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Fig. 1. Sampling regions 1–6 used in the 13th National Forest Inventory (NFI13) of Finland. Regions 1–3 form the reporting domain South Finland and regions 4–6 form the reporting domain North Finland.

Table 1. Stratification of first-phase clusters in Northernmost Lapland in the 13th National Forest Inventory of Finland. The proportions were computed from MS-NFI-2019 (multi-source NFI 2019; Mäkisara et al. 2022) land pixels in 4 × 4 pixel windows containing the plot centres. If the mean canopy height (CHM, https://www.metsakeskus.fi/fi/avoin-metsa-ja-luontotieto/metsatietoaineistot/metsavaratiedot) of the pixels around a sample plot location was less than 5 m, then all pixels of that window originally classified as ‘poorly productive forest’ were re-classified into ‘unproductive land’. 
At least one pixel classified into other land use than forestry in MS-NFI-2019 Stratum 0
No other land use pixels Proportion of productive forest
≤1%  >1%, ≤25% >25%, ≤85% >85%
Proportion of unproductive land <10%  Stratum 3  Stratum 3  Stratum 2  Stratum 1 
≥10%, <40% Stratum 3  Stratum 3  Stratum 2  Stratum 2 
≥40%, ≤85% Stratum 3  Stratum 3  Stratum 3  Stratum 3 
>85%  Stratum 4  Stratum 3  Stratum 3  Stratum 3 
Table 2. Land areas represented by strata, number of second-phase clusters and field sample plots on land, as well as land areas represented by one plot in Northernmost Lapland of Finland in NFI13. 
Stratum  Area, km2  Number of clusters  Number of land plots  area/plot, km2 
0  2552 29 188  13.574 
1  1975 27 168  11.756 
2  6193 67 457  13.551 
3  8834 62 442  19.986 
4  8603 11 80  107.54 
Northernmost Lapland total  28 158 196 1335 
Table 3. Number of plot centres in the NFI13 of Finland for the old permanent, new permanent and temporary clusters by land use class.
Land use class  Temporary clusters  Old permanent clusters  NFI12 established permanent clusters  New NFI13 permanent clusters  Total 
Productive forest  8641  9693  16 654  8756  43 744 
Poorly productive forest 768  927  1169  846  3710 
Unproductive land  714  786  1193  904  3597 
Other forestry land  98  106  180  111  495 
Forestry land total  10 221  11 512  19 196  10 617  51 546 
Agriculture land  1440  1610  2423  1302  6775 
Built-up land  561  630  1073  515  2779 
Roads  191  200  322  174  887 
Power lines  48  56  115  60  279 
Land total  12 461  14 008  23 129  12 668  62 266 
2

Fig. 2. An illustration of Voronoi-neigbourhoods of some field plot clusters (black dots) of NFI13 in Finland. Light orange lines show the boundaries of Voronoi-tiles associated to each cluster and dark orange lines connect neighouring clusters, i.e., those with a common boundary segment.

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Fig. 3. Area by land use class in 1921–2023 in Finland according to NFI1–NFI13.

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Fig. 4. The proportions of site types (fertility classes) (Hotanen et al. 2018b) on mineral soils of the productive forest a) in South and b) in North Finland according to NFI13: 1 = herb-rich forests, 2 = herb-rich heath forests, 3 = mesic heath forests, 4 = sub-xeric heath forests, 5 = xeric heath forests, 6 = barren heath forests, 7 = rocky and sandy soils and alluvial lands.

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Fig. 5. The proportions of grouped site types (fertility classes) (Hotanen et al. 2018) on mineral soils in 1951–2023 a) in South Finland and b) in North Finland according to NFI3–NFI13.

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Fig. 6. Area of the drained mires by drainage stages and area of drained mineral soil sites in 1951–2023 in Finland according to NFI3–NFI13.

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Fig. 7. Area of productive forest by age class and dominant tree species in a) South Finland, b) North Finland, and c) whole Finland according to NFI13.

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Fig. 8. Proportion of dominant tree species in productive forests 1921–2021 in a) South Finland, and b) North Finland according to NFI13.

