Nutrient limitation of forest growth, especially nitrogen (N) deficiency, is widespread in the boreal region. N fertilisation has thus become a common silvicultural practice in Fennoscandian Norway spruce stands, but to what extent phosphorus (P) is co-limiting productivity and how initial basal area affects the growth response to N addition remains unresolved. To address these questions, two experiments were established in mid-rotation Norway spruce stands in southern Sweden where decades of high atmospheric N deposition have reduced the severity of N-limitation. In a P experiment initiated in 2011, we tested P addition alone (two applications of 200 kg P ha⁻¹) and in a second study also starting in 2011 (NP experiment), a single dose of N was administered alone (200 kg ha⁻¹ in thinned and unthinned stands, hereafter called N and N-unthinned treatments) and in combination with P (N+P = one-time 200 kg N ha⁻¹, two applications of 200 kg P ha⁻¹ in thinned stands). P addition alone increased PAI (periodic annual increment) significantly by 21 % during the first, moister assessment period up to 2014 and by 18 % in the drier 2015–2019 period, resulting in a 10 % increase in final stem volume yield. In the NP experiment, significant PAI increases under favourable meteorological conditions up to 2014 occurred in all fertilisation treatments. The strongest effects were seen in the N-unthinned treatment while no significant additive effect resulted from the joint addition of N and P (N: +20 %, N-unthinned: +38 %, N+P: +23 %). In the drier 2015–2019 period, only the N+P treatment caused significantly greater PAI (+29 %). Final stem volume yield in the NP experiment significantly increased by 10 %, 39 % and 16 % in the N, N-unthinned and N+P plots, respectively. In both experiments, foliar P and thus P/N rose drastically in response to P addition alone or in combination with N. Minor increases in leaf area index (LAI) only occurred in P-containing treatments. Our findings indicate that Norway spruce productivity in southern Sweden is constrained to a similar extent by both N and P. Sustainable nutrient management in Norway spruce growing regions with high N loading (like southern Sweden) should prioritise P over N supply.

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.

Silvicultural techniques aimed at promoting forest biomass production can help meet the growing demand for renewable materials and mitigate climate change. One-time nitrogen (N) addition late in the rotation is a well-established method to stimulate growth in coniferous forests in northern Europe, but the potential gains from earlier and repeated fertiliser application remain uncertain. Here, we tested the impact of repeated fertilisation in juvenile Norway spruce stands across 9 sites covering a wide range of growing conditions over a 700 km stretch from central to southern Sweden. We tested the fertilisation effects using two separate studies: i) an interval trial with a fertilisation frequency of one (F1), two (F2), or three years (F3) performed at plot-level across five sites (2002–2014), and ii) a practice-oriented trial with a two-year fertilisation interval (F2) applied at stand-level and replicated at four sites (2003–2013). The composition of the nutrient mix in each plot was optimised based on foliar nutrient analyses. In the interval trial, all three fertilisation schedules strongly increased periodic annual increment (PAI) (F1: 105 %, F2: 93 %, F3: 79 %) relative to the unfertilised control, resulting in more than a doubling of stem volume yield in the F1 and F2 treatments (120 % and 110 %, respectively) and a significantly smaller but still sizeable yield stimulation of 82 % in the F3 treatment. Nitrogen use efficiency (NUE, stemwood volume increase per unit mass of N added) was similar among fertilisation intervals (on average 130 m³ ha−1 1000 kg N⁻¹), indicating that the extra N provided through yearly fertilisation (F1) is redundant given the similar stemwood yields in the F2 treatment. In the practice-oriented trial, the sole F2 treatment increased PAI by 95 % over the control, translating into a yield stimulation of 114 % and an almost identical NUE to that of the interval trial. NUE greatly exceeded the figures typically observed with traditional late-rotation fertilisation and correlated inversely with baseline site productivity (using site index as a proxy) in the F1 and F2 treatments (the latter pooled across the two trials). Our results clearly indicate that nutrient limitation restricts growth and carbon (C) capture in young Norway spruce plantations in northern Europe to less than half of their potential, highlighting repeated fertilisation at nutrient-poor sites as an effective management tool to support a growing bioeconomy and enhance C sequestration.

Low nutrient availability often limits productivity in northern forests. In a nutrient optimisation trial, we investigated the effects of fertilisation and irrigation on soil moisture, leaf area index (LAI) as well as height and radial growth in 25-year-old stands of pedunculate and sessile oak (Quercus robur L., Q. petraea (Matt.) Liebl.) growing on abandoned farmland in southwestern Sweden. Control (C), fertilisation (F), irrigation (I), and irrigation +fertilisation (IF) treatments were arranged in a randomized complete block design. End of growing season analysis of foliar nutrients guided the quantitative composition of next year’s fertiliser mix. Volumetric soil moisture (VWC) was significantly higher in the I and IF treatments compared to the C and F treated stands of both oak species. We did not observe a fertiliser-related reduction in VWC (except for 2015, when VWC in F- treated Q. robur stands was significantly lower than the control by about 18 %). This is in line with the unaffected LAI estimates (5.3–5.9) suggesting no stimulation of leaf production that could drive increases in transpiration with subsequent soil moisture depletion. There was no treatment ×year interaction for any of the growth-related variables. Treatments had no significant effects on basal area growth, which increased annually by 1.72 and 1.54 m² ha^-1 on average for Q. petraea and Q. robur, respectively. Pre-treatment height differences in Q. petraea stands (7–12 % taller trees in the C and IF plots) persisted throughout the study resulting in significant effects, while no significant differences in height occurred in Q. robur. Periodic annual volume increment varied more strongly following drier periods but there were no significant differences among treatments. Our findings indicate that fertilisation causes no or only minor increases in oak water use, suggesting that nutrient addition in oak stands within this precipitation regime does not require simultaneous irrigation. Most importantly, our data implies that the soil nutrient legacies of past agricultural use suffice to maximise the productivity of oak stands on abandoned farmland typical of the main oak growing region in southwestern Sweden.