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

We studied effects of repeated applications of nitrogen on stem growth of mature Scots pine at four sites with yield potential around 3 - 4.5 m3 ha‑1 yr‑1. Treatments included control (0) and fertilization with ammonium nitrate at 100 and 200 kg N ha‑1. Some plots were supplied with corresponding doses of urea. Applications were repeated up to five times, at five to twenty years intervals. Plot size was 22 x 22 m with buffer zones of 4 m. Measurements were made on an inner plot, 14 x 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 were larger to 200 than to 100 kg N ha‑1. Annual volume increment over the experimental period was 1.5 to 2.0 m3 ha-1 larger in plots provided with 200 kg N ha-1 than in control plots, or even more as the buffer zones of 4 m did not completely exclude tree roots from growing into neighbouring plots.