Above-belowground carbon allocation in earth system models
Above-belowground carbon allocation is one critical mechanism for plant growth and its adaptation to the changing environmental condition. The model representation of carbon allocation significantly influence the simulated carbon stock and further land-atmosphere exchange. How well the above-belowground carbon allocation is represented in System Models (ESMs) will be fundamentally important for the model applications to evaluate and project climate changer and terrestrial feedbacks. However, very few studies have investigated and evaluated the above-belowground carbon allocation in the ESMs. In this study, we analyzed vegetation root:shoot ratio, carbon stock and fluxes of ten ESMs simulations from the Coupled Model Inter-comparison Project Phase 5 (CMIP5) which were used for the recently released IPCC report. We used three observational datasets to evaluate ESMs results for the mean measurements during 1980-1999 based on historical experiments. Two compiled data sets were from two review papers about root:shoot ratios and root turnover rates in terrestrical biome, and another dataset came from IPCC (Good Practice Guidance for Land Use, Land-Use Change and Forestry) which was generated from national and regional survey data. Each compiled dataset provided individual value of around 300 observational sites, and the IPCC dataset supplied an average value for special vegetation type.
The two observational datasets are consistent in biome-level estimation of root:shoot ratio, while the ESMs slightly underestimated the belowground carbon portion in forest ecosystems and significantly underestimate the belowground carbon portion in grassland ecosystems. General agreement between the models and observational data for root turnover times are true for all key biomes. Evidence suggests that ESMs significantly underestimated vegetation root carbon flux. The findings of this study suggest that the above-belowground carbon allocation in current ESMs is largely uncertain, indicating the importance of model improvement and field measurement of above-belowground carbon allocation.