Simulating Dynamic Roots in the Energy Exascale Earth System Land Model
This study integrates a dynamic root structure in the E3SM Land Model such that the vertical distribution of fine roots can respond to water and nitrogen heterogeneity in the soil to meet the demands of the plant.
The simulated root profiles generally agree with observations and although simulated productivity declines, model agreement with observations improves. The model response highlights additional model processes that should be the focus of future work, including climate dependent root depth, root hydraulics, root form and function, and better nitrogen uptake.
Most Earth System Models, including the Energy Exascale Earth System Model, do not include time-varying root distribution, despite evidence that indicates roots respond to their environment with foraging strategies to increase uptake of resources. I have modified the E3SM Land Model (ELM) to accommodate the root response to heterogeneity in the soil column of water and nitrogen by adding a new root growth algorithm that distributes fine roots in soil layers weighted by the water and nitrogen availability, with a preference given to water stress. Furthermore, crops were given a root depth that changed over the growing season to simulate the rapid growth of roots for crop species. This study describes the new model and evaluates how the root distribution and gross primary productivity are changed along with several sensitivity experiments with different levels of minimum water stress. The model simulated root profiles generally agree with observations, showing shallow roots in water saturated systems and in dry and boreal ecosystems. However, the model did not capture the deep roots in the dry season tropics. Furthermore, the model estimated gross primary productivity (GPP) decreased compared with the default root model, especially in the dry season tropics and other wet seasonal regions. Increases in GPP are in dry and boreal ecosystems. Despite the loss of GPP, the model agreement with observations is improved slightly. The model response is the result of a stronger impact on GPP to increases in water rather than nitrogen. When roots are weighted toward nitrogen layers, water uptake decreases, therefore causing a decrease in GPP. The opposite response occurs when water stress is simulated. When a higher weight of root profiles is in soil layers with water, the model responds with deeper roots and increased water uptake. The model response also highlights processes that are missing in ELM.