Exploring Hydrological Influences on Tree-Driven Bedrock Physical Weathering at Two Karst Sub-Watersheds

Trees are found in almost every environment, and in thin-soil settings, their extensive root systems can occupy pre-existing rock fractures to access water and nutrients.While we ha ve visual evidence of trees detaching bedrock ( i.e. turning rock into soil) via tree -throw, recent work suggest s that trees may also exert sub-critical forces on bedrock that weaken or break apart the surrounding rock over time . We speculate that sub-critical forces by trees on the surrounding bedrock are generated by root torque driven by minor wind gusts and root swelling during water-uptake. However, the frequency and magnitude of tree-driven bedrock physical weathering mechanisms may vary depending on water availability, wind intensities, tree species, and size. This study investigates how water availability differences at two karst sub-watersheds may influence tree-driven forces on bedrock generated by wind gusts and water uptake . We hypothesize that water availability differences between the sites will affect water uptake, resulting in variations in tree properties such as mass and elasticity that govern tree-sway patterns as well as the degree of diurnal root swelling and thus determine the frequency and magnitude of biotic forces on the bedrock. W e instrumented six trees in the thin-soil-mantled karst of Savoy Experimental Watershed, in Savoy, Arkansas. The two sub-watersheds are drained by two springs with contrasting rates of discharge providing an ideal location to observe forces at the root-rock interface at sites with similar lithology, aspect, and climate but varying hydrologic conditions. We monitored three trees at each spring consisting of h ackberry, b oxelder, and e lm trees with heights of 13 - 28m and diameters ranging from 0.7 - 2.2m. We installed force sensors between roots growing into bedrock and tree trunks wedged against bedrock to monitor forces generated at the tree-bedrock interface and accelerometers on the trunk to measure tree movement. Using tree sway, wind speed, and precipitation along with spring flow and water levels measurements, as a proxy for water availability, we examine the influence of water availability on tree properties that may control root-forces on the bedrock to build relationships between above-ground tree sway and below-ground forces for trees of different species and sizes.