RATES OF SEDIMENT FLUX FROM THREE THROW IN THE SHALE HILLS CRITICAL ZONE OBSERVATORY AND ASSOCIATED SATELLITE SITES IN THE APPALACHIAN MOUNTAINS

Arborturbation, or tree throw, the upheaval of soil and sometimes bedrock in the root mass of a fallen tree, has been identified as a major process in the overturn and downslope transport of soil and shallow bedrock in mountainous regions. We report on measurements of tree throw that were made as part of a broader effort to quantify erosion rates on shale slopes, information that is applicable to understanding the evolution of topography and regolith thickness on shale landscapes. In our study at the Shale Hills Critical Zone Observatory (CZO) and associated satellite sites in the central and southern Appalachian Mountains spanning from Alabama to New York, slope and prevailing wind direction, while important in places, did not control the majority of tree throw events. The depth to a root limiting layer and the distance from the center of a root wad to the center of an excavated pit increased from north to south – suggesting that deeper roots excavate more soil and deeper soils generally exist in warmer climates. The calculated sediment flux rates range from 1.8 x 10^-5 m³/m/y to 2.1 x 10^-4 m³/m/y. The observations double in number and verify formulations of sediment flux due to tree throw cited in the literature. The values exceed by several orders of magnitude values for sediment flux rate by soil creep on slopes.

Here we expand upon an initial study presenting an analysis of nearly 500 tree throw measurements from the Shale Hills CZO, an 8-hectare headwater catchment. The catchment is oriented in a dominantly east-west direction opening to the west into the greater Shaver Creek watershed. We present a sediment flux rate from the catchment ranging from 2.9 to 3.2 x 10^-4 m³/m/y, describe the evolution of that rate through an estimated 60 year period, and present new sediment flux rate calculations from a massive tree blow down event that occurred in late summer 2017 in and adjacent to the catchment.