A mechanistic nitrogen limitation model for CLM(ED)

Abstract:

Photosynthetic capacity is a key plant trait that determines the rate of photosynthesis; however, in Earth System Models it is either a fixed value or derived from a linear function of leaf nitrogen content. A mechanistic leaf nitrogen allocation model have been developed for a DOE-sponsored Community Land Model coupled to the Ecosystem Demography model (CLM-ED) to predict the photosynthetic capacity [V_c,max25 (µmol CO₂ m^-2 s^-1)] under different environmental conditions at the global scale. We collected more than 800 data points of photosynthetic capacity (V_c,max25) for 124 species from 57 studies with the corresponding leaf nitrogen content and environmental conditions (temperature, radiation, humidity and day length) from literature and the NGEE arctic site (Barrow). Based on the data, we found that environmental control of V_c,max25 is about 4 times stronger than the leaf nitrogen content. Using the Markov-Chain Monte Carlo simulation approach, we fitted the collected data to our newly developed nitrogen allocation model, which predict the leaf nitrogen investment in different components including structure, storage, respiration, light capture, carboxylation and electron transport at different environmental conditions. Our results showed that our nitrogen allocation model explained 52% of variance in observed V_c,max25 and 65% variance in observed J_max25 using a single set of fitted model parameters for all species. Across the growing season, we found that the modeled V_c,max25 explained 49% of the variability in measured V_c,max25. In the context of future global warming, our model predicts that a temperature increase by 5^oC  and the doubling of atmospheric carbon dioxide reduced the V_c,max25 by 5%, 11%, respectively.