A simple approach to apply an eco-evolutionary optimality model with a global climatological aridity function to predict the spatial and seasonal dynamics of net ecosystem exchange

The P model is a parameter-sparse model for gross primary production (GPP) based on eco-evolutionary optimality principles. Here we describe a global implementation of the sub-daily P model, which separates the acclimation response of photosynthetic parameters to environmental variations (with an e-folding time scale of 15 days) from the rapid response of photosynthesis (with a time step of 30 minutes), together with a daily soil-moisture accounting scheme (SPLASHv1.0) and a semi-empirical response function describing how the influence of soil moisture on GPP varies systematically with climatic aridity. We assess the model’s ability to reproduce seasonal cycles of net ecosystem exchange (NEE), as inferred from spaceborne atmospheric CO₂ measurements via the Global Carbon Assimilation System version 2 (GCAS2021). For simplicity, we assume net primary production (NPP) is a constant fraction of GPP, and constrain total annual heterotrophic respiration (R_H) to match total annual NPP at each 0.5˚ grid cell. The response of R_H to environmental variations is represented via a model that links R_H to physical and biological processes involving oxygen transport and microbial activity, influenced by the soil water content and the temperature. This model mechanistically represents the nonlinear coupling of moisture and temperature dynamics, replacing the canonical “function times function” approach. The coupled model reproduces the observed spatial variation in amplitude and timing of NEE, with excellent agreement in extratropical regions. It also captures the interannual differences (over a time span of 10 years) in the seasonal cycle aggregated by the Fifth Assessment Report (AR5) geographic reference regions. In the tropics and some Southern regions, however, the large interannual variability in the inversion products results in a signal of the climatological seasonal cycle of NEE that is too small to assess model performance. Our results suggest that the temperature and moisture dependences of heterotrophic respiration, as well as primary production, are major controls of the seasonal cycle of NEE and that the observed global patterns in this cycle can be well captured by an extremely parameter-sparse model.