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Biomechanical Effects of Trees on Soil and Regolith: Beyond Treethrow


Correspondence: Department of Geography, University of Kentucky, Lexington, KY 40506-0027, e-mail: (Phillips); Southern Research Station, USDA Forest Service, P.O. Box 1270, Hot Springs, AR 71902, e-mail: (Marion).


Forest soils are profoundly influenced by the biomechanical as well as the chemical and biological effects of trees. Studies of biomechanical impacts have focused mainly on uprooting (treethrow), but this study shows that at least two other effects are significant: physical displacement of soil by root growth, and infilling of stump rot pits. Rocky soils in the Ouachita Mountains in Arkansas were studied because they allow for the use of rock fragments as a tracer of displacement. Rock fragments displaced by tree growth (baumsteins) are ubiquitous here, and displacement shows characteristic differences between pines and hardwoods. Hardwoods promote primarily lateral displacement, with a higher probability of displaced rock fragments eventually falling into stump holes. Pine displacement has a significant vertical component associated with basal mounding, and a lower probability of baumstein deposition in stump holes. Obvious stump holes are relatively rare, but the high ratio of stumps and snags to uprooted trees indicates that standing dead trees, which would ultimately result in a stump hole, are quite common. This, plus the presence of numerous duff-filled depressions, suggests that such holes are filled rapidly. The presence of surface-derived rock fragments and thick litter and duff accumulations indicate that at least some of the fill is external, as opposed to soil detachment from the pit walls. The primary influence of stump holes, as reflected by rock fragment distributions, is localized subsurface stone accumulations that do not extend laterally. The total area affected by uprooting is larger than that of stump holes, despite the lower frequency, due to the greater area of disturbance per event. Estimated turnover times (time for 100 percent of the forest floor to be affected) are shortest for soil displacement, intermediate for uprooting, and longest for stump hole effects. Although contemporary rates cannot be confidently extrapolated, the geomorphological efficacy of these processes is reflected by the fact that they are rapid enough to result in complete regolith turnover over time scales comparable to the Holocene. Displacement, stump holes, and uprooting help to maintain a continuously mixed surface biomantle, and may in some cases result in distinctive pedological features, local spatial variations in soil morphology, and divergent evolution of the soil cover.