Dendrologic Effects on Soil Evolution Timescales and System Behavior in the Ouachita Mountains, USA

Soils are a primary medium through which ecological forms such as trees interact with landscape geology and geomorphic processes. Trees have a profound effect on soil development and pedodiversity in the Ouachita Mountains of Arkansas and Oklahoma, manifesting their impacts over relatively short time spans, at non-random locations, and producing unstable, divergent evolution of the landscape regolith. The landscape of the Ouachita Mountains provides a surprisingly active environment for these dendrologic effects to occur with a complex geology; a climate conducive to rapid weathering and frequent meteorological disturbances; and a fast growing, nearly continuous, forest cover that has existed in roughly its current form and composition for the last 4,000 yr. Within this landscape, we conducted a series of related studies looking at factors that effect soil formation processes and spatial variability in soil types. We find that despite nearly uniform slope, aspect, parent material, vegetation cover, and site history, pedodiversity within 1.27-ha plots is remarkably high with 4-11 (mean = 6.4) different soil types occurring per plot. This high local diversity, and variability in other persistent soil features, is best explained by the point-centered effects of trees, and to a lesser degree by local parent material differences. In addition to promoting accelerated chemical weathering, trees produce biomechanical effects on soils through (1) the vertical displacement of soil via root growth; (2) the occurrence of stump holes that infill and mix surficial and subsoil materials; and (3) through excavation of soil volumes and "mining" of bedrock via tree throw. Moreover, trees facilitate rapid rates of soil production on fresh bedrock surfaces (5-10 mm/yr). These effects are enhanced where steeply dipped beds of varying lithologic composition provide fractures and differential weathering opportunities that tree roots can exploit. The combined disturbance by biomechanical effects could potentially produce 100 percent turnover of the entire surface area within a time span of 3,000 yr. These dendrologic effects operate in ways that have direct bearing on ecogeomorphic system behavior, and have important implications for system modeling. Despite their widespread influence, there is strong evidence that trees have a non-random spatial impact over time due to a tendency for trees to reoccupy favorable sites. This results in dendrologic perturbations that are individually small and short lived, but whose effects are self-reinforcing and extend over time spans greatly exceeding that of the tree producing the initial effect. This dynamical instability produces non-equilibrium soil thicknesses and high pedodiversity within the region that is uncorrelated to landform-scale factors such as slope, aspect, vegetation type, or general lithology of underlying formations. While our results are contingent upon the landscape conditions of the Ouachita Mountains, they certainly suggest that dendrologic effects can be very important in understanding soil evolution in mountain forest systems and deserve serious consideration in any modeling effort.