A prognostic phenology scheme for global terrestrial carbon cycle models

Prognostic and mechanistic schemes for the determination of plant phenological stages from environmental conditions and for the estimation of net primary production (NPP) are presented. The new schemes account for different biomes and are included in a global model of carbon cycling in the terrestrial biosphere. The capability of such a model to simulate the seasonal cycle of atmospheric CO2 is explored. The model is forced by mean monthly climate variables (temperature, precipitation and Light) and the mean annual CO2 concentration. It predicts atmosphere-biosphere CO2 exchange fluxes, leaf area index (LAI), and the times of budburst and leaf abcission. The predicted variables can be validated against data on the observed annual cycle of atmospheric CO2 concentration and against observations of LAI derived from satellite data. The estimated annual NPP of forests appeared realistic; however, the NPP of grass-dominated biomes was greatly underestimated. This seems to be related to the fact that belowground biomass is not explicitly considered in the model. The results of a simulation of the seasonal cycle of atmospheric CO2 concentration using a 3-dimensional atmospheric transport model were in satisfactory agreement with the observations.