Fourteen-year growth response of young lodgepole pine to repeated fertilization

Four rates of N (0, 50, 100, and 150 kg·ha–1) with and without a fertilizer mix containing P, K, S, Ca, Mg, and micronutrients were applied to a stand of 8-year-old lodgepole pine (Pinus contorta var. latifolia Engelm.) in four applications during 7 years. Fourteen years after the first fertilization, mean tree basal area and volume were significantly increased in the treatments containing the two highest rates of N plus the fertilizer mix. Mean tree basal area was 38% greater and mean tree volume was 42% greater in the highest N (525 kg·ha–1) plus mix treatment than in the control treatment. Stand basal area increased 48% over the control in the highest N plus mix treatment. Mean tree basal area and volume were 27 and 25% greater than the control under the intermediate N rate (350 kg·ha–1) with mix, respectively. Height growth was not significantly increased by fertilization. Both higher rates of N (four applications of >100 kg·ha–1) and the fertilizer mix were required for significant growth responses to occur. Addition of N alone or mix alone had a negative effect on mean tree basal area and volume. Addition of higher N rates without the mix decreased foliar total S and SO4-S concentration and increased the N/S ratio. Addition of the higher rates of N without the mix aggravated slight K deficiencies. The responses to the higher N rates in combination with the fertilizer mix are mainly attributable to S.