Expect the unexpected: Four hypotheses to explain unexpected critical
zone symmetry in hillslopes with opposing aspect
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.