Effects of slope angle on mass movement by tree uprooting

Scott A. Norman, Randall J. Schaetzl and Leon R. Follmer

An examination of 189 well-delineated mounds and pits in sandy soils of northern lower Michigan, all presumably formed by tree uprooting, was used to determine the effects of slope angle on morphology and volume, and to assess the potential importance of uprooting to mass movement. Slopes ranged from zero to 54%. Data indicate that mound and pit volumes increase with increasing slope angle, suggesting that on gentle slopes more of the disturbed soil wastes off the mound, back into the new pit. Mounds are often elongated in the downslope direction on steep slopes. Based on regression analyses, slopes of right harpoon-up over left harpoon-down47° are generally sufficient for all mound soil to slump or wash off in a downslope direction, rather than into the upslope pit. Thus, on steep slopes pit volumes provide a better representation of root plate volume. Pit depth can also be used as a surrogate for rooting depth on steep slopes where infilling from the mound is minimal.