Abstract
Projecting the dynamics and functioning of the biosphere requires a holistic consideration of whole‐ecosystem processes. However, biases toward leaf, canopy, and soil modeling since the 1970s have constantly left fine‐root systems being rudimentarily treated. As accelerated empirical advances in the last two decades establish clearly functional differentiation conferred by the hierarchical structure of fine‐root orders and associations with mycorrhizal fungi, a need emerges to embrace this complexity to bridge the data‐model gap in still extremely uncertain models. Here, we propose a three‐pool structure comprising transport and absorptive fine roots with mycorrhizal fungi (TAM) to model vertically resolved fine‐root systems across organizational and spatial–temporal scales. Emerging from a conceptual shift away from arbitrary homogenization, TAM builds upon theoretical and empirical foundations as an effective and efficient approximation that balances realism and simplicity. A proof‐of‐concept demonstration of TAM in a big‐leaf model both conservatively and radically shows robust impacts of differentiation within fine‐root systems on simulating carbon cycling in temperate forests. Theoretical and quantitative support warrants exploiting its rich potentials across ecosystems and models to confront uncertainties and challenges for a predictive understanding of the biosphere. Echoing a broad trend of embracing ecological complexity in integrative ecosystem modeling, TAM may offer a consistent framework where modelers and empiricists can work together toward this grand goal.