Root system hydraulic architecture is a key determinant of plants’ ability to withdraw water from the soil, satisfying transpirational demand. Presently, the representation of this component of the hydrological cycle in large-scale models is generally very simplistic, even though transpiration accounts for much of the terrestrial heat and water surface fluxes, exercises control over photosynthetic uptake of CO2, and regulates the distribution of plant communities across the earth’s surface. My dissertation work aims to close this gap by defining simple functions that characterise the results of explicit 3D simulations of root system growth and water uptake.
The central research question my research addresses: what is the effect of root system architecture on plant uptake of water? As the question is motivated by finding a more accurate representation of this element of the water cycle in Land Surface Models and the associated Dynamic Vegetation Models, a related question addressed is: how is the effect of root system architecture on plant water uptake best represented in large-scale models? I hypothesise that root system architecture affects water uptake by two separate mechanisms: (1) the distribution of root surface absorbing area spatially throughout the soil, and (2) affecting the pressure at the absorbing surfaces, thereby modifying the rate of uptake per unit area.
My methodological approach relies on several separate components: (a) RootGrow, original MATLAB code that simulates the stochastic growth of a root system as a function of an intrinsic set of parameters as well as its environment; (b) parametrising RootGrow based on quantitative descriptions of root systems observed empirically with innovative imaging techniques such as Magnetic Resonance Imaging (MRI); (c) a finite-element simulation of the physics and physiology of water transport in the soil and root system using COMSOL; and (d) Ecosystem Demography Model v2.1 (ED2), a large-scale Dynamic Vegetation Model, based on a method of upscaling individual-scale models of plant ecology. The first three components are used for small-scale simulations of coupled root-system growth and water uptake. The outputs of these simulations will be used to define and parametrise simple relationships characterising water uptake as a function of root system architecture. These will in turn be implemented in ED2 with an aim to improve its performance under water-limited and transitional conditions.
Several products are expected from my dissertation research. The first is developing a theoretical approach to conceptualising root system architecture that is based on quantitative empirical measurement and can be used directly to inform and constrain mathematical representation of root system geometry. Second, from results of simulations of individual plant water uptake, this work will advance the basic understanding of water relations of vascular plants – particularly the place of root system architecture in ecological strategies. Finally, the product of the modelling work at the larger scale will be a functional representation of root systems that is directly relatable to the results of smaller-scale work, thus increasing the realism of these models.