MODELING DAILY GROWTH AND NITROGEN TURNOVER FOR A SHORT-ROTATION FOREST OVER SEVERAL YEARS

The growth and nitrogen now 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 w as divided into compartments of leaves, stems, roots and easily availa ble assimilates. The soil was divided into horizontal layers. Input va riables represented climate, management and soil physical variables. T he last mentioned variables were simulated using an associated model. The model was tested using measured values of leaf biomass, stem bioma ss, leaf area and soil mineral nitrogen from a Salix viminalis stand i n central Sweden. The measurements and simulation were made from 1985 (the year following planting) until 1988. Growth-related processes of the model evaluated for I-year-old shoots were applied to 2-year-old s hoots with good results, both before and after harvest and fungal infe ction. According to regression analysis for the whole simulation perio d, the model explained 98% of the variation in stem biomass and 86% of the variation in the leaf biomass. The corresponding values for growt h rates, expressed as changes between successive measurement occasions , were 88% and 62%, respectively. High leaching rates were simulated w ith the multilayered soil submodel during September 1986 (resulting in toxic nitrate concentrations), as a consequence of intensive irrigati on.