QUANTIFYING THE ROLE OF TREE THROW IN HILLSLOPE PROCESSES OF THE WESTERN APPALACHIAN PLATEAU

Tree throw is a dominant mechanism of bioturbation that generates and mixes soils on forested hillslopes. When trees topple and uproot - often as the result of high-wind events - root plates rip up soil and underlying bedrock and, depending on hillslope gradient as well as direction of fall, transport sediment downslope. There have been many studies investigating the impact of tree throw on forests globally, but additional empirical data are needed to inform process models for hillslope sediment transport. This project quantifies the role of tree throw in the production and transport of soils in the western Appalachian Plateau of Central Ohio, USA.

Our area of study is Denison University’s Biological Reserve, a 140 hectare parcel in Granville, OH, that is a mix of deciduous and conifer forest as well as open fields. The Reserve was likely fully cleared for agriculture during the 1800s and parts of our study area were clear cut as recently as sixty to seventy years ago. We surveyed a number of key hillslopes and valley floors throughout the Reserve looking for tree throw events. For each event, we recorded the length of the tree, diameter at breast height, a decay rating, root plate dimensions, direction of tree fall, and species, if possible. We also extracted a tree core to determine age at the time of topple if the tree was not too decayed.

We observed a total of 222 events over 27.2 hectares and were able to quantify rates of soil turnover and hillslope sediment flux. We found that steeper slopes and more frequent tree throw events drive faster rates of soil mixing with a range from 7600 to 19000 years for total soil mantle mixing in our study area. Hillslope sediment flux rates range from 7 x 10^-7 m³ m^-1 yr^-1 to 1.4*10^-3 m³ m^-1 yr^-1. Wind is a dominant factor in this geomorphic regime, causing trees to fall primarily in a SSE driving many trees on NW-facing slopes to topple upslope. Using our limited tree core data (n = 24), we determined a mean age of 51.5 years with a standard deviation of 16.8 years for recently toppled trees. Moving forward, we plan to compare these results with decadal-scale hillslope sediment transport rates quantified using fallout radionuclides such as Cs-137.