THE INFLUENCE OF PLANTS ON ELEVATION- AND ASPECT-DEPENDENT EMERGENCE OF BEDROCK TORS IN THE RAMPART RANGE, CO

While plants play myriad potential roles in landscape evolution (e.g., adding cohesion to soils, converting rock into regolith, mediating runoff generation), both ecological and topographic structure are emergent features of landscapes that can be generated via alternative process pathways. This leads to ambiguity in determining the net influence of plants on landscape development. As such, it is essential to identify natural experiments capable of testing alternative models of landscape evolution. We identify one such landscape, the Rampart Range in Colorado, USA, where forest structure, soil cover and depths, and topographic form covary. This setting is ideally suited to examine feedbacks among forest and hillslope dynamics. It is unglaciated, underlain by a uniform lithology, and, because it is perched near the erosional limit for its soil mantle, exhibits an abundance of bedrock tors. At the landscape scale, an increase in base level fall along bedrock rivers has set up gradients in climate, vegetation, hillslope erosion, and topographic relief. We show that this transient signal of fluvial adjustment results in hillslopes that are steeper, rougher, and less soil mantled lower in the stream network. However, at the local scale, aspect exerts an influence on hillslope morphology of comparable magnitude. Equator-facing slopes are not as steep as their pole-facing counterparts, but are much rougher, have more exposed bedrock, and support less forest biomass. In this work, we quantify both elevation- and aspect-dependent patterns using 1-m lidar topography and forest canopy height. Uncertainty in lidar-based analyses is characterized using drone and field surveys at select locations. The lidar, drone, and field surveys presented here provide the essential data (including tree densities, above-ground biomass, high resolution topography, and bedrock outcrop maps) needed to test numerical simulations that allow forest structure and topographic form to coevolve. We argue from this analysis that the critical link between forest structure and topography lies in the relative balance among plants as soil producers (e.g., via root growth), soil stabilizers (e.g., via root cohesion), and soil movers (e.g., via tree throw).