Table 1. Description of the research sites where the effects of nitrogen fertilization on ground vegetation were studied. The research sites were located in Eastern-Finland. The mean annual air temperature and the annual precipitation are the 30-year average from 1990–2020 (Finnish Meteorological Institute 2023). Stand characteristics were calculated as averages of 27 plots (200 m2).
Research site Liperi Ilomantsi Juuka Savonranta
Latitude 62,55846 N 62,85543 N 63,066199 N 62,139028 N
Longitude 29,06265 E 30,67705 E 28,874640 E 29,149086 E
Altitude (m a.s.l.) 160 180 170–200 120–140
Dominant tree species Pinus sylvestris Pinus sylvestris Picea abies Picea abies
Pinus sylvestris volume (%) 78 86 0 11.6
Picea abies volume (%) 19.3 12.5 86.6 86.4
Betula spp. volume (%) 2.7 1.5 4.6 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
Date of N application 27.06.2018 20.07.2018 25.–27.07.2019 10.08.2019
Means of application Forwarder Forwarder Helicopter Helicopter
Stand density (stems ha–1) 513 687 619 455
Mean diameter at breast height (cm) 23 18 21 21
Initial volume (m3 ha–1) 167 170 187 159
Range of the N fertilizer dose
(kg N ha–1) (> 0 in plots*) 		110–288 109–369 48–441 50–426
*) In Ilomantsi in one plot and in Savonranta in two plots planned to be fertilized the funnel-trap-observed amount of fertilizer was zero due to problems in fertilizing and they were excluded from range values.

Fig. 1. The location of the four research sites in Eastern Finland, a schematic presentation of the block-design including three replicates of the fertilization treatments (150 kg N ha–1 and 200 kg N ha–1) and unfertilized control blocks. Each block contained three circular plots (200 m2) from where the tree characteristics were measured. Each plot further contained three subplots (0.5 m2) for measuring the cover and species composition of the ground vegetation. Every plot (200 m2) also contained four mineral soil sample points and sixteen mor humus layer sample points.

2

Fig. 2. The development of mean cover of mosses and lichens, dwarf shrubs and herbaceous plants in unfertilized control plots (left) and in fertilized plots (right) during the study period. Standard deviations are presented in parenthesis. The plots were fertilized in 2018 (Liperi, Ilomantsi) and 2019 (Juuka, Savonranta). An asterisk in the year 2022 indicates statistically significant change in the mean cover from 2018 to 2022 (Liperi, Ilomantsi) and from 2019 to 2022 (Juuka, Savonranta).

Table 2. The change in the ground vegetation and coarse woody debris cover expressed as slopes (β1, Eq. 1) from linear regression analysis. The slope describes the arcsine-square root transformed cover change per amount of funnel-trap-observed N (Mg ha–1). If the slope is negative the cover is reduced by the fertilization and if the slope is positive the cover increases. Statistically significant slope values (p < 0.05) are shown as bold text.
Vegetation cover Liperi (Pine) Ilomantsi (Pine) Juuka (Spruce) Savonranta (Spruce)
2018–2021 2018–2022 2021–2022 2018–2021 2018–2022 2021–2022 2019–2021 2019–2022 2021–2022 2019–2021 2019–2022 2021–2022
Tree seedlings 0.01 –0.05 –0.06 0.17 0.52 0.24 0.00 0.03 0.03 0.05 0.13 0.08
Herbaceous plants 0.11 0.04 –0.08 0.56 0.36 –0.08 0.12 0.21 0.09 0.39 0.50 0.11
Grasses –0.42 –0.50 –0.08 0.35 0.19 –0.08 0.34 0.40 0.07 0.17 0.23 –0.06
Herbs –0.08 0.18 0.26 0.32 0.28 0.02 –0.18 –0.07 0.08 0.06 0.10 0.07
Pteridophytes 0.48 0.27 –0.21 0.03 –0.02 –0.02 –0.08 –0.08 0.00 0.22 0.38 0.16
Dwarf shrubs –0.25 –0.39 –0.15 0.00 0.01 0.18 –0.04 –0.02 0.02 –0.05 –0.18 –0.12
Vaccinium myrtillus –0.18 –0.33 –0.15 0.09 0.32 0.36 –0.01 –0.03 –0.01 –0.04 –0.13 –0.09
Vaccinium vitis-idaea –0.15 –0.11 0.04 0.02 –0.06 –0.09 –0.03 –0.02 0.01 0.01 0.00 –0.01
Mosses and lichens –0.39 –0.23 0.13 –0.55 –0.57 0.11 –0.60 –0.29 0.30 –0.08 –0.29 –0.21
CWD 0.52 0.13 –0.39 –0.28 0.38 0.62 0.23 0.41 0.18 –0.04 0.17 0.21
3

Fig. 3. The relationship between funnel-trap-observed N amount (in plots with area of 200 m2) and vegetation cover change (%) of the mosses and lichens, dwarf shrubs and herbaceous plants in vegetation sub-plots between 2018 (Liperi, Ilomantsi) or 2019 (Juuka, Savonranta) and 2022. The points indicate the measured change in cover with corresponding funnel-trap-observed N amount.

Table 3. Concentration of nitrogen (N) (% of dry mass) and carbon to nitrogen (C:N) ratio in the mor humus layer three to four years after fertilization in unfertilized control plots and fertilized plots.
Research site N C:N
Control Fertilized Control Fertilized
Liperi (Pine) 1.1 1.3 26 23
Ilomantsi (Pine) 0.9 0.9 29 32
Juuka (Spruce) 1.2 1.5 26 24
Savonranta (Spruce) 0.9 1.2 27 25
4

Fig. 4. Nitrogen (N) concentration (% of dry mass (d.w.)) and carbon to nitrogen (C:N) ratio in the mor humus layer in unfertilized and fertilized study plots (200 m2) as a function of the funnel-trap-observed N fertilizer dose.

Table 4. The estimated amount of nitrogen (N) in the ground vegetation and mor humus layer kg ha–1 in 2022, three to four years after the fertilization. Statistically significant (p < 0.05) differences between the means of the unfertilized control plots and the fertilized plots within the research site are shown as bold text.
Research site Treatment Herbaceous plants Dwarf shrubs Mosses and lichens All ground vegetation Mor humus layer All
Liperi Control 2.3 5.2 5.2 12.7 813.9 826.6
Fertilized 4.9 4.0 1.2 10.1 1117.0 1127.1
Ilomantsi Control 0.2 4.7 3.1 8.0 404.2 411.6
Fertilized 0.3 4.2 1.9 6.4 447.5 453.5
Juuka Control 3.2 1.2 7.4 11.8 683.2 695.0
Fertilized 2.2 0.8 7.1 10.1 752.8 762.8
Savonranta Control 2.8 1.4 7.5 11.8 489.1 500.9
Fertilized 3.6 0.6 5.3 9.5 588.5 598.0