Optimal nitrogen fertilization of boreal conifer forest

Background: Forest fertilization offers a means to increase the production of renewable resources. Nitrogen is the most common fertilizer in boreal upland forests. There is plenty of research on the effect of nitrogen fertilization on volume growth, but less research on the optimal timing of fertilization and optimal management of fertilized stands. Methods: This study used simulation and optimization to analyze the profitability of fertilization, optimal management of fertilized stands and the effects of fertilization on cash flows and timber yields. The management of 100 stands representing the most common growing sites of Scots pine and Norway spruce was optimized. Results: Fertilization improved profitability in most of the analyzed stands. Profitability improved most in spruce stands growing on mesic site. Improving stem quality increased the economic benefit of fertilization. The timber yields of medium-aged conifer stands can be increased by almost 1 m(3)ha(-1)a(-1) (15%) in sub-xeric pine and mesic spruce sites and about 0.5 m(3)ha(-1)a(-1) (5%) in mesic pine and herb- rich spruce sites when the recommended nitrogen dose (150 kg ha(-1)) is applied once in 30 years. Conclusions: Nitrogen fertilization of boreal conifer forest should be used mainly in spruce-dominated stands growing on medium sites. The gains are the highest in stands where the mean tree diameter is 16-20 cm and stand basal area is 14-20 m(2)ha(-1).