Table 1. Description of fertilization experiments at the different study sites with climate and initial stand attributes. The mean annual air temperature and annual precipitation were 30-year averages from 1990 to 2020 (Finnish Meteo­rological Institute 2023). During the study period, the mean annual air temperature was 0.8 °C higher, and the annual precipitation slightly lower (592–667 mm) than the 30-year average (Finnish Meteorological Institute 2023). The stand characteristics were calculated as the averages of the 27 circular plots (trees with Dbh > 8 cm) for each study site.
Study site Liperi Ilomantsi Juuka Savonranta
Location 62.55846 N, 29.06265 E 62.85543 N, 30.67705 E 63.066199 N, 28.874640 E 62.139028 N, 29.149086 E
Altitude (m above sea level) 160 180 170–200 120–140
Main tree species Pinus sylvestris Pinus sylvestris Picea abies Picea abies
Pinus sylvestris, Picea abies,
Betula spp. volume (%) 		78.0, 19.3 and 2.7 86.0, 12.5 and 1.5 8.8, 86.6 and 4.6 11.6, 86.4 and 2.0
Mean annual air temperature (°C) 3.5 3.0 2.8 3.4
Effective temperature sum (d.d.) 1200–1300 1100–1200 1100–1200 1200–1300
Annual precipitation (mm) 640 644 661 670
Initial stand attribute measurements 24 May–7 Jun 2018 8–21 Jun 2018 13–22 May 2019 23–31 May 2019
Fertilization 27 Jun 2018 20 Jul 2018 25–27 Jul 2019 10 Aug 2019
Stand density (stems ha–1) 513 687 619 455
Mean basal area (m² ha–¹) 19 20 20 17
Mean stand diameter (cm) 23 18 21 21
Initial stand volume (m3 ha–1) 167 170 187 159
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Fig. 1. Layout of the fertilization experiments using ground spreading in the Scots pine study sites at Liperi and Ilomantsi and helicopter spreading in the Norway spruce study sites at Juuka and Savonranta. Each of the three fertilization treatments (target doses of 0, 150 and 200 kg N ha–1) were applied to three replicate 1-ha blocks, each containing three circular plots (radius 7.98 m, area 200 m2).

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Fig. 2. Average ± standard deviation for measured fertilizer dose (kg N ha–1) over three blocks (each containing three circular plots) for each fertilization treatment delivered via ground spreading in the Scots pine study sites at Liperi and Ilomantsi and via aerial spreading in the Norway spruce study sites at Juuka and Savonranta in comparison to the fertilizer targets of 150 and 200 kg N ha–1.

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Fig. 3. Initial and final volume (V) of living trees at the end of the study period (left) and the corresponding volume growth (ΔV) of the trees (right) over three blocks (each containing three circular plots) for each fertilization treatment in the Scots pine study sites at Liperi and Ilomantsi and in the Norway spruce study sites at Juuka and Savonranta for fertilizer targets of 0, 150 and 200 kg N ha–1.

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Fig. 4. Relationships between realised fertilizer dose based on block-level averages from the circular plots and measured concentrations of N in the needles (left) and the humus layer (middle) in the Scots pine and Norway spruce study sites, and the relationship between the humus and needle N concentrations (right). The optimal diagnostic level of needle N concentration is >1.5%.

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Fig. 5. Volume growth (ΔV) as a function of fertilizer dose (top left), initial volume of living trees at the end of the study period (top right) and measured concentrations of N in the needles (bottom left) and humus layer (bottom right) in the Scots pine study sites (Liperi and Ilomantsi together) and in the Norway spruce study sites (Savonranta and Juuka together). Data points represent average values from the three circular plots in each block (three replicates per treatment).