EFFECT OF NITROGEN-FERTILIZATION ON GROWTH IN A SALIX-VIMINALIS STANDUSING A RESPONSE-SURFACE EXPERIMENTAL-DESIGN

In order to find a nitrogen fertilization regime that is economically feasible in commercial short-rotation forest stands of basket willow ( Salix viminalis L.), a field trial was established on a clay soil near Vasteras, central Sweden, in early 1990. A response surface design wa s used in which three levels of treatment were chosen for each single year: year 1-0, 30, 60, kg N ha(-1); year 2-0, 60, 120, kg N ha(-1); y ear 3-0, 90, 180, kg N ha(-1); year 4-0, 60, 120, kg N ha(-1). Thirty- two combinations of these levels, out of 81 possible, were chosen and divided into four blocks. Thus: eight unique treatments were randomize d to the plots within each block, providing no replicates at the end o f the experimental period. This gave us a more flexible and informativ e experiment than many of those used earlier with rather few treatment s, although they were replicated. After 4 years the willow stand was h arvested and the accumulated stem production of each treatment was ass essed. The effect of nitrogen fertilization on accumulated stem growth over the experimental period was found to be significant only for nit rogen applied in years 2 and 3, with a somewhat (statistically signifi cant) larger effect in year 2. Growth response to nitrogen fertilizati on was best approximated with a Function including terms with applied amount of nitrogen in years 2 and 3 and, furthermore, a term of intera ction with negative coefficient between these years. The interaction b etween these years was interpreted as the system's ability to recycle nitrogen from one year to another.