The necessity for climate change mitigation and the growing demand for renewable materials calls for silvicultural approaches that increase forest biomass production. In northern conifer forests growth is normally limited by nutrients, primarily nitrogen (N). Potential advantages of early and repeated fertiliser applications needs to be studied and compared with traditional late-rotation fertilisation.  Fertilisation influences earlywood proportion and wood density, but responses are variable. The role of phosphorus (P) in areas with high N deposition remains unclear. Moreover, the growth potential of fertilised oak remains underexplored. 

This thesis investigates the effects of nutrient optimisation on Norway spruce and oak growth, as well as its influence on earlywood proportion and basic wood density in spruce stem wood. Also, growth effects of P, N, and NP fertilisation were studied in mid-rotation Norway spruce stands.

Paper I examines the impact of repeated fertilisation at nine young Norway spruce stands comprising an interval study with different fertilisation frequencies across five sites, and a practice-oriented study with two-year fertilisation interval at four sites. All fertilisation treatments significantly increased periodic annual increment (PAI) by 79-105 % over the control. 

Paper II studied the influence of fertilisation on the proportion of earlywood and wood density using X-ray densitometry. Fertilisation caused a reduction in wood density and increases in earlywood proportion. 

Paper III evaluated the effects of fertilisation, irrigation and their combination on pedunculate and sessile oak (Quercus robur L., Q. petraea (Matt.) Liebl.) on abandoned farmland. None of the treatments resulted in a significant growth stimulation indicating that, under the current precipitation regime, the soil nutrient legacies from former agricultural use are sufficient to leverage the maximum growth potential.

Paper IV examined the effects of P, N, and NP fertilisation in two experiments in mid-rotation Norway spruce stands in an area with elevated N deposition. The pure P treatment increased PAI by 21% in assessment period 1 and 18% in period 2. In the NP experiment, all treatments increased PAI in period 1 (N: +20%, N+P: +23%), but in period 2 only the N+P treatment showed a significant 29% increase.