Taming root systems heterogeneity to reduce Earth System Model uncertainty

Earth System Models (ESMs) are confronted with huge structural uncertainty with respect to the terrestrial biosphere. As photosynthesis and aboveground activities in general have received the most attention, root representation lags behind and is still rudimentary in ESMs. Primary functions of nutrients and water acquisition by fine roots bridge the aboveground and the soil-microbiome complex. However, theoretical and observational advances in the last two decades have revealed that root systems develop structural and functional differentiation to fulfill their functioning. We hypothesize that an appreciation of root systems heterogeneity holds promise to further reduce the structural uncertainty in ESMs. To tame the complexity, instead of one single fine-root pool adopted by current ESMs and ecosystem models we propose a new, function-oriented framework with two pools: absorptive and transportive fine roots. With this two-pool structure, vertically resolved controls on fine-root dynamics are updated with respect to pool-specific phenology (asynchronicity with leaf phenology), photosynthate allocation and partitioning, and turnover (longevity and mortality). We implemented this framework in the Energy Exascale Earth System Model (E3SM) Land Model (ELM) at two sites (Howland Forest AmeriFlux site and Spruce Experiment site) with parameterization facilitated by the Fine-Root Ecology Database (FRED), and conducted uncertainty and sensitivity analyses. Our preliminary results show that this new implementation can dramatically alter above- versus below-ground carbon allocation and ecosystem carbon turnover, but with large uncertainty attributed to indeterminate parameters associated with controls on root phenology, allocation partitioning, and longevity. These uncertainties result from root-specific data inadequacy and process indetermination (e.g., root phenology and mortality), challenging experimentalists to conduct more detailed and extensive field observations. This new representation provides a closer approximation of the theoretical and observational evidence for root system heterogeneity that has developed over the past two decades. Capturing this complex reality robustly in ESMs, though entailing more dedicated efforts, is a promising path to reduce uncertainty.