The Calhoun Critical Zone Observatory: understanding the evolution of the critical zone after centuries of anthropogenic degradation

Abstract:

The Piedmont region of the southeastern United States is characterized by rolling topography, high precipitation, heavily weathered soils, and, after the immigration of European settlers, heavy anthropogenic degradation. Starting in the early eighteenth century and lasting until the early twentieth, intensive cultivation of cotton and tobacco dramatically altered the physical and biogeochemical landscape. Over roughly two centuries it has been estimated that the southern Piedmont lost 17 cm of soil to erosion due to the aforementioned cultivation, combined with poor erosion controls and annual precipitation of 1300 mm. The 20 km² Calhoun Experimental Forest was opened in 1947 as a unit of the Sumter National Forest in upstate South Carolina to study what we would now term restoration ecology on such a heavily degraded landscape, which it did until it was closed as an active research site in the 1960’s. It left behind an extensive hydrologic and climatologic record in four small watersheds over the roughly twenty year period of active research, flow gauging infrastructure in the form of four v-notch weirs, and a compelling but currently incomplete story of the critical zone lying at the intersection of long-term critical zone evolution and rapid, anthropogenic degradation of that same critical zone. With the designation of Calhoun as the newest Critical Zone Observatory we now begin to address the questions originally sought by the Calhoun experimental forest: how the critical zone evolves after centuries of heavy degradation and to what degree that evolution could be considered recovery. Here we present preliminary findings derived from historic data and from the initial stages of a multi-faceted field campaign including soil and geophysical surveys, spatially distributed soil and stream hydrologic measurements, and high-resolution terrain analysis to characterize the spatial extent of degradation or recovery and begin to constrain the temporal trajectory of this heavily impacted critical zone.