Congaree National Park's Plants, 2021: A Baseline for the Old-Growth Floodplain

Congaree National Park, in South Carolina's inner Coastal Plain about 30 km (18.6 miles) southeast of Columbia, protects the largest remaining contiguous tract of old-growth bottomland forest in the United States. It is an International Biosphere Reserve, a National Natural Landmark, a Globally Important Bird Area, a Ramsar Wetland of International Importance, and in part a Wilderness Area.

Most of the park lies in the vast floodplain of the Congaree River, with upland pine woodlands along its northern edge. In summer 2021, the National Park Service's Southeast Coast Network (SECN) began monitoring the park's vegetation for the first time — so these data are the baseline.

Vegetation plots established in summer 2021. NPS

How it's done

The network uses a peer-reviewed protocol, like other NPS monitoring networks in the East, modeled on the Carolina Vegetation Survey. In permanent, randomly placed plots across the main habitat types, it records:

1. every plant species, native and non-native, and how abundant each is;
2. trees' basal area, density and health;
3. fuel on the forest floor;
4. disturbances, including insect pests and disease;
5. physical conditions — landform, soil nutrients, canopy cover and height.

At Congaree, 64 plots were set up in two broad habitats: Coastal Plain alluvial (floodplain) wetlands and forests, and Coastal Plain upland open woodlands.

Why the floodplain is so diverse

• Subtle terrain. The floodplain's complex landforms create many small elevation differences, giving species with different tolerances for flooding each a place.
• Wind. Storms large and small open gaps in the canopy, letting in light and changing soil moisture, so shade-intolerant trees can regenerate.
• Fire, in the uplands. Frequent, low-severity fires once kept the upland woods open and rich in species.

The plots recorded windthrow, feral hog rooting, and signs of recent and past fire.

What they found

Average species richness (± standard deviation) by plant growth form, by habitat. NPS

Dominant trees:

• floodplain plots — sweetgum (Liquidambar styraciflua), water tupelo (Nyssa aquatica), green ash (Fraxinus pennsylvanica) and bald-cypress (Taxodium distichum);
• upland plots — loblolly pine (Pinus taeda).

Invasive plants

Non-native species were fairly common in all plots and habitats. Seven were species the South Carolina Exotic Pest Plant Council classes as invasive:

Native versus non-native species, by habitat. NPS

All plots are due to be resampled in summer 2025. Full report: Terrestrial Vegetation Monitoring at Congaree National Park, 2021 Data Summary.

Why monitor

Plant communities change with the environment and with natural and human disturbance. In the southeastern Coastal Plain they provide ecological services, habitat and food for wildlife, and the scenery that draws people outdoors. Tracking them over time, and spotting what stresses them, guides management that keeps parks ecologically healthy.

The work is part of the NPS Vital Signs Monitoring program, begun in 1999 so that more than 270 park units could track their most important resource measures. Its goals: find status and trends in park ecosystems, give early warning of abnormal conditions, improve understanding of how ecosystems work, meet legal and congressional requirements, and measure progress toward goals. The Southeast Coast Network covers twenty park units in eighteen administrative areas — more than 184,000 acres of federal land in North Carolina, South Carolina, Georgia, Alabama and Florida, and more than 253,000 acres counting jointly managed non-federal land.

Sources

Based on "Terrestrial Vegetation Monitoring 2021 Data Summary for Congaree National Park," Southeast Coast Network resource brief, National Park Service; a work of the United States government in the public domain. The brief spells sweetgum's species name "syraciflua"; it is styraciflua.