In this paper it was investigated to what extent it is necessary to include estimates of site nitrogen (N) conditions for prediction of biomass production of short rotation willow (Salix sp.) forests in Sweden. At the same time it was tested a growth model, that can be parameterised from a very limited amount of soil information of the site concerned and still is believed to mimic major effects of N dynamics on growth. The willow growth model consists of three main modules: one for simulation of annual biomass production, one for daily water conditions and one for daily soil nitrogen conditions. Stem biomass production is predicted from the fraction of solar radiation intercepted, radiation use efficiency, shoot age and mortality and water and nitrogen availability. Nitrogen limitation to biomass production, was estimated as a seasonal N availability factor. This factor is estimated as the difference between seasonal plant N demand, and the seasonal net supply of N to the soil mineral N pool. The water and nitrogen modules were parameterised and calibrated for a clay soil and for a loamy sandy soil, as simulated by detailed soil heat, water, nitrogen and carbon models (SOIL and SOILN), that were tested elsewhere. Parameter values of other soil types were estimated with help of USDA soil classification. The model was applied to 22 field experiments located at five different sites in southern Sweden. They differed in weather conditions, soil type and fertilisation level (N treatment). The model was able to explain 82 - 86 % of the observed variations in harvested biomass and 33 - 41 % of the variations in annual production. Taking into account the soil N conditions improved the model predictions of variations in harvested biomass as caused by N treatments, from 70 % to 82 %. Model predictions of the variations in annual production, or in harvest production of stands within the same N treatment, were not improved. It is concluded, that for site conditions of willow forest plantations in Sweden, differences in N treatments are more important than differences in natural soil N availability between sites, and that differences in N availability between sites, due to fertilisation, are more important than differences in water availability or solar energy absorption, in determining variations in harvested biomass
