Effect of repeated fertilization on stem growth in old stands of Pinus sylvestris in South East Norway

We studied effects of repeated nitrogen applications on the stem growth of mature Scots pine at four sites with the yield potential of around 3-4.5 m3·ha-1·yr-1. Treatments included control (0) and fertilization with ammonium nitrate at 100 and 200 N kg ha-1. Some plots were supplied with corresponding doses of urea. Each treatment was normally performed five times at six or seven year intervals. At all sites fertilization was omitted in one block once or twice to study the response to delayed application. The plot size was 22 × 22 m with buffer zones of 4 m. Measurements were made on an inner plot, 14 × 14 m. Tree ring widths usually reached a maximum two to four years after fertilization, returning to about the original level after seven to ten years. Responses to ammonium nitrate were significant, and larger to 200 than to 100 N kg ha-1. Responses to urea were significant at 200 N kg ha-1 only. The annual volume increment over the experimental period was 1.5 to 2.0 m3·ha-1 larger in plots provided with 200 N kg ha-1 than in control plots, or even more as the buffer zones of 4 m did not completely prevent tree roots from growing into neighbouring plots.