BIOTURBATION AND EROSION BY TREE THROW IN FORESTED LANDSCAPES, APPALACHIAN MOUNTAINS

Tree throw, the upheaval of bedrock and soil in the root mass of a fallen tree, has been suggested to be a major process in the overturn and downslope transport of soil and shallow bedrock in mountainous regions. We report on an effort to quantify the effects of tree throw along a climosequence of five sites in the Appalachian Mountains associated with the Susquehanna-Shale Hills Critical Zone Observatory (SSHO) - sites were studied in central New York, central Pennsylvania, west central Virginia, eastern Tennessee, and northern Alabama. The study included field measurements of tree throws within a 120-meter diameter search area centered on soil pits on ridge tops on the Silurian Rose Hill Shale and coeval strata of similar composition. The following observations were made for each tree throw observed at each study site: GPS location, tree girth, relative tree age, tree type, dimensions of pit, azimuth of fall, and slope and azimuth of maximum slope. These observations allowed quantification of the volume and distance of transport of sediment per event, and the number of events/area/time.

The measurements of tree throw were made as part of a broader effort to characterize erosion rates on shale slopes, information that is applicable to understanding the evolution of topography and regolith thickness on shale landscapes. Specifically, the observations are used to verify formulations of sediment flux due to tree throw as presented in Gabet et al., 2003, The effects of bioturbation on soil processes and sediment transport, Annu. Rev. Earth Planet. Sci., 31, 249-73. The sediment fluxes reported here range from 1.8 X 10^-5 m²/m/y to 2.1 X 10^-4 m²/m/y. The higher values are comparable to those sediment flux rates from tree throw reported in the literature whereas the lower values compare well to long-term erosion rates for the Appalachian Mountains determined using cosmogenic radionuclide analyses.