Daniel A. Marion

Biomechanical effects of trees on soil and regolith: beyond treethrow

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.

Equal-mobility bed load transport in a small, step-pool channel in the Ouachita Mountains

Equal-mobility transport (EMT) of bed load is more evident than size-selective transport during near-bankfull flow events in a small, step-pool channel in the Ouachita Mountains of central Arkansas. Bed load transport modes were studied by simulating five separate runoff events with peak discharges between 0.25 and 1.34 m3/s (1.0- to 1.6-year recurrence intervals) in a natural channel using controlled releases from a storage tank. EMT occurrence was investigated using four different bed load relationships suggested by previous research. With each of these approaches, the relationship of a given bed load characteristic (Dmax, distribution percentile, displacement distance and skewness) to some independent factor (τc*, τ and grain size) was assessed to determine which transport mode was evident. Regression models derived using combinations of these four relationships with different datasets provide seven separate tests. Five of the seven tests indicate that EMT occurred or was predominant. Several reasons may explain the apparent contradictory results, but the confounding effects of changes in the structural arrangements of bed material prior to or during the events seem particularly important.

Nonequilibrium Regolith Thickness in the Ouachita Mountains

Interpretations of regolith and soil thickness in the context of landscape evolution are typically based on the notion that thickness is controlled by the interaction of weathering rates and erosion and tuned to topography. On sideslopes of the Ouachita Mountains, Arkansas, however, there is a high degree of local spatial variability that is largely unrelated to topography. This indicates nonequilibrium in the sense that there is no evidence of a balance between rates of weathering and removal, as is postulated in some conceptual models in geomorphology and pedology. Johnsons soil thickness model is applied as an alternative to interpret local variations in regolith thickness. At the study sites, regolith thickness is not generally related to slope, curvature, elevation, or pedogenic development in the solum. This indicates that variability in thickness is related chiefly to processes and controls acting in the lower regolith, below the solum. The primary controls of variability are local lithological variation, variable structural resistance associated with fractures and bedding planes in strongly tilted Paleozoic sedimentary parent material, and point‐centered pedological influences of trees. A steady state regolith may be relatively rare. Results of this study suggest that an equilibrium regolith thickness is most likely in uniform lithology with a high degree of lithologic purity, less likely in interbedded sedimentary rocks, and more unlikely still if the latter are titled and fractured. Equilibrium thickness would also be more likely where the effects of bioturbation are more areally uniform (as opposed to the point‐centered effects of individual trees) and where the biomantle is above the weathering front.

Residence times of alluvium in an east Texas stream as indicated by sediment color

Abstract The relationships between sediment production, storage, and transport in fluvial systems are complex and variable. Key issues in addressing these relationships are the residence times of sediment delivered to the channel, and the proportion derived from recent upland erosion as opposed to remobilized alluvium. The systematic changes in iron geochemistry often experienced by sediments deposited in an anaerobic environment, such as a stream channel or waterlogged floodplain, are used here as an indicator of residence time over contemporary time scales. In areas such as east Texas, where upland soils are high in iron oxide content, these changes are reflected in soil color. Alluvium with red, yellow, or brown colors indicating ferric (oxidized) iron and sufficient organic matter for reduction to occur indicates a short ( 1 year). Sediments with the longest residence times in alluvial environments (≫1 year) will not oxidize on exposure to the atmosphere due to the loss of iron, while those with ferrous iron remaining will experience oxidation and color change on exposure. In Loco Bayou, Texas, these indicators of residence time are shown to be generally consistent with other field evidence of erosion and sedimentation. Further, the color indicators correctly indicate the residence time in several cases where the latter is known from field observations.

A history of watershed research in experimental forests of the Interior Highlands

The history of watershed research in the Interior Highlands can be divided into four periods: Initial Start and Stumble (1930s–1950s), Reestablishment and Renewal (1960–1980), Partnerships and Expansion (1980–1990), and New Scales and Paths (1990–present). While each of these periods was marked by different societal concerns and scientific questions, experimental forests played a central role in accomplishing watershed research during all of these periods. Unlike other regions of the country, there was no dominating theme or inspirational leader to focus watershed research in one particular experimental forest; rather the work shifted between several experimental forests over time. Despite many changes in personnel and research direction, a significant body of knowledge has been developed over the past 70 years that has benefited scientists, forest managers, and the public. Fundamental knowledge has been gained regarding the components of the hydrologic system and how these components are affected by natural disturbances. Timber harvesting impacts on soil and water resources have been quantified and shown to be short-lived. Concerns about acid rain and road erosion also have been addressed and shown to be less severe than initially thought. These findings, coupled with the discovery that, in general, small watersheds responded in similar ways across the Interior Highlands, have been the basis for forest planning across the region. As new research challenges arise, experimental forests will continue to play a critical role in addressing these needs.