Symmetry in Hillslope Steepness and Saprolite Thickness Between Hillslopes With Opposing Aspects

Abstract

The structure of the critical zone (CZ) is a product of feedbacks among hydrologic, climatic, biotic, and chemical processes. Past research within snow-dominated systems has shown that aspect-dependent solar radiation inputs can produce striking differences in vegetation composition, topography, and soil depth between opposing hillslopes. However, far fewer studies have evaluated the role of microclimates on CZ development within rain-dominated systems, especially below the soil and into weathered bedrock. To address this need, we characterized the CZ of a north-facing and south-facing slope within a first-order headwater catchment located in central coast California. We combined terrain analysis of vegetation distribution and topography with soil pit characterization, geophysical surveys and hydrologic measurements between slope-aspects. We documented denser vegetation and higher shallow soil moisture on north facing slopes, which matched previously documented observations in snow-dominated sites. However, average topographic gradients were 24° and saprolite thickness was approximately 6 m across both hillslopes, which did not match common observations from the literature that showed widespread asymmetry in snow-dominated systems. These results suggest that dominant processes for CZ evolution are not necessarily transferable across regions. Thus, there is a continued need to expand CZ research, especially in rain-dominated and water-limited systems. Here, we present two non-exclusive mechanistic hypotheses that may explain these unexpected similarities in slope and saprolite thickness between hillslopes with opposing aspects.