Abstract
Large uncertainties in Earth System Models (ESMs) results for terrestrial carbon processes have been attributed to a number of mechanisms, and vegetation carbon allocation algorithm is considered as one key mechanism for the uncertainties.
Above- and below-ground vegetation biomass simulated by nine ESMs was evaluated by comparing model outputs against observational datasets at site, biome and global levels. Three parameters, roots carbon (including fine and coarse roots), total vegetation carbon, and the root: total vegetation carbon ratios (R/T ratios), were chosen for evaluating carbon allocation algorithms in ESMs.
Substantial discrepancies between modeled and observed R/T ratios were found: the R/T ratios from ESMs are relatively constant, approximately 0.2, which are significantly different from a large variation in the R/T ratios (0.1 ~ 0.8) shown in the observational data. ESMs performed well in simulating spatial distribution of carbon densities, however, ESMs have significant biases in simulating root and total vegetation biomass across land ecosystems, especially in tropical, and arctic ecosystems.
The poor performance of ESMs in simulating vegetation carbon allocation suggests model improvements on carbon allocation algorithms are highly needed. Additionally, we also call for intensive field campaign targeting understanding carbon allocation in a complex of climatic and environmental conditions.
