NFI13


NFI data

The sample plot data of the 13th national forest inventory (NFI13) from the years 2019–2023 (Korhonen et al. 2024) were used in the calculations.

MELA data

The input data for MELA, based on the NFI13 field measurements, covers the stands that include a NFI plot center and are on forest land or on poorly productive forest lands. The total number of calculation units in the input data is 51051.

The trees of input data were generated using a model-based approach based on the measurements of stand-level characteristics (the stratum-level basal-area, mean diameter and mean height) and measured NFI sample trees. The stratum-level measurements describe the growing stock of the stand realistically, but the measurements are not fully objective and therefore not unbiased. The tree-level measurements from NFI plots are based on objective procedures and yield unbiased estimates at area-level, but due to the small sample size from only one location within the stand, they include a large amount of random variability at stand level. The model-based approach aims in such description that is as close to the stratum-level measurements within stand as possible, but also utilizes the measