Park Restores Unique Riverside Habitat by Managing Invasive Plants

The Delaware River is a beautiful spot for fishing, paddling, bird watching, and… rare plants?

Many unique native plants grow on the Calcareous Riverside Outcrops and Seeps in the Delaware Water Gap National Recreation Area

NPS / Perles

Calcareous Riverside Outcrops and Seeps

In places where limestone bedrock juts out of the shoreline and groundwater seeps out over the rock, you’ll find special plant communities called Calcareous Riverside Outcrops and Seeps. The places are unique partly because they resemble miniature American tallgrass prairies winding along the rivershore. Even though Calcareous Riverside Outcrops and Seeps are too small for herds of bison, they contain the same tall prairie grasses as found in the Midwest, such as big bluestem, Indian grass, switchgrass, and little bluestem. Plus, Calcareous Riverside Outcrops and Seeps are globally-rare plant communities that support 26 state-rare plant species within the Delaware Water Gap National Recreation Area. There are only a few Calcareous Riverside Outcrops and Seeps in the park, the longest of which is near the Dingmans Ferry bridge.

Decreasing Ice Scour and Increasing Invasive Plants

Historically, the Calcareous Riverside Outcrops and Seeps remained as open sunny miniature prairies because winter ice scoured the rock outcrops during spring floods. Any trees and shrubs that sprouted in rock crevices would be scraped off by huge blocks of ice floating down the river that was swollen with snow melt. As the climate changes, the Delaware River will experience warmer, shorter winters with fewer days with temperatures below freezing. Warmer, shorter winters mean less river ice will be formed and less ice scour to keep the Calcareous Riverside Outcrops and Seeps free from trees and shrubs. Without ice scour, it’s important that trees and shrubs (especially invasive ones!) are removed from the Calcareous Riverside Outcrops and Seeps every 5 – 10 years to keep the habitat open and protect the rare plants.

Group of adults and children standing at the edge of the Port Jervis ice dam.

R.G. Tarbell, Courtesy of the Minisink Valley Historical Society, Port Jervis, NY

One of the biggest threats to Calcareous Riverside Outcrops and Seeps are invasive plants. These species often originate from outside North America, but its not their country of origin that is problematic. Invasive plants spread rapidly and aggressively out compete native plants, especially rare plants that need unique habitats to survive.

Left image
View of a transect showing exotic shrubs forming a dense shady thicket.
Credit: NPS

Right image
View of a transect showing an open sunny prairie-like outcrop.
Credit: NPS

Monitoring Special Habitats

To better understand threats to these unique habitats, park staff and biologists from the New Jersey Department of Environmental Protection Office of Natural Lands Management began monitoring the Calcareous Riverside Outcrop and Seep at Dingmans Ferry in 1998. They set up 150 plots that could be relocated using marked trees along the river’s floodplain. Since then, scientists from the National Park Service’s Eastern Rivers and Mountains Inventory and Monitoring Network have revisited the same plots in 2010 and 2020 to collect information on how the rare plant communities are changing.

Monitoring which plants are present and thriving in the Calcareous Riverside Outcrops and Seeps provide clues to the health of the rare community.

NPS / Callahan

Fifteen new invasive plant species established at the Dingmans Ferry Calcareous Riverside Outcrops and Seeps since 1998. Some of the most aggressive invaders were exotic shrubs such as autumn olive, bush honeysuckles, and multiflora rose. These hardy shrubs transformed parts of the open sunny prairie-like outcrops into dense shady thickets where many of the rare plants cannot persist.

The invasive reed canarygrass showed the largest increases over the two decades of monitoring. Reed canarygrass is becoming so common at Dingmans Ferry, it is threatening to change the rare Calcareous Riverside Outcrops and Seeps into a weedy Reed Canarygrass Floodplain Grassland – a common plant community that is spreading along rivers in the mid-Atlantic region. Common wormwood (Artemisia vulgaris) and Japanese knotweed were two other problematic invasive plants that showed up at the Dingmans Ferry site over the monitoring period.

Importance of invasive plant species at the Dingmans Ferry Calcareous Riverside Outcrops and Seeps between 1998 and 2020.  Importance measures the dominance of a plant species within the community.

NPS

The native prairie grasses like big bluestem, little bluestem, and sedges (Carex viridula, C. granularis, and others) were less common in 2020 than in 1998. More than 140 native wildflowers were observed at the Dingmans Ferry site, however, over half of those species became less common, including many species of asters and goldenrods. Native shrubs like common ninebark (Physocarpus opulifolius), alders (Alnus incana and A. serrulata), dogwoods (Cornus racemosa and C. sericea), and viburnums (Viburnum acerifolium and V. lentago) also declined. As these native plants are replaced by invasive species, the Calcareous Riverside Outcrops and Seeps lose their unique character that supports rare plants.

While native plant cover has remained stabled since 1998, non-native and invasive plants now cover more area than native plants in the Dingmans Ferry Calcareous Riverside Outcrops and Seeps.

NPS

Biological Controls Knock Back Invasive Plants

Don’t let these beautiful blooms fool you.  Purple loosestrife pushes out native plants and animals from wetland and riverside habitats.

L. Wilson / University of Idaho / Bugwood.org

A lush wetland packed with bright purple flowers waving in the breeze… Though this scene may be aesthetically pleasing to some, it is actually an ecological nightmare for native plants and animals! Those beautiful flowers are purple loosestrife (Lythrum salicaria), an aggressive invasive plant brought to the United States from Europe in the nineteenth century. Purple loosestrife invades wetlands and riparian areas, outcompeting and replacing the native grasses, sedges, and other flowering plants. In some wetlands, purple loosestrife forms dense, homogeneous stands, which degrades the habitat for numerous birds, amphibians, and rare plants.

As part of an integrated pest management approach to control purple loosestrife in the Delaware Water Gap National Recreation Area, three different species of purple loosestrife-eating beetles were released in the park between 1996 and 2007. These biocontrol beetles were effective at reducing purple loosestrife in many of the park’s wetlands, allowing a diversity of native plants to re-establish.

Galerucella beetles eating purple loosestrife leaves (left) and weevil insect (Rhinoncomimus latipes) feeding on mile-a-minute weed (right)

NPS / Shreiner (left) and NPS (right)

Mile-a-minute weed (Polygonum perfoliatum) is a fast-growing vine with many sharp barbs along the stem that can smoother plants in open habitats. This invasive first appeared in the park in the early 2000’s, and park staff released a weevil insect approved as a biological control for mile-a-minute weed to help manage the plant’s spread. At the Dingmans Ferry site, mile-a-minute weed declined between 2010 and 2020 likely because weevil is effective at keeping the vine in check.

At the Dingmans Ferry Calcareous Riverside Outcrops and Seeps, purple loosestrife showed the largest declines of any plant species observed. Signs of feeding by the Galerucella beetle, one of the biocontrol species, were seen widely throughout the site. Although the beetles have not eliminated purple loosestrife, they help reduce the cover of that invasive species and keep it from expanding.

Park Removes Invasive Plants to Protect Rare Habitats

Park staff restore the Calcareous Riverside Outcrops and Seeps at Smith’s Ferry by removing invasive shrubs.

NPS

Without regular winter ice scour, it is up to park staff and their partners to remove invasive trees and shrubs from the Calcareous Riverside Outcrops and Seeps to maintain the open prairie habitat and protect the rare plants. Over the past few years, park staff worked with the regional Invasive Plant Management Team, as well as an invasive plant control company, to remove invasive shrubs like autumn olive, honeysuckles, and multiflora rose from the Calcareous Riverside Outcrops and Seeps in the Delaware Water Gap National Recreation Area.

As of 2024, nearly all invasive shrubs were removed from all three sites, restoring the Calcareous Riverside Outcrops and Seeps there to open prairie-like habitat where a diversity of unique native plants can thrive.

For More Information

Stephanie Perles
Ecologist, Project Lead
Rare Riparian Prairies Monitoring
814-441-9643
[e-mail]

Eastern Rivers & Mountains Network
420 Forest Resources Building
University Park, PA 16802