FVSBGC: a hybrid of the physiological model STAND-BGC and the forest vegetation simulator

The physiological model STAND-BGC was linked to the forest vegetation simulator (FVS) as a system extension. With the linked model, an FVS user can invoke STAND-BGC to obtain tree- and stand-level physiological output in addition to standard FVS mensurational output. An FVS user may choose to have increments in diameter, height, crown ratio, and mortality from STAND-BGC replace those generated by FVS. This option essentially replaces the empirical growth engine of FVS with the physiological engine from STAND-BGC. Physiological and mensurational outputs were generated for an existing, fully stocked, Pinus contorta Dougl. ex Loud. stand, with and without thinning, using the hybrid model. The STAND-BGC engine produced results similar to FVS for the unthinned stand but predicted more rapid tree growth than FVS following thinning. Simulations for a newly regenerated stand using the linked model allowed assessment of the predicted effects of grass competition and drought on stand production. Comparisons of model predictions to remeasured permanent plot data showed the empirical and process growth engines had similar precision, but that STAND-BGC substantially overpredicted growth, while FVS slightly underpredicted growth. The need for model calibration and opportunities for more sophisticated communication between models is discussed.