Establish the Physical Link between Tree Growth and Critical Zone Processes Using a Coupled Hydrology-root system Optimization Model

Tree growth is limited by physical environment including soil depth and water/nutrients holding capacity, the product of long term landscape development. Meanwhile trees affect landform through regulating weathering and erosion processes. Root regulated below ground processes is the key link in the feedback between tree growth and geomorphic processes. We present a dynamic root system model with a mechanistic weathering function.

The root system model describes the root architecture by a dendritic network system where all the fine roots are in first order and coarse roots are in higher order. The variation of the distribution of coarse and fine roots over depth in response to belowground water profiles is modeled through the optimization process to give the maximum net plant primary productivity of a given size tree.

The resulting distribution of coarse roots and fine roots and soil water content are integrated into the parameters of a coupled physical-chemical weathering model. The physical weathering model calculates the rate of fragmentation of a give size particle from its size and strength, and the external stress. The chemical weathering model calculates the dissolution (or production of a secondary mineral) rate of a candidate mineral by the surface area of the particle, reaction kinetics, and water content of the saprorite. Then, the chemical weathering rate is used to calculate the decreasing of particle strength. Once integrated into a dynamic vegetation model, the root system model provides a physical link between tree growth and weathering and erosion processes.