Coevolving topography, patchy soils, and forest structure

The transition from soil-mantled to bedrock-dominated landscapes is important to hydrology, ecology, and landscape evolution. Where classical theory predicts that this transition abruptly occurs once erosion rates exceed soil production ones, actual landscapes show a much more gradual transition. The Rampart Range, CO, is an unglaciated, forested mountain range with uniform lithology that spans this transition, providing an opportunity to interrogate how topography, soil cover, and forest structure coevolve. Using drone imagery and airborne lidar, we show the utility of object-based methods that segment the lidar topography into individual outcrops. Lower elevation hillslopes are rockier (~14% rock) than their higher elevation counterparts (~2% rock) due to late Cenozoic incision of a regional, high elevation, low relief surface. After accounting for the erosional control on rockiness, we show that bedrock exposure also depends on topographic aspect. Equator-facing hillslopes are rockier (~20% rock at low elevations; ~3% rock at high elevations) than their more heavily forested, pole-facing counterparts (~11% rock at low elevations; ~1% rock at high elevations). To simulate these patterns, hillslope evolution models need "humped" soil production functions that can generate rocky outcrops within a soil mantle. When combined with depth-dependent soil creep, such models readily produce large gradients in the height and frequency of bedrock outcrops in response to aspect-dependent weathering. Given the correspondence between higher forest biomass (e.g., increased tree density, canopy height) with lower bedrock exposure on pole-facing hillslopes, we argue that forest dynamics may be an underappreciated regulator of hillslope morphology via root zone weathering.