Equilibrium analysis of a decomposition and yield model applied to Pinus radiata plantations on sites of contrasting fertility

Recent models of growth and nutrient cycling relate forest productivity to canopy photosynthesis, as influenced by the effect of nutrient cycling on foliar nitrogen concentration. A useful approach for analysing the impact of elevated CO2 or altered nitrogen inputs on production is to consider model solutions where recycling leaves, fine roots, litter and soil organic pools of intermediate turnover time are in equilibrium, while tree stems and recalcitrant humus are accumulating or releasing carbon and nitrogen. This equilibrium analysis, employed by the Generic Decomposition and Yield (GʹDAY) model, was applied to Pinus radiata plantations growing on an infertile site in Australia and a fertile site in New Zealand. ted productivities and foliar nitrogen concentrations were substantially lower than observed for the young (12-year-old) stands, particularly for the fertile site. The model predictions were closer to values expected for older stands late in the commercial rotation cycle when reduced wood production rates reduce the net nitrogen requirements for growth. These results underscore the importance of the net release of nitrogen from soil organic matter early in the life of a stand and suggest that care should be taken in using equilibrium analyses to estimate the impacts of elevated [CO2] on forest production.