Which way do you lean? Using slope aspect variations to understand Critical Zone processes and feedbacks

Abstract

Soil‐mantled pole‐facing hillslopes on Earth tend to be steeper, wetter, and have more vegetation cover compared with adjacent equator‐facing hillslopes. These and other slope aspect controls are often the consequence of feedbacks among hydrologic, ecologic, pedogenic, and geomorphic processes triggered by spatial variations in mean annual insolation. In this paper we review the state of knowledge on slope aspect controls of Critical Zone (CZ) processes using the latitudinal and elevational dependence of topographic asymmetry as a motivating observation. At relatively low latitudes and elevations, pole‐facing hillslopes tend to be steeper. At higher latitudes and elevations this pattern reverses. We reproduce this pattern using an empirical model based on parsimonious functions of latitude, an aridity index, mean‐annual temperature, and slope gradient. Using this empirical model and the literature as guides, we present a conceptual model for the slope‐aspect‐driven CZ feedbacks that generate asymmetry in water‐limited and temperature‐limited end‐member cases. In this conceptual model the dominant factor driving slope aspect differences at relatively low latitudes and elevations is the difference in mean‐annual soil moisture. The dominant factor at higher latitudes and elevations is temperature limitation on vegetation growth. In water‐limited cases, we propose that higher mean‐annual soil moisture on pole‐facing hillslopes drives higher soil production rates, higher water storage potential, more vegetation cover, faster dust deposition, and lower erosional efficiency in a positive feedback. At higher latitudes and elevations, pole‐facing hillslopes tend to have less vegetation cover, greater erosional efficiency, and gentler slopes, thus reversing the pattern of asymmetry found at lower latitudes and elevations. Our conceptual model emphasizes the linkages among short‐ and long‐timescale processes and across CZ sub‐disciplines; it also points to opportunities to further understand how CZ processes interact. We also demonstrate the importance of paleoclimatic conditions and non‐climatic factors in influencing slope aspect variations. Copyright © 2017 John Wiley & Sons, Ltd.

Designing a network of critical zone observatories to explore the living skin of the terrestrial Earth

“…Now, geophysicists travel among CZOs to image the subsurface with a battery of instruments to image the belowground landscape (St. Clair et al, 2015). In another example, after the Boulder Creek CZO began emphasizing slope aspect as a useful natural experiment to examine controls on CZ architecture and function in 2009, similar analyses at other CZOs led to highlighted linkages among aspect, water, biota, regolith structure, and episodic events (West et al, 2014;Ebel et al, 2015;Pelletier et al, 2017). Finally, a deep drilling project ("drill the ridge") was proposed and then pursued at many CZOs, and these data in turn led to a special issue describing regolith formation (Riebe et al, 2016).…”

“…A full elucidation of hypotheses is beyond the scope of this paper and only a subset is shown in Table 4. Many have been published in collaborative papers (Rempe and Dietrich, 2014;Riebe et al, 2016;Li et al, 2017;Pelletier et al, 2017;Yan et al, 2017;Brantley et al, 2017a). Here we summarize three multidisciplinary discoveries that have large implications for the prediction of flow paths relevant to the largest supply of accessible and drinkable water available to humans -water contained in rock and regolith (Fetter, 2001;Banks et al, 2009).…”

Investigating Microtopographic and Soil Controls on a Mountainous Meadow Plant Community Using High‐Resolution Remote Sensing and Surface Geophysical Data

“…Recent studies conducted in Arctic regions—using high‐resolution geophysical and remote sensing data—have revealed that microtopography could produce significant spatial heterogeneity in soil moisture and plant community distributions at several‐meter or submeter scales (S. S. Hubbard et al, ; Dafflon et al, ; H. M. Wainwright et al, ). While the slope aspect of the hillslopes can be considered as a first‐order large‐scale control (Pelletier et al, ; Yetemen et al, ), the controlling factors within each hillslope have not yet to the authors' knowledge been investigated. Given that soil moisture is very sensitive to microtopography, it could be an important factor governing the spatial organization of the ecosystem in a water‐limited region.…”

Microclimate–forage growth linkages across two strongly contrasting precipitation years in a Mediterranean catchment

“…Given their inherently complex topography, microclimates vary at short distances (10s of meters), as both air and soil temperatures have been documented to differ on north versus south aspects in California range improvement studies (Evans, Kay, & Young, 1975;Raguse & Evans, 1977). Moreover, differences in microclimate (including soil climate) across complex topography have been recognized as a force affecting ecohydrology and vegetation, which convey their effects on landscape evolution, for example, the steepening of north-facing slopes in the northern hemisphere midlatitudes (Poulos, Pierce, Flores, & Benner, 2012;Yetemen, Istanbulluoglu, Flores-Cervantes, Vivoni, & Bras, 2015;Pelletier et al, 2018). These differences extend into the soil mantle, affecting soil temperature, soil moisture, and plant growth.…”