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

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. Plain Language Summary: Small differences in solar radiation and water availability between hillslopes facing opposite directions may lead to distinct vegetation and hillslope structures. However, more research is needed to understand the controls and extent of structural differences in the subsurface, especially in rain‐dominated landscapes. To investigate the physical and ecohydrologic characteristics between hillslopes that face opposite directions, we combined terrain analysis, soil pit characterization, geophysical surveys and hydrologic measurements taken from two hillslopes facing opposite directions. We found that the hillslope that faced north had higher oak tree density, and higher soil moisture than the hillslope that faced south. These observations match other published studies from a range of landscapes and climates in the northern hemisphere. However, contrary to expectations based on other studies, we found that the surface slope and weathered bedrock thickness were similar between the two hillslopes. Similarities in soil water at 50 cm and increased perched groundwater response on the hillslope that faces south suggest that how water moves within the hillslope and what water is available to plants may alter how rock breaks down. In addition, historic climate and water availability may be important to understand the present‐day hillslope structure. Key Points: Aspect‐dependent microclimatic and vegetative differences do not correspond to physical structure between hillslopesSimilar physical structure between hillslopes may be driven by various aspect‐dependent processes leading to similar material loss and productionA more comprehensive understanding of critical zone development requires integration of past and present biotic and climatic processes [ABSTRACT FROM AUTHOR]