EAR-PF Cracking the Critical Zone: Tree roots in fractures and a proposed mechanistic soil production function

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Dr. Jill Marshall has been granted an NSF EAR Postdoctoral Fellowship to carry out a research and education plan at the University of California, Berkeley, in collaboration with the University of Colorado, Boulder. She will develop numerical models to describe the effect tree roots have on soil production. The investigation is important across a broad range of problems in critical zone science, from understanding landscape evolution to quantifying soils sustainability. The investigation will use data collected at three Critical Zone Observatories (Eel River, Boulder Creek, and Southern Sierra CZOs) that have patches of terrain with thin to no soils and bedrock exposures. This enables quantitative observations of tree roots growing in fractures across sites underlain by different lithologies and fracture patterns. The development of a biomechanical soil production model will allow for improved insight into the role of vegetation in soil sustainability and resilience. The education plan, based in Round Valley, CA (a rural region in the Eel watershed encompassing the Round Valley Indian Tribal Lands) has two components: multi-age field-based science activities directed at the school district's students and a LiDAR workshop for the Tribes' Natural Resource Department. The educational plans were developed with extensive community input and these synergistic activities meet the community's needs of expanding access to science activities and providing tools for future endeavors.Research in this project will test the hypothesis that depth-dependent soil production is a function of root pressures, variations in root density with depth, and tree sway modulated by rock properties. To develop a geomorphic transport law that describes how and why soil production rates vary with depth, this project will combine Discrete Element Method simulations incorporating bonded grains, rock properties, and field-derived values for root pressures at the bedrock interface from data collected at the three Critical Zone Observatories. A suite of bedrock-impeded tree roots will be monitored with force sensors to measure external root pressures generated from growth, diurnal fluctuations due to tree water fluxes, and torque about the tree base from wind and snow events. Hydrologic routing, net primary productivity, carbon storage in soils, and mineral supplies for the geochemical reactor are all linked to rates of soil production. By including tree-driven processes in a geomorphic process law for soil production, this project will enable numerical experiments that explore how the presence and absence of vegetation control surface and near-surface processes and landscape form.