Topographic Roughness on Forested Hillslopes: A Theoretical Approach for Quantifying Hillslope Sediment Flux From Tree Throw

Wind-driven tree throw is an observable and consequential process that suddenly moves soil downslope, inverts the soil column, and roughens the surface with pit-mound topography. Quantifying fluxes due to tree throw is complicated by its stochastic nature and estimation requires averaging over a large area or long time. Here, we develop a theory that leads to a dimensionless metric directly measurable from high resolution topographic data. The theory explains the flux and topographic roughness as a function of tree throw production and decay rate by creep-like processes. We then form a measurable dimensionless variable that is the ratio of fluxes due to tree throw versus creep-like processes. Applying the theory to hillslopes in Southern Indiana, we find that tree throw accounts for 11%-18% of the hillslope sediment flux. The theoretical and observational findings provide important constraints on quantifying Critical Zone function from topographic parameters such as roughness. Plain Language Summary When trees fall on hillslopes, they often uproot a volume of soil that is attached to the roots. Because trees usually fall downslope, this uprooted soil also moves down the hillslope, contributes to erosion, and leaves characteristic pit and mound shapes on the surface. Despite the topographic signature of the process, quantifying how much dirt trees move downslope is complicated by the randomness that drives the process. We develop a theory that explains the roughness of hillslope topography and how it relates to sediment transport rates driven by tree throw. We then map topographic roughness over a county in southern Indiana and demonstrate that tree throw accounts for 11%-18% of the sediment motions on hillslopes. Further, we demonstrate that east-facing hillslopes tend to have more tree throw events, which coincides with the dominant wind directions and illustrates that extreme wind events drive most tree throw events in southern Indiana. Key Points Tree throw accounts for 11%-18% of the hillslope sediment flux in southern Indiana The expected topographic variance is a function of the ratio of tree throw rates to creep-like diffusivity Tree throw occurs more frequently on steep, east-facing hillslopes, which is consistent with the dominant wind directions in Indiana