enough to keep up with recent annual sea level rise.



The stump of a red cedar in Withlacoochee Gulf Preserve [left]. Dead and dying trees on the shores of the Albemarle Sound in North Carolina [right].