Modelling daily growth and nitrogen turnover for a short-rotation forest over several years

The growth and nitrogen flow dynamics of a short-rotation willow forest were simulated with a model in which the assimilation rate determined the nitrogen uptake, which in turn, determined the assimilation rate. The stand was assumed to be horizontally homogeneous, and the plant was divided into compartments of leaves, stems, roots and easily available assimilates. The soil was divided into horizontal layers. Input variables represented climate, management and soil physical variables. The last mentioned variables were simulated using an associated model. The model was tested using measured values of leaf biomass, stem biomass, leaf area and soil mineral nitrogen from a Salix viminalis stand in central Sweden. The measurements and simulation were made from 1985 (the year following planting) until 1988. Growth-related processes of the model evaluated for 1-year-old shoots were applied to 2-year-old shoots with good results, both before and after harvest and fungal infection. According to regression analysis for the whole simulation period, the model explained 98% of the variation in stem biomass and 86% of the variation in the leaf biomass. The corresponding values for growth rates, expressed as changes between successive measurement occasions, were 88% and 62%, respectively. High leaching rates were simulated with the multilayered soil submodel during September 1986 (resulting in toxic nitrate concentrations), as a consequence of intensive irrigation.