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Fig. 9. Proportion of age classes in productive forests 1921–2023 in a) South Finland and b) North Finland according to NFI1-NFI13.

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Fig. 10. Total volume of growing stock in a) whole Finland by tree species group and b) South and North Finland according to NFI1–NFI13. Scots pine group in a) includes other conifers than Norway spruce.

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Fig. 11. The mean annual volume increment in productive forests by tree species groups according to NFI13 in South Finland, North Finland, and whole country.

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Fig. 12. The annual volume increment (M m3) by tree species in a) South Finland, b) North Finland, and c) the whole country 1921–2023 according to NFI1–NFI13. NFI1 estimate is for the territory of Finland of those times, NFI2 estimate for both the current and historical territory of Finland.

Table 4. Average growth indices for the increment measurement periods of NFI13 and NFI12 in South Finland and in North Finland (100 = average level of 1990–2023).
Scots pine Norway spruce Birch
NFI South Finland North Finland South Finland North Finland South Finland North Finland
NFI12 104 101 113 101 97 89
NFI13 93 89 110 103 90 90
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Fig. 13. Annual diameter increment indices in South and North Finland for a) Scots pine b) Norway spruce, and c) birch. Index value 100 is the average in 1990–2023.

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Fig. 14. Estimates of annual natural losses (M m3) and its proportion of annual volume increment (%) 1920–2016 according to National Forest Inventories (NFIs) of Finland. Until the 1970s the estimates of annual natural losses are excluding bark.

Table 5. Forest balance between NFI12 and NFI13 mid years (2016–2021) for South Finland, North Finland, and the whole country. Notation: GS12 = Volume for growing stock in NFI12, GS13 = Volume for growing stock in NFI13, Difference = Increment – Drain – (GS13 – GS12). All values are in M m3. 
GS12  Increment Drain GS12 + Increment – Drain GS13 Increment – Drain GS13 – GS12 Difference Standard error of change
A  B  C  D = A + B – C E  F = D – A = B – C G = E – A  F – G
South Finland                   
   Scots pine  713  133  121  726  719  13  6  7  9.2 
   Norway spruce  572  146  130  588  586  16  14  2  9.9 
   Broadleaves  339  87  89  337  350  –2  11  –13  5.2 
   Total  1624  366  339  1651  1655  27  31  –5  15.2 
North Finland                 
   Scots pine  530  90  56  564  564  34  34  0  9.0 
   Norway spruce  168  31  15  183  177  15  9  7  5.2 
   Broadleaves  153  29  25  157  156  4  3  1  3.6 
   Total  852  149  96  905  897  53  45  8  11.0 
Whole country                   
   Scots pine  1244  223  177  1292  1283  48  39  8  12.9 
   Norway spruce  740  176  145  772  763  32  23  8  11.2 
   Broadleaves  492  115  113  495  507  3  17  –13  6.3 
   Total  2475  515  436  2558  2552  83  77  3  18.8 
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Fig. 15. Proportion of damaged area by causing agent group in productive forest available for wood supply in a) South Finland, and b) North Finland according to NFI13.

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Fig. 16. Proportion of the most significant causing agents in terms of damaged area in a) pine dominated, b) spruce dominated, and c) broadleaf dominated productive forest available for wood supply in Finland according to NFI13. Proportions are calculated from the forest area dominated by the tree species group in question.

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Fig. 17. Proportion of stand quality classes on productive forest available for wood supply in Finland according to NFI3–NFI13.

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Fig. 18. Observed area (M ha) of annual thinnings and regeneration cuttings in Finland according to NFI9–NFI13.

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Fig. 19. Mean volume of lying and standing dead wood in productive forests according to NFI9–NFI13 in a) South Finland, b) North Finland and c) whole Finland.

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Fig. 20. Mean volume of lying and standing dead wood in productive forests available for wood supply (FAWS) according to NFI9–NFI13 in a) South Finland, b) North Finland and c) whole Finland.

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Fig. 21. Mean volume of lying and standing dead wood in productive forests not available for wood supply (FNAWS) according to NFI9–NFI13 in a) South Finland, b) North Finland and c) whole Finland.