STABLE VS. EVOLVING HILLSLOPES: SETTING THE GEOMORPHIC STAGE FOR INTERPRETING SOILS

Pedologists have traditionally constrained soil development processes by studying constructional features that can be aged and that have minimal soil erosion. Yet, these settings represent a subset of geomorphic features whose characteristics are not readily transferrable to eroding landscapes. In contrast, geomorphologists target hillslopes to understand the rates and mechanisms of erosion. Yet, soil development on hillslopes is often assumed a function of balanced erosion and rejuvenation processes, which minimizes spatial complexity even as observation suggests otherwise. When erosional processes are balanced by rejuvenation of the soil, hillslope morphology is unchanging. However, when perturbations such as base-level fall or land use change disturb this balance, hillslopes and their soils evolve accordingly: e.g., erosion dominated systems will see roughening in morphology and loss of soil. Our work seeks to bridge these two disciplines and advance soil development theory by providing the necessary spatial and temporal geomorphic context for interpreting hillslope soils when driving processes are out of balance. This study uses lidar point cloud and high resolution (0.25 and 1 m) digital elevation model analysis to segment hillslopes by their degree of stability. Perturbations which trigger hillslope evolution occur on multiple timescales. At our study site – Santa Cruz Island (SCI), CA – short term (~10² years) overgrazing couples with long term (~10⁵ years) sea level and climate fluctuations to drive increases in erosional pressure. Erosion rates from cosmogenic radionuclides and dated markers within fill terraces set constraints on the erosional forcing affecting the region. Actively evolving hillslopes are identified by increases in hillslope topographic roughness. We ascertain roughness by comparing changes in hillslope attributes, such as significant breaks in slope, curvature, and hilltop curvature and increases in relief, drainage density, and flowpath length. In statistical distributions of these properties, regions of active evolution display wider ranges in values compared to stable hillslopes. Our landscape analysis provides a robust spatial indicator of relative hillslope stability against which differences in soil morphology and chemistry can be assessed.