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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.
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Throughout Earth’s mountainous terrain, forest productivity responses to aspect in the Anthropocene are migrating.
Aspect influences critical zone (CZ) function, particularly in mountainous terrain where it is an ecosystem-defining geographical feature. Distinct insolation across aspects is linked to differences in water availability and flows, land cover and vegetation productivity, soil thickness and rooting depths, frost cracking, weathering rates, and solute concentrations. Relatively few studies have explored any changing influence of aspect on vegetation productivity, which governs soil water storage and runoff. We probe the hypothesis that the productivity benefit of growing on aspects with greater radiation inputs in mountain systems has been declining over the past few decades as warming has accelerated. We quantify how forest productivity varies with aspect from 1985 to 2021 across the world’s mountain ranges using a monthly-averaged, satellite-derived measure of greenness (NDVI). Globally, most montane forests exhibited increasing greenness over time. Mountainous forests ~15° to ~40° latitude N or S of the equator exhibited behavior consistent with our hypothesis by increasingly favoring shadier aspects, particularly during growing seasons when rainfall and soil moisture can be limiting to productivity. In contrast, closer to the poles where climates are coolest and aspect has an even greater influence on annual solar radiation, the benefit of a sun-facing aspect appears to be increasing across all seasons, consistent with poleward forest community migration hypotheses. We also demonstrate greater increases over time in montane forest greenness on east-facing slopes compared to west-facing slopes; north of ~40° latitude this pattern appears less robust. These observations reveal that it is increasingly disadvantageous for montane forests growing on sunnier, hotter aspects at relatively low latitudes during the hottest times of the year. Given known linkages between ecosystem productivity and CZ functions like water storage, provision, and flows, soil development, solute production, and regolith thickness, these analyses cast light on yet-underappreciated consequences of a rapidly warming climate on Earth’s montane forests and their capacity to shape CZ processes.
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- Award ID(s):
- 2121639
- PAR ID:
- 10505020
- Publisher / Repository:
- AGU abstract
- Date Published:
- Journal Name:
- AGU abstract
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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