Plants of Ocmulgee Mounds: The First Vegetation Survey, 2021

Ocmulgee Mounds National Historical Park lies just below the “fall line” in Macon, Georgia. Its Main Unit covers 266 hectares (657 acres) along the Ocmulgee River and its main tributary there, Walnut Creek. Wetland forests line the river and streams; upland forests sit on the slopes and ridges above the floodplain.

In 2021 the National Park Service’s Southeast Coast Network (SECN) sampled the park’s vegetation for the first time under its long-term monitoring protocol. The results form the baseline against which future change will be measured.

Vegetation plots established in May 2021. Image from NPS’s page.

How the monitoring works

Plant communities change with their environment and with disturbance, natural and human. Tracking them shows how healthy a park’s land ecosystems are and helps guide management. The network’s peer-reviewed protocol, modeled on the Carolina Vegetation Survey, uses permanent plots placed at random within broad habitat types. In each plot, crews record:

1. every vascular plant species, native and non-native, and how abundant it is
2. the size, density and health of every tree
3. the amount of fuel on the forest floor
4. disturbances such as insect pests and disease
5. physical conditions — landform, soil nutrients, canopy cover and height

At Ocmulgee, eight plots were set up in two habitats: Coastal Plain alluvial vegetation (floodplain forest) and Coastal Plain upland forests.

A landscape shaped by people

Like most of the inner Coastal Plain, the park’s land has been heavily altered by people before and after European settlement: forests cleared for farms and grazing, demands from industry, railroads and roads, and the suppression of natural fire. The floodplain forests are now dominated by hardwoods that tolerate wet ground, and may once have held extensive canebrakes of giant cane (Arundinaria gigantea). The upland forests are in early to middle stages of regrowth.

The effects show up in the plots. One floodplain plot beside Interstate 16 (OCMU003) was completely underwater when sampled — most likely because the interstate’s berm has changed how water drains across the floodplain. The soils are acidic, as elsewhere in the inner Coastal Plain, with uplands generally less fertile than the sometimes nutrient-rich floodplains.

What grows there

Crews found 142 plant species, 15 of them not recorded in earlier plant surveys of the park.

• Floodplain plots held 20 to 73 species each; upland plots 24 to 32.
• Floodplain forests were most diverse in trees and herbs; upland forests in trees and vines.

Average species richness (± standard deviation) by plant growth form. Image from NPS’s page.

Invasive plants

Non-native species turned up often in every plot and habitat, including eight ranked invasive by the Georgia Exotic Pest Plant Council. They appeared to hit the floodplain forests hardest, where three were among the most frequent and abundant species of all:

• Chinese privet (Ligustrum sinense)
• Chinese tallow (Triadica sebifera)
• Japanese honeysuckle (Lonicera japonica)

Mean occurrence of native and non-native species by habitat. Image from NPS’s page.

Other findings

• No plants listed as rare by the Georgia Department of Natural Resources were found in the plots.
• There was very little disturbance — little or no deer browsing, feral hog rooting, or diseased and dying trees.
• All plots were scheduled to be sampled again in spring or summer 2024.

About the program

The plots are part of the NPS Vital Signs Monitoring program, begun in 1999 to let more than 270 park units track the condition of their most important natural resources over the long term. It aims to measure status and trends, give early warning of problems, improve understanding of park ecosystems, meet legal requirements and measure progress toward goals. The Southeast Coast Network covers parks in North Carolina, South Carolina, Georgia, Alabama and Florida.

Sources

Based on "Terrestrial Vegetation Monitoring 2021 Data Summary for Ocmulgee Mounds NHP," Southeast Coast Network, National Park Service; a work of the United States government in the public domain. Map and charts from the same page.