Effects of high levels of fertilization with urea on growth of thinned and unthinned Douglas-fir stands

Abstract

The diameter and volume growth of Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) were documented 9 years after thinning and fertilization. The treatments involved three levels of thinning (0, 1/3, and 2/3 of basal area removed) and six levels of fertilization (0–1344 kg N/ha) with urea. Increments for both diameter and gross volume increased with the rate of fertilization and responses were still apparent 9 years after treatment. For unthinned plots, the 9-year volume growth responses were 30, 50, and 80% with fertilizer rates of 224, 448, and 896 kg N/ha, respectively. The efficiency of fertilizer use, measured as stem volume response per unit of nitrogen applied, decreased with the rate of fertilization, but this picture may change over a longer response period. There was a positive interaction between fertilization and thinning such that high levels of both mutually enhanced growth. Mortality increased with fertilization, but only noticeably in unthinned plots.