LISTENING FOR HOW TREES, WIND, WATER, AND TEMPERATURE PROPAGATE AND AMPLIFY NEAR-SURFACE FRACTURING

Fractures provide first order control on the rate bedrock is liberated to become erodible sediment. Thus, understanding fracture growth is foundational to understanding and modeling interactions between erosion and tectonics. Near-surface bedrock fractures propagate due to environmental and tectonic stresses, yet there are few studies that document the key driving factors propagating this fracture growth. For example, both biotic and abiotic mechanisms (e.g. plants and temperature) produce sufficient stresses to crack near-surface rock, but no study has been designed to discern how these stresses combine in a natural setting. Here we seek to quantify how tree-driven stresses combine with temperature cycling to accelerate, amplify and/or uniquely generate cracks due to roots embedded in near-surface bedrock. In the southern Piedmont of NC, we instrumented along contour (thus controlling for aspect and water differences) two beech trees with roots extending into competent quartzite, and an outcrop of fractured, tree-free quartzite flanked by the two trees. We inserted six ~9 mm force sensors at root-rock boundaries, and three in cracks on the tree-free rock, to measure forces generated by wind, temperature fluctuations, root plumping due to water uptake, and precipitation events. Alongside the force sensors we installed 1) acoustic emission sensors to record real-time elastic energy release from both subcritical and critical cracking and, 2) type-T thermocouples to record rock surface temperatures. In addition, we deployed a full meteorological station to collect air temperature, wind, precipitation and shortwave radiation data for the site. We hypothesize that cracking associated with solar-related diurnal temperature cycling as well as faster temperature fluctuations caused by wind and precipitation events will be amplified in the tree-hosting rock (due to tree-torque and water-driven root swelling). We further predict that higher root pressures at night due to water uptake will provide for a biotic contribution to rock cracking not recorded in the tree-free rock. Additionally, we speculate that post-big wind events, fresh mineral surfaces exposed due to tree-torque may become hot spots of subsequent cracking activity due to fine root and microbial expansion into these freshly exposed surfaces