As sea levels rise along East Coast, salt water intrusion is creating 'ghost forests'

In the coastal forest at the Pine Neck Nature Sanctuary on Shinnecock Bay, oaks and pitch pines give way to red maples and tupelos closer to the open salt marsh. Beyond the shade of the trees, bright green Spartina grasses spread on the soggy ground, all the way to the shoreline. Stranded among the grasses is a grove of bleached-gray trunks, the skeletal remains of tupelo trees that thrived and then died there decades ago.

At Pine Neck and other tidal wetlands up and down the Atlantic seaboard, king tides are flowing higher, storm surges are breaking deeper inland and salt water is creeping into the groundwater below. Trees that can withstand occasional flooding, such as tupelos, cannot manage chronic flooding with salt water and are slowly dying.

"This ghost forest is just an example of what's happening all over the world thanks to climate change," said Alison Branco, climate adaptation director at the New York branch of The Nature Conservancy, as she stood at the edge of the grove.

"The tree next to me probably died, I don't know, 50 or more years ago. But up at the very edge of the healthy forest, we're seeing trees that have died probably in the last 10 or 20 years. So it's been slowly marching upward."

Sea level rise is expected to accelerate over the next several decades, inundating salt marshes and eventually the coastal forests that border them. In some places, if there’s enough undeveloped land upslope, the marsh will migrate into the woodlands, and the woodlands will retreat as well. But a lot of this inundation is happening in areas, such as much of Long Island, that are bordered by roads and neighborhoods.

"Eventually, we could lose a great deal of our coastal forest," said Kevin Munroe, Long Island preserve director at The Nature Conservancy, which owns Pine Neck. "Because where's the forest going to go when it hits a highway or someone's backyard?"

Can marsh migration keep pace with rising seas?

Polar ice is melting at an ever-quickening pace, and warmer ocean waters expand, both of which cause sea levels to rise everywhere. But they are rising three to four times faster along the East Coast than the global average, possibly, according to some climate researchers, because ancient ocean circulation patterns are slowing, creating a swell of water. 

By 2050, sea levels on the East Coast are expected to rise by about a foot, placing thousands of miles of coastal wetlands at risk. Already, as marshes are flooding more frequently, ground-nesting shorebirds such as rare saltmarsh sparrows, which hide their nests among the marsh grasses, are losing their nesting sites.

Marshes also absorb rainwater and storm surges, lessening flooding further inland. And they capture and store 10 times more carbon per acre than tropical rainforests, locking it away in the boggy soil, which climate scientists say makes them a highly effective defense against further global heating. Their loss, then, would contribute to the feedback loop of accelerating climate change.

Even as the nearshore areas are drowned, Branco said, the marsh ideally can retreat inland. Plants that thrive in the wettest and saltiest soil, like Spartina alternifolia, would spread into the high marsh, and the plants that dominate the high marsh, like Spartina patens, also would migrate inland.

Farther upland, first the oaks, pines and blueberries, and then red maples, tupelos and bayberries, could succumb to the salt water; eventually, marsh grasses could thrive where tupelos now grow, and the tupelos could take over where the oaks and pines flourished.

That, at least, is the most optimistic scenario, considering global greenhouse gas emissions haven’t been curbed.

If the forests can’t move inland, Munroe said, "we don't know what all the end results are going to be." But it’s clear the loss of trees will mean a loss of carbon sequestration, he noted, as well as poorer soil health, more erosion, less habitat and fewer food sources for wildlife.

The abundant acorns in the coastal forest at Pine Neck, for example, along with the blueberries and huckleberries in the understory, are an important source of nourishment for birds that nest and raise their young there, Munroe said. "And we're going to lose the shade," he said, "which means the temperature goes up even higher."

Declining salt marshes

Elizabeth Watson, an associate professor of ecology at Stony Brook University, is studying marsh migration at six sites on Long Island’s South Shore, including Pine Neck, Ludlow's Creek in Oakdale and the Wertheim National Wildlife Refuge in Shirley.

She regularly monitors the salinity of the soil and returns once a year to measure the circumference of the trees, to see if salt water is slowing their growth. Ultimately, she hopes her data will help her to "understand how quickly the forest is retreating" and "whether the marshlands are moving upslope," she said.

In early July, Watson followed a boardwalk at the South Shore Nature Center in East Islip to a stand of dead tupelos, most likely killed by the slow intrusion of salt water from the Great South Bay.

At this site, marsh migration could prove difficult. "Our main finding is that the forest is retreating at a pretty constant rate," Watson said. But dense stands of invasive reeds have spread in the upper marsh, so that "the more salt-tolerant plants are not moving in as quickly."

The small ghost forest still serves an ecological purpose, she pointed out: It’s now "a rookery for all kinds of birds," including egrets and ospreys, which use the dead trees and stumps as perches and nesting sites. As she spoke, a woodpecker ducked into a hole chiseled into a dead tupelo.

Long Island lost roughly 2,700 acres of intertidal, high and coastal fresh marsh between the mid-1970s and the early 2000s, which Branco said is because of development, shoreline hardening, poor water quality and encroaching seas. And New York could lose more than half of its remaining coastal wetlands by the end of the century if emissions are not sharply cut, according to the research group Climate Central.

But conserving land to allow for marsh migration could reverse those losses, the group’s models show, possibly even increasing wetlands by 20%. 

"We need to address climate change and try to get that problem under control before it's too late," Branco said.

Just as crucial, she added, is the need to preserve open land at the edges of Long Island’s marshes, so that they can move and keep pace with sea level rise. "We need to give it some space, so we can continue to have this important salt marsh habitat."