Soil Production Mechanisms: Can Tree Roots Significantly Weaken Rock Through Daily Water Uptake?
Abstract
We depend on soil but still have yet to fully understand the mechanics of what converts bedrock into soil. Trees have a significant portion of their mass underground as roots. This means the roots can plumb and alter the unseen landscape below while seeking out water and nutrients. Every night, during their active season, tree roots will swell as the take in water from the surrounding soil and fractured rock, in preparation for transport to their photosynthesizing leaves. Previously, we have quantified the cyclic rise and fall of forces at the root-rock interface on both Douglas fir and Ponderosa pines, coincident with daily shortwave radiation cycles. If rooted in bedrock, this swelling exerts a small force on the surrounding material. Taking the field data measurements from root-generated force monitoring sites at the Eel River Critical Zone Observatory (ERCZO) and applying it to a physical experiment, we tested if trees, with their extensive root systems, can weaken bedrock through daily water uptake and transpiration. We hypothesize that, over the course of a Douglas fir's lifetime, this small repetitive force can lower the rock's original tensile strength, through cyclic stress fatigue, which leads to subcritical cracking. In other words, there is the potential for these small daily forces to significantly reduce the tensile strength of the enfolding bedrock. We characterized a representative sample set of sandstone cores taken from the ERCZO and determined their initial tensile strength to gain a baseline for the study. Using micro CT scans, we characterized porosity, grain size, and pre-existing micro-fractures of the samples. For our physical experiment, we applied a force of 0.56 kN over 104 cycles, based on field measurements from a 5 cm diameter Douglas fir root, to the experimental samples to model the root pressures over the lifespan of a Douglas fir tree ( 103 to 104 days). Finally, we quantified the tensile strength, porosity, and occurrence of micro-fractures within our experimental samples to determine if root water uptake can indeed weaken rock over time.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMEP11D2080C
- Keywords:
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- 0416 Biogeophysics;
- BIOGEOSCIENCESDE: 1826 Geomorphology: hillslope;
- HYDROLOGYDE: 1835 Hydrogeophysics;
- HYDROLOGYDE: 5415 Erosion and weathering;
- PLANETARY SCIENCES: SOLID SURFACE PLANETS