A process-based, terrestrial biosphere model of ecosystem dynamics (Hybrid v3.0)

A numerical process-based model of terrestrial ecosystem dynamics is described and tested. The model, Hybrid v3.0, treats the daily cycling of carbon, nitrogen, and water within the biosphere and between the biosphere and the atmosphere. It combines a mass-balance approach with the capacity to predict the relative dominance of different species or generalised plant types (such as evergreen needleleaved trees, cold deciduous broadleaved trees, and C3 grasses). The growth of individual trees is simulated on an annual timestep, and the growth of a grass layer is simulated on a daily timestep. The exchange of carbon, nitrogen, and water with the atmosphere and the soil is simulated on a daily timestep (except the flux of tree litter to the soil, which occurs annually). Individual trees and the grass layer compete with each other for light, water, and nitrogen within a ‘plot’. Larger and taller plants shade smaller ones; they also take up a greater proportion of the available water and nitrogen. The above-ground space in each plot is divided into 1 m deep layers for the purposes of calculating irradiance interception; horizontal variation in the plot environment is not treated. The soil is represented as a single layer, with a daily hydrological budget. Decomposition of soil organic matter is calculated using an empirical sub-model. The initial size of each tree seedling is stochastic. To predict the mean behaviour of the model for a particular boundary condition it is necessary to simulate a number of plots. Hybrid v3.0 has been written with three major requirements in mind: (i) the carbon, water, and nutrient cycles must be fully coupled in the soil-plant-atmosphere system; (ii) the internal constraints on the modelʹs behaviour, and the driving forces for the model, must be the same as those which operate in nature (e.g., climate, nitrogen deposition, and the atmospheric concentrations of CO2 and O2); and (iii) the model must be constructed so that it is capable of predicting transient as well as equilibrium responses to climate change. These conditions have largely been met by constructing the model around a set of fundamental hypotheses regarding the general constraints under which plants and soils behave, independently of any particular location or time. The model is thus potentially capable of making reliable predictions of ecosystem behaviour and structure under future, new, atmospheric conditions. The model is tested for a site in eastern North America. A quasi-equilibrium is reached after approximately 250 years with 10 plots. It is found that more plots are not necessary in order to obtain a reliable estimate of mean behaviour. Predictions of productivity, leaf area index, foliage nitrogen, soil carbon, and biomass carbon are all within the range expected for this location. Mortality is shown to be a necessary model component; without it large trees reach a maximum size, and then remain in dynamic equilibrium with the climate, without dying. The model runs at a rate of 0.176 s plot−1 year−1 on a workstation (a 500 year simulation, with 10 plots, thus takes approximately 15 min). A sensitivity analysis demonstrates the importance of the parameterisation of phenology, photosynthesis, and foliage/fine root carbon and nitrogen partitioning for the overall carbon balance of the modelled ecosystem. Hybrid v3.0 has been written with the intention of using it to represent the terrestrial biosphere in a total earth system model. This would be achieved by linking it to models of other components of the earth system, such as the climate and the oceans, in a fully coupled manner. This total earth system model could then be used to answer a large range of questions concerning global environmental change.