Impact of Spin-up Forcing on Vegetation States Simulated by a Dynamic Global Vegetation Model Coupled with a Land Surface Model

A dynamic global vegetation model (DGVM) coupled with a land surface model(LSM) is generally initialized using a spin-up process to derive aphysically-consistent initial condition. Spin-up forcing, which is theatmospheric forcing used to drive the coupled model to equilibrium solutionsin the spin-up process, varies across earlier studies. In the present study,the impact of the spin-up forcing in the initialization stage on thefractional coverages (FCs) of plant functional type (PFT) in the subsequentsimulation stage are assessed in seven classic climate regions by a modifiedCommunity Land Model's Dynamic Global Vegetation Model (CLM-DGVM). Resultsshow that the impact of spin-up forcing is considerable in all regionsexcept the tropical rainforest climate region (TR) and the wet temperateclimate region (WM). In the tropical monsoon climate region (TM), the TR andTM transition region (TR-TM), the dry temperate climate region (DM), thehighland climate region (H), and the boreal forest climate region (BF),where FCs are affected by climate non-negligibly, the discrepancies ininitial FCs, which represent long-term cumulative response of vegetation todifferent climate anomalies, are large. Moreover, the large discrepancies ininitial FCs usually decay slowly because there are trees or shrubs in thefive regions. The intrinsic growth timescales of FCs for tree PFTs and shrubPFTs are long, and the variation of FCs of tree PFTs or shrub PFTs canaffect that of grass PFTs.