Fig. 1. In 2022, study sites (A) were established in eastern Finland (27°53′58″–28°06′58″ E, 63°44′25″–63°48′06″ N). The drill diameter was 40 cm (B). Sites 1 and 2 were planted with Norway spruce seedlings, and sites 3–5 were planted with Scots pine seedlings. Panel (C) shows a Norway spruce seedling planted in late summer. The map was created using ArcGIS Pro (version 3.1.0). Photos (B and C): Eetu Tarvainen.
| Table 1. Site details for Norway spruce and Scots pine seedlings planted in different seasons during the summer of 2022. In the first measurement (2022), all planting spots were measured (n = 4120). The follow-up measurements in 2023 and 2024 were based on 60 permanent circular sample plots (r = 5.64 m). Planting season: early (ES), mid (MS), and late (LS) summer. Growing season: the number of growing seasons at the end of the study period (2024). Site type: herb-rich heath (Oxalis-Myrtillus type [OMT]), mesic heath (Vaccinium myrtillus type [MT]), and sub-xeric heath (Vaccinium vitis-idaea type [VT]) forests. | ||||||||
| Area (ha) | Spots and seedlings | Sample plots | Growing seasons | Site type | ||||
| Site | Tree species/Season | 2022 | 2023 | 2024 | ||||
| 1 | Norway spruce | 0.6 | 1251 | 376 | 337 | 18 | ||
| ES | 0.2 | 414 | 102 | 82 | 6 | 2.0 | MT | |
| MS | 0.2 | 393 | 125 | 114 | 6 | 1.5 | MT | |
| LS | 0.2 | 444 | 149 | 141 | 6 | 1.0 | MT | |
| 2 | Norway spruce | 0.6 | 1250 | 374 | 279 | 18 | ||
| ES | 0.2 | 406 | 125 | 92 | 6 | 2.0 | OMT | |
| MS | 0.2 | 404 | 118 | 98 | 6 | 1.5 | OMT | |
| LS | 0.2 | 440 | 131 | 89 | 6 | 1.0 | OMT | |
| 3 | Scots pine | 0.2 | 400 | 111 | 78 | 6 | ||
| ES | 0.2 | 400 | 111 | 78 | 6 | 2.0 | VT | |
| 4 | Scots pine | 0.2 | 422 | 124 | 74 | 6 | ||
| ES | 0.2 | 422 | 124 | 74 | 6 | 2.0 | VT | |
| 5 | Scots pine | 0.4 | 797 | 289 | 221 | 12 | ||
| LS | 0.4 | 797 | 289 | 221 | 12 | 1.0 | VT | |
| Total | 2.0 | 4120 | 1274 | 989 | 60 | |||
| Table 2. Spot- and seedling-specific notes that were documented in 2022, 2023, and 2024. Notes that were not recorded in 2022 are marked with “-”. In the first measurement, all planting spots were measured and evaluated (n = 4120). The follow-up measurements in 2023 (n = 1274) and 2024 (n = 989) were based on 60 permanent circular sample plots (r = 5.64 m) systematically placed at the study sites. | ||||||
| Note | Measurement year | |||||
| 2022 | 2023 | 2024 | ||||
| n | % | n | % | n | % | |
| No notes | 2864 | 70 | 408 | 32 | 351 | 35 |
| Stumpa | 24 | 1 | 12 | 1 | 11 | 1 |
| Roota | 203 | 5 | 221 | 17 | 83 | 8 |
| Stonea | 351 | 9 | 158 | 12 | 183 | 19 |
| Logging residuesa | 95 | 2 | 59 | 5 | 57 | 6 |
| Humus layer residuesb | 518 | 13 | 267 | 21 | 1 | 0 |
| Waterc | 53 | 1 | 46 | 4 | 22 | 2 |
| Logging traild | 12 | 0 | 1 | 0 | 2 | 0 |
| Peate | - | - | 40 | 3 | 0 | 0 |
| Brushwoodf | - | - | 1 | 0 | 273 | 28 |
| Tree-top browsingg | - | - | 52 | 4 | 6 | 1 |
| Tree-top breakageg | - | - | 2 | 0 | 0 | 0 |
| Loose seedlingg | - | - | 7 | 1 | 0 | 0 |
| Total | 4120 | 100 | 1274 | 100 | 989 | 100 |
