Modelling forest ecosystem net primary production: the hybrid simulation approach used in forecast

In order to evaluate the impacts of alternative stand-level management scenarios on long-term site productivity, forest resource managers need ecologically based forest growth models. The forecast forest ecosystem management simulation model combines the traditional bioassay modelling approach with process-based simulation modelling to provide a method of projecting future forest biomass yield as well as a variety of other ecosystem variables and social values under a range of management conditions. A review of the hybrid simulation approach to modelling forest ecosystems is provided, and the representation of stand-level net primary production and nutrient cycling in forecast is described. The major driving function in the model (shade-corrected foliage nitrogen efficiency), the concept of site quality, and the simulation of site quality change are discussed. The value of an accurate definition of the state of the simulated ecosystem at the start of a run is emphasized, and the use of the ecostate (state of the ecosystem, as generated by forecast) file in this process is described. Limitations of the forecast modelling approach are reviewed.