The role of residence time in diagnostic models of global carbon storage capacity: model decomposition based on a traceable scheme

As a key factor that determines carbon storage capacity, residence time (tau(E)) is not well constrained in terrestrial biosphere models. This factor is recognized as an important source of model uncertainty. In this study, to understand how tau(E) influences terrestrial carbon storage prediction in diagnostic models, we introduced a model decomposition scheme in the Boreal Ecosystem Productivity Simulator (BEPS) and then compared it with a prognostic model. The result showed that tau(E) ranged from 32.7 to 158.2 years. The baseline residence time (tau'(E)) was stable for each biome, ranging from 12 to 53.7 years for forest biomes and 4.2 to 5.3 years for non-forest biomes. The spatiotemporal variations in tE were mainly determined by the environmental scalar (xi). By comparing models, we found that the BEPS uses a more detailed pool construction but rougher parameterization for carbon allocation and decomposition. With respect to xi comparison, the global difference in the temperature scalar (xi(t)) averaged 0.045, whereas the moisture scalar (xi(w)) had a much larger variation, with an average of 0.312. We propose that further evaluations and improvements in tau'(E) and xi(w) predictions are essential to reduce the uncertainties in predicting carbon storage by the BEPS and similar diagnostic models.