Simulating dynamic roots in the Energy Exascale Earth System Land Model
Fine roots function to provide nutrients and water for plant growth, and have a strong influence on surface energy, photosynthesis, and the carbon cycle. Although there is evidence indicating that roots respond to their environment with foraging strategies to increase uptake of resources like water and nitrogen, most Earth System Models do not take into account root distribution (growth) over time. Beth Drewniak, a climate scientist in EVS, has modified the Energy Exascale Earth System Model (E3SM) Land Model (ELM) to accommodate root response to the heterogeneity of water and nitrogen in the soil column by adding a new root growth algorithm that distributes fine roots in soil layers weighted by water and nitrogen availability, giving preference to water stress. This modification also gives crops a vertical root profile that changes over the growing season so that the rapid growth of roots for crop species can be simulated.
The study, recently published in the Journal of Advances in Modeling Earth Systems, describes the new model and evaluates how the root distribution and gross primary productivity (GPP) are changed relative to the default root model. It also presents several sensitivity experiments with varying levels of minimum water stress. Simulated root profiles generally agreed with observations, showing shallow roots in water saturated systems and in dry and boreal ecosystems, but did not capture the deep roots in the dry season tropics. The estimated GPP decreased in the dry season tropics and other wet seasonal regions, and increased in dry and boreal ecosystems. The model response is the result of a stronger impact on GPP to increases in water than to nitrogen. When root profiles are weighted toward soil layers with nitrogen, water uptake decreases, causing a decrease in GPP. The opposite response occurs when water stress is simulated. When root profiles are weighted toward 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, including climate dependent root depth, root hydraulics, root form and function, and better nitrogen uptake.
Read the full article in the Journal of Advances in Modeling Earth Systems.