Pushing Boundaries in Ecohydrological Geophysics to Test the Two-Water Worlds Model

Recent isotopic (δ18O and δ2H) research into the nature of soil/ground water uptake by trees has shown that two separate “water worlds” exist in the sub-surface where water fluxes associated with trees, through the process of transpiration, are different to those related to groundwater and streamflow. Such compartmentalised fluxes suggest that trees utilise soil waters that are not part of the overall, ‘precipitation-recharge-streamflow’ water reservoir that is traditionally used to assess ecohydrological water demand and cycling with the critical zone – therefore impacting on all current water flux models. Despite the clear evidence of isotopic separation between these two ‘water world’ pathways, there are still important, unanswered questions about the nature of the segregation process. For instance,

Is the process persistent or temporal? (the latest research suggests a dominant seasonal element).

Does it depend on a) the tree/climate type, b) the distribution & extent of the root structure and c) the degree of leaf coverage?

Does Oxygen-Carbon flux in the local area influence the degree of water take-up and therefore isotopic differentiation?

To address these key issues, this project will pioneer investigations into the nature of water take-up by mature woodland trees in temperate climates and the environmental conditions that control the isotopic separation processes. Using the latest in 3D near-surface geophysical investigation techniques and soil-tree water isotope analysis, it will attempt to develop a forest stand model of moisture content distribution and isotopic (δ18O and δ2H) differentiation across control and enhanced CO2 tree test sites at the University of Birmingham’s BIFoR FACE forest research facility.

(https://www.birmingham.ac.uk/research/activity/bifor/face/index.aspx).

The research will investigate the impact of seasonality and leaf loss on the isotopic differentiation mechanism, how the nature and extent of tree root spatial heterogeneity affects measurement analysis and whether elevated levels of CO2 have any bearing on the isotopic separation process and its timing seasonally.

Multi-frequency, 3D ground penetrating radar (GPR) surveys will be conducted to determine the spatial extent (and volume) of the tree root mass whilst high-resolution, total-waveform Lidar surveys will be collected seasonally to determine the change in leaf coverage across each tree in the forest stands.

At the same time, 3D Spectral Electrical Resistivity Imaging surveys and direct sampling for isotopic water analysis will be conducted to provide the crucial baseline and temporally constrained data of soil moisture content distribution/change in the tree root zone. In combination, this information will be used to develop a conceptual model for the volume uptake of water from the soil, its nature (either as bound moisture in the soil fraction or free water in the open pore spaces) and the seasonal and/or environmental impacts on this process. In addition to developing cutting-edge, in-situ sensing technologies, the project will also establish new methodologies and protocols for the on-going analysis of soil/xylem hydrogen-oxygen isotopes in complex forest stand environments.