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Expect the unexpected: Four hypotheses to explain unexpected critical zone symmetry in hillslopes with opposing aspect
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  • Amanda Marie Donaldson,
  • Margaret Ann Zimmer,
  • Mong-Han Huang,
  • Kerri N Johnson,
  • Berit Hudson-Rasmussen,
  • Noah Joseph Finnegan,
  • Nerissa Barling,
  • Russell Callahan
Amanda Marie Donaldson
University of California, Santa Cruz

Corresponding Author:[email protected]

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Margaret Ann Zimmer
University of California - Santa Cruz
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Mong-Han Huang
University of Maryland, College Park
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Kerri N Johnson
University of California - Berkeley
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Berit Hudson-Rasmussen
University of Maryland, College Park
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Noah Joseph Finnegan
University of California, Santa Cruz
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Nerissa Barling
University of California, Santa Cruz
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Russell Callahan
University of California, Santa Cruz
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Abstract

The structure of the critical zone is a product of feedbacks between hydrologic, climatic, biotic, and chemical processes. Ample 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, more research is needed to understand the role of microclimates on critical zone development within rain-dominated systems, especially below the soil and into weathered bedrock. To address this need, we characterized the critical zone of a north-facing and south-facing slope within a first-order headwater catchment located in central coastal California. We combined terrain analysis of vegetation distribution and topography with field-based soil pit characterization, geophysical surveys and hydrologic measurements between slope-aspects. We observed thicker soil profiles, higher shallow soil moisture, and denser vegetation on north facing slopes, which matched previously documented observations in snow-dominated sites. However, average topographic gradient and saprolite thickness were uniform across our study hillslopes, which did not match common observations from the literature. These results suggest dominant processes for critical zone evolution are not necessarily transferable across regions. Thus, there is a continued need to expand critical zone research, especially in rain-dominated systems. Here, we present four non-exclusive, testable hypotheses of mechanisms that may explain these unexpected similarities in slope and saprolite thickness between hillslopes with opposing aspects. Specifically, we propose both past and present ecohydrologic processes must be taken into account to understand what shaped the present day critical zone.
24 Jan 2023Submitted to ESS Open Archive
24 Jan 2023Published in ESS Open Archive