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Individual trees may have significant impacts on soil morphology. If these impacts are non-random such that some microsites are repeatedly preferentially affected by trees, complex local spatial variability of soils would result. A model of self-reinforcing pedologic influences of trees (SRPIT) is proposed to explain patterns of soil variability in the Ouachita Mountains, Arkansas. SRPIT postulates that trees are preferentially established on patches that are nutrient-rich and rock fragment poor relative to adjacent sites. The biomechanical effects of trees on soil, and decomposition of roots then maintain and reinforce the rock fragment and nutrient differences relative to surrounding soils, increasing the likelihood of successful future tree establishment. The links hypothesized in the SRPIT model are dynamically unstable, which would be necessary for the self-reinforcing mechanisms to operate. Soil variability in 16 study plots is dominated by local, within-plot variability, pointing to highly localized biological effects and consistent with the SRPIT model. Within each 0.127 ha plot, 4-11 different series, and 4-9 different rock fragment classes were found. Of the 10 paired pits at each plot, 3-7 pairs had different series in pits typically less than 1 m apart. On average, each of the 16 plots had 6.3 different soil types, 6 different rock fragment classes, and 60% of the sample pairs differing in soil series. Richness-area analysis of soil series, and of rock fragment classes, both indicate that pedodiversity is dominated by within-plot rather than between-plot variability. The vertical variations in the concentration of rock fragments in 40 of 58 soil pits is consistent with redistribution of soil material by tree throw, and there is also evidence of rock fragment displacement by tree growth and deposition in stump holes. Overall, results suggest that soil morphological effects of individual trees are an important source of soil spatial variability in forests, and that such effects are non-random over time. Thus even relatively homogeneous areas may be characterized by tree-rich patches which support repeated generations of trees, and tree-poor patches which more rarely host trees. #

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