Long-term Effects of Nitrogen Fertilization on Productivity of Subsequent Stands of Douglas-fir in the Pacific Northwest.

The long-term (Carryover) effects of nitrogen (N) fertilization on productivity of five stands of Douglas-fir in the Puget Sound Region of western Washington were quantified. Average height and diameter at breast height (dbh) of 11-12 year old Douglas-fir trees and biomass and N-content of understory vegetation were assessed on control (untreated) and N fertilized plots that had been fertilized with urea 15-22 years ago (total amount of applied N 810-1120 kg ha-1). This block design experiment consisted of two 0.04 ha measurement plots with one control and one fertilized plot in each of the five blocks. Overall productivity was significantly greater on the previously fertilized plots than on the controls. In 2006, the last growth measurement year, mean seedling height was 15% greater (p = 0.06) and mean dbh was 29% greater (p = 0.04) on previously fertilized plots than on control plots. Understory vegetation biomass was 73% greater (p = 0.005), and N-content was 97% greater (p = 0.004) than on control plots. These results show that past N fertilization markedly increased seedling growth, and biomass and N-content of understory vegetation in a subsequent rotation. These findings suggest that N fertilization can potentially increase site productivity of Douglas-fir stands in the Pacific Northwest 15-22 years after application. Thus having multiple implications for natural resource managers. These trends should continue to be monitored, and similar studies should be established, to further understand the Carryover effects of N fertilization on subsequent stands.