Fire Island's Sunken Forest: old hollies under pressure from deer, flooding and salt water

Fire Island National Seashore protects an old-growth holly forest that the National Park Service says is so rare that only New Jersey and New York have anything like it. The Sunken Forest was first set aside as a preserve in 1960 and was once about one-and-a-half times the size of Ellis Island. A park biologist has found that it is shrinking. A National Park Service article, written by Jamie Remillard and Caroline Gottschalk Druschke of SEAcomm URI, explains why, and how new survey tools are showing where the forest is most at risk.

How a pile of sand became a forest

Long before the forest, the article says, Fire Island was little more than a pile of sand, and a forest cannot take root in sand. One plant community grew there, then another, in a process called succession, until about a thousand years ago the island could support trees: first black cherry and pitch pine, and later American hollies. The secondary dunes gave shelter to the first holly seedlings some 300 years ago. Walking across the island at Sailor's Haven, you cross one habitat at a time from beach to forest, a lesson in succession that the article likens to tree rings.

The forest matters beyond its rarity. It shelters birds and mammals, and as a stable landscape behind the secondary dune it prevents more dramatic erosion and lessens the impact of storm surge on neighboring coastal communities.

Surveying the Sunken Forest. Image from NPS.

Three pressures

The park's research builds on data that scientists collected more than 50 years earlier, a baseline for seeing how the forest has changed. Before the biologist could use it, weeks of work went into restoring neglected research sites, including vegetation plots and wells that track groundwater. The work shows three pressures at once:

• Wet ground and salt water. Erosion and sea level rise are saturating the ground beneath parts of the forest, and salt water leaches into groundwater and disturbs the hollies' deep roots. Holly trees at low elevations are dying off.
• Deer. The white-tailed deer population on the island rose from zero in the late 1960s to nearly seven hundred in 2003. The deer browse new growth so hard that whole generations of native plants, including American hollies, are lost, leaving an empty understory.
• Loss of new trees. New holly saplings are not surviving as they once did. The researchers have found whole sections of the forest where American holly saplings might never grow again.

Deer browse has removed new growth of hollies and left an empty understory. Image from NPS.

Holly trees at low elevations are dying off where the ground is unusually wet and salt water reaches their roots. Image from NPS.

American holly (Ilex opaca). Image from NPS.

The park's managers are charged with keeping the forest much as it was in 1964, when Congress passed legislation to preserve it. So the forest has to be protected because it is a key natural resource and because it is the law.

Mapping the forest floor

To decide how to help the forest recover, managers first need to know how intense the threats are in each part of it. Federal funding awarded after Hurricane Sandy for coastal resilience research in the Northeast Coastal and Barrier Network parks bought a set of advanced geodetic survey instruments for capturing very precise ground elevations; some are designed for forested areas and have been especially useful here. The article says more than $300 million allocated by the U.S. Department of the Interior for coastal resilience research, and $400,000 worth of new survey equipment, are part of that effort.

With the new equipment, park scientists and researchers from the University of Rhode Island (URI) have measured elevation with millimeter accuracy and located the places where holly trees are at greatest risk of dying from sea level rise. Continued research should let managers decide how to protect the forest from further decline, restore its ecological balance and sustain its part in the resilience of the surrounding coast.

The Sunken Forest boardwalk at Fire Island National Seashore. Image from NPS.

Sources

• National Park Service, Fire Island National Seashore and Northeast Coastal and Barrier Network, article on preserving the Sunken Forest (https://www.nps.gov/articles/ncbn-quest-to-preserve-the-sunken-forest.htm), written by Jamie Remillard and Caroline Gottschalk Druschke (SEAcomm URI); the names of staff and a photograph of a staff member are left out; rewritten in hubnx's own words.