The Calhoun Critical Zone Observatory

Recovery after extreme soil erosion and land degradation

As this picture from the 1950s shows, the Calhoun Experimental Forest suffered from severe erosion. Photo by U.S. Forest Service.

From surface to core, the Earth’s radius is almost 4,000 miles, but only the uppermost sliver of that rocky expanse, called the critical zone, sustains life. This zone extends from the base of weathered rock to the treetops, and includes water, soils, vegetation, and animals.  

A new study, led by Duke University and funded with a $5 million National Science Foundation grant, will bring university and U.S. Forest Service researchers and managers together to examine the critical zone and its recovery after extreme soil erosion and land degradation. Mac Callaham, a team lead at the Southern Research Station (SRS) Center for Forest Disturbance Science, will be one of a group of scientists involved in the study. Other participating institutions in the new Calhoun Critical Zone Observatory include the University of Georgia, Georgia Tech, University of Kansas, Mississippi State University, and Roanoke College.

The study will take place in the Calhoun Experimental Forest (Calhoun) located in Sumter National Forest in South Carolina. The Calhoun was established in 1947, a time when much of the Piedmont faced severe degradation. After well over a century of intensive crop production, erosion was so severe that nearly 6 inches of top soil had been washed into the rivers, while the abandoned cropland was furrowed by gullies and ravines. The land where the Calhoun was established represented the poorest of poor Piedmont conditions.

Today, almost 70 years later, the gullies and ravines are blanketed with leaf litter and shaded by trees, but the scars remain. Some experts have suggested that the reforestation indicates natural restoration processes, but the new study takes a more critical perspective. “Our study is guided by a hypothesis that the impressive-looking reforestation masks fundamental alterations to the local and regional hydrology, biology and chemistry,” says Daniel Richter, a researcher at Duke University and lead investigator of the study. “Much of the Piedmont may not be recovered so much as it has been re-stabilized in a highly altered state.”

Because the Calhoun Critical Zone Observatory involves studies of  historical and current geological and hydrological processes, along with land uses, vegetation patterns, soil dynamics, and a number of other factors, researchers from many disciplines will pool their expertise to discover how above and belowground systems are functioning and may have recovered.

Researchers will gather information about the site from a range of sources, from library archives to networks of wireless sensors—and  from the more than 60 years of data collected by the Forest Service and Duke University at the site. “We’ll also be digging holes to collect soil animals and doing some field studies,” says Callaham, who will be focusing on how different soil animal communities – especially earthworms – affect soil function. Callaham and his colleagues found earlier that different land uses led to different earthworm communities. “Native worms were reliably found in relatively undisturbed oak-hickory forest fragments, and occasionally in planted pines, but almost never in pastures or plowed fields,” says Callaham. “We are interested in the role of different earthworm communities in water infiltration, macropore density, and gas diffusion rates.”

Researchers are also interested in understanding how land degradation and reforestation affects the relationship between aboveground and belowground systems, as well as the long-term effects of erosion on organic carbon dynamics, how human-caused changes in the critical zone might affect human livelihoods, and how human-influenced critical zones might be forever altered, yet still stabilize and enter new steady states.

The Calhoun project is the newest addition to a network of six Critical Zone Observatories in the U.S., with more developing across the globe. Although the U.S. observatories are funded separately and focus on different aspects of the critical zone, their common goal is to develop predictive abilities for how the critical zone will respond to projected changes in climate and land use. The Calhoun study adds crucial information about how severely degraded land can recover and continue to provide clean water, air, and other ecosystem services.

Visit the Critical Zone Observatory website.

For more information, email Mac Callaham at

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