| a Ground and soil obstacles reducing planting spot quality b Humus layer residues within the planting spot c The presence of water within the planting spot d Machine tracks formed during the harvesting e Paludified areas where planting spots were covered with peat f Naturally regenerated broadleaf trees g Seedling damage affecting vitality | ||||||
| Table 3. Details of planting spots (depth, diameter, and area) by season (early [ES], mid [MS], and late [LS] summer) for Norway spruce and Scots pine planting sites (n = 4120). Data are presented as the mean ± standard deviation. | ||||
| Season/ | Planting spot | |||
| Tree species | Depth (cm) | Diameter (cm) | Area (cm2) | Number |
| ES | 10.1 ± 4.4 | 41.9 ± 3.8 | 1388 ± 255 | 1642 |
| Norway spruce | 8.9 ± 3.8 | 42.0 ± 3.4 | 1393 ± 210 | 820 |
| Scots pine | 11.3 ± 4.5 | 41.8 ± 4.2 | 1384 ± 293 | 822 |
| MS | 9.8 ± 4.6 | 46.8 ± 4.8 | 1735 ± 359 | 797 |
| Norway spruce | 9.8 ± 4.6 | 46.8 ± 4.8 | 1735 ± 359 | 797 |
| Scots pine | - | - | - | - |
| LS | 10.9 ± 4.6 | 52.0 ± 6.2 | 2156 ± 539 | 1681 |
| Norway spruce | 8.9 ± 3.4 | 49.8 ± 4.2 | 1960 ± 321 | 884 |
| Scots pine | 13.2 ± 4.6 | 54.5 ± 7.0 | 2373 ± 640 | 797 |
| All | 10.4 ± 4.5 | 47.0 ± 6.8 | 1768 ± 537 | 4120 |
| Norway spruce | 9.2 ± 4.0 | 46.3 ± 5.3 | 1702 ± 384 | 2501 |
| Scots pine | 12.2 ± 4.7 | 48.0 ± 8.6 | 1871 ± 700 | 1619 |
Fig. 2. Proportions of planting spots classified as good, satisfactory, and poor by study site and season (n = 4120). Season: early (ES), mid (MS), and late (LS) summer. Site type: herb-rich heath (Oxalis-Myrtillus type [OMT]), mesic heath (Vaccinium myrtillus type [MT]), and sub-xeric heath (Vaccinium vitis-idaea type [VT]) forests.
| Table 4. Cumulative link model (CLM) for the quality classification of the planting spots based on data from 2022. The CLM predicts the probability of belonging to the quality classes poor, satisfactory, or good. The threshold coefficients determine the cutoffs between quality classes (poor-to-satisfactory and satisfactory-to-good). The cumulative odds ratios (CORs) are shown in the last column. Site type × Season: interaction coefficients between site types and planting seasons. Site type: herb-rich heath (Oxalis-Myrtillus type [OMT]), mesic heath (Vaccinium myrtillus type [MT]), and sub-xeric heath (Vaccinium vitis-idaea type [VT]) forests. Season: early (ES), mid (MS), and late (LS) summer. | ||||
| Parameter | Estimate | Std. error | p-value | COR |
| Spot diameter | 0.023 | 0.008 | 0.007 | 1.023 |
| Spot depth | 0.134 | 0.021 | <0.001 | 1.144 |
| Spot depth2/100 | −0.333 | 0.076 | <0.001 | 0.716 |
| Notes (ref. No notes) | ||||
| Stump or roota | −3.168 | 0.166 | <0.001 | 0.042 |
| Stonea | −3.293 | 0.156 | <0.001 | 0.037 |
| Logging residuesa | −4.959 | 0.261 | <0.001 | 0.007 |
| Waterb | −7.134 | 0.350 | <0.001 | 0.001 |
| Humus layer residuesc | −4.609 | 0.150 | <0.001 | 0.010 |
| Site type × Season (ref. MT × ES) | ||||
| MT × MS | 1.974 | 0.197 | <0.001 | 7.199 |
| MT × LS | 1.451 | 0.195 | <0.001 | 4.266 |
| OMT × ES | 0.433 | 0.161 | 0.007 | 1.541 |
| OMT × MS | 1.402 | 0.192 | <0.001 | 4.064 |
| OMT × LS | 1.231 | 0.193 | <0.001 | 3.425 |
| VT × ES | −0.257 | 0.135 | 0.057 | 0.773 |
| VT × LS | 0.834 | 0.190 | <0.001 | 2.303 |
| Threshold coefficients | ||||
| Poor | Satisfactory | −3.065 | 0.398 | ||
| Satisfactory | Good | 1.073 | 0.390 | ||
| a Ground and soil obstacles reducing planting spot quality b The presence of water within the planting spot c Humus layer residues within the planting spot | ||||
Fig. 3. Soil compaction around the planted seedlings (compacted or uncompacted), seedling stem angle (upright [<20°], askew [20°–40°], or prone [>40°]), seedling position (planted in the middle, side, or outside of the planting spot), and planting depth (proper [2–3 cm], shallow [root plug shallow and/or exposed], or deep [shoot clearly less visible than usual]) based on follow-up measurements in 2023 (n = 1274) and 2024 (n = 989). Unstable refers to seedlings that were loose from the soil.
Fig. 4. Observed seedling vitality (healthy, weakened, or dead) by planting season, tree species, site type, and measurement year (2023: n = 1274 and 2024: n = 989). The number of missing seedlings (n = 285) was calculated by subtracting the measured number of seedlings in 2024 from the measured number of seedlings in 2023. Planting season: early (ES), mid (MS), and late (LS) summer. Site type: herb-rich heath (Oxalis-Myrtillus type [OMT]), mesic heath (Vaccinium myrtillus type [MT]), and sub-xeric heath (Vaccinium vitis-idaea type [VT]) forests.
Fig. 5. Seedling number (all seedlings) and number of healthy seedlings per hectare based on the measured permanent sample plots in 2023 and 2024. Planting season: early (ES), mid (MS), and late (LS) summer. Site type: herb-rich heath (Oxalis-Myrtillus type [OMT]), mesic heath (Vaccinium myrtillus type [MT]), and sub-xeric heath (Vaccinium vitis-idaea type [VT]) forests.
| Table 5. Cumulative link mixed model (CLMM) for the vitality classification of the planted seedlings based on data collected in 2023 and 2024. The CLMM predicts the probability of belonging to the seedling vitality class dead, weakened, or healthy with the sample plot-level random effect (variance). The threshold coefficients determine the cutoffs between vitality classes (dead-to-weakened and weakened-to-healthy). The cumulative odds ratios (CORs) are shown in the last column. Tree species × Site type × Season × Year: interaction coefficients among tree species, site types, planting seasons, and measurement years. Site type: herb-rich heath (Oxalis-Myrtillus type [OMT]), mesic heath (Vaccinium myrtillus type [MT]), and sub-xeric heath (Vaccinium vitis-idaea type [VT]) forests. Planting season: early (ES), mid (MS), and late (LS) summer. | ||||
| Parameter | Estimate | Std. error | p-value | COR |
| Tree species × Site type × Season × Year (ref. Norway spruce × MT × ES × 2023) | ||||
| Norway spruce × MT × ES × 2024 | −0.307 | 0.412 | 0.457 | 0.736 |
| Norway spruce × MT × MS × 2023 | −0.880 | 0.409 | 0.031 | 0.415 |
| Norway spruce × MT × MS × 2024 | −0.279 | 0.433 | 0.519 | 0.756 |
| Norway spruce × MT × LS × 2023 | 1.774 | 0.526 | 0.001 | 5.894 |
| Norway spruce × MT × LS × 2024 | 0.337 | 0.442 | 0.446 | 1.401 |
| Norway spruce × OMT × ES × 2023 | 0.400 | 0.470 | 0.394 | 1.492 |
| Norway spruce × OMT × ES × 2024 | 0.192 | 0.484 | 0.692 | 1.212 |
| Norway spruce × OMT × MS × 2023 | 0.070 | 0.444 | 0.875 | 1.072 |
| Norway spruce × OMT × MS × 2024 | 2.153 | 0.804 | 0.007 | 8.612 |
| Norway spruce × OMT × LS × 2023 | 1.193 | 0.536 | 0.026 | 3.296 |
| Norway spruce × OMT × LS × 2024 | 0.227 | 0.488 | 0.642 | 1.255 |
| Scots pine × VT × ES × 2023 | −1.573 | 0.369 | <0.001 | 0.207 |
| Scots pine × VT × ES × 2024 | −1.320 | 0.388 | 0.001 | 0.267 |
| Scots pine × VT × LS × 2023 | −0.124 | 0.375 | 0.741 | 0.883 |
| Scots pine × VT × LS × 2024 | −0.622 | 0.374 | 0.096 | 0.537 |
| Seedling anglea (ref. Upright) | ||||
| Askew | −0.940 | 0.125 | <0.001 | 0.391 |
| Prone | −1.376 | 0.292 | <0.001 | 0.253 |
| Soil compactionb (ref. Compacted) | ||||
| Uncompacted | −0.749 | 0.182 | <0.001 | 0.473 |
| Notes (ref. No notes) | ||||
| Tree-top browsingc | −2.296 | 0.298 | <0.001 | 0.101 |
| Waterd | −0.839 | 0.283 | 0.003 | 0.432 |
| Threshold coefficients | ||||
| Dead | Weakened | −4.388 | 0.347 | ||
| Weakened | Healthy | −2.086 | 0.326 | ||
| Random effects | Variance | Std. dev | ||
| Sample plot (n = 60) | 0.147 | 0.384 | ||
| a Upright (<20°), askew (20°–40°), or prone (>40°) b Soil compaction around the planted seedling c Seedlings with eaten tops d The presence of water within the planting spot | ||||
Fig. 6. Observed seedling heights, excluding those that were dead, had broken or eaten tops, or were loose from the soil (2023: n = 1209; 2024: n = 690). The value at the bottom of the dark-gray bar is the mean height in 2023, the value in the light-gray bar is the mean height difference between 2023 and 2024, and the value above the bar is the mean ± standard deviation of height in 2024. Site type: herb-rich heath (Oxalis-Myrtillus type [OMT]), mesic heath (Vaccinium myrtillus type [MT]), and sub-xeric heath (Vaccinium vitis-idaea type [VT]) forests. Planting season: early (ES), mid (MS), and late (LS) summer.
| Table 6. F-tests for the fixed main effects and two-way interaction effects of year, season, and site type in a linear mixed model for Norway spruce height. Year: 2023 and 2024. Site type: herb-rich heath (Oxalis-Myrtillus type [OMT]) and mesic heath (Vaccinium myrtillus type [MT]) forests. Planting season: early (ES), mid (MS), and late (LS) summer. Seedling vitality: healthy and weakened. Degrees of freedom: numerator (numDF) and denominator (denDF). | ||||
| Parameter | numDF | denDF | F-value | p-value |
| Intercept | 1 | 1302 | 790.423 | <0.001 |
| Seedling vitality | 1 | 1302 | 58.274 | <0.001 |
| Year | 1 | 1302 | 97.124 | <0.001 |
| Season | 2 | 30 | 26.932 | <0.001 |
| Site type | 1 | 30 | 0.471 | 0.498 |
| Interaction | ||||
| Year × Season | 2 | 1302 | 16.552 | <0.001 |
| Year × Site type | 1 | 1302 | 39.953 | <0.001 |
| Season × Site type | 2 | 30 | 0.439 | 0.634 |