Tree throw, the upheaval of bedrock and soil in the root mass of a fallen tree, has been suggested as a major process in the overturn and downslope transport of soils in mountainous regions. The process typically leads to an excavated pit, often with exposed bedrock, and a large mass of rock and soil in the exposed root mass. Through time, the pit fills and soil and rock from the root mass move down slope as the tree and roots decay.
Reported here is an effort to quantify the effects of tree throw along a climosequence of sites on shale in the Appalachian Mountains associated with the Susquehanna-Shale Hills Critical Zone Observatory (SSHO). The study includes the following field measurements for tree throws within a 120 meter diameter search area centered on soil pits on ridge tops on the Silurian Clinton Group shale: GPS location, tree girth, relative tree age, tree type, dimensions of pit, azimuth of fall, and slope and azimuth of maximum slope.
Five sites were studied: central New York, central Pennsylvania (SSHO), west central Virginia, eastern Tennessee, and northern Alabama. A general north-to-south decreasing trend in total number of tree throws was observed excluding the Virginia site. In Virginia, the total number of tree throws was twice the number observed in New York, which we attribute to the higher elevation setting subjected to the steadiest winds as well as the shallowest soils. The relatively high number of throws in New York is likely tied to glacial till at the site – rooting depth appears to be limited by rock fragments, abundant clay and periodic soil saturation. Trees with the largest girths tend to excavate the largest tree throw pits, a relationship best defined in Alabama where the deepest pits were excavated by large trees that had fallen most recently.
Most of the observed tree throws occurred on slopes ranging from 15-31 degrees except in Alabama where tree throws fall on a range of slopes with the highest number at 45 degrees. No strong relationship was observed between the azimuth of fallen trees and the azimuth of maximum slopes. At the New York site, all of the tree throws fell toward the east, suggestive of control by prevailing wind direction on the direction of fall. In Virginia, most of the trees fell to the west, the prevailing slope direction of the study site, though a significant number fell to the northeast, again suggestive of influence by prevailing wind direction. In Tennessee most of the trees fell to the south-southeast, the prevailing slope direction, while in Alabama most of the trees fell to the northwest, compatible with prevailing wind directions; the broader range of azimuth of falls in Alabama is most likely due to the effect of chaotic falls associated with recent tornadoes.
These observations of tree throw have been made as part of a broader effort to characterize rates of erosion on shale hill slopes, information that is applicable to understanding the evolution of topography and regolith thickness on shale landscapes. Specifically, our observations are used to verify formulations of volumetric regolith flux due to tree throw.
Mann, E., Downy, K., Dere, A.L., White, T. (2011): Bioturbation of forested shale soils by tree throw in the Appalachian Mountains. AGU Annual Fall Conference Proceedings.
This Paper/Book acknowledges NSF CZO grant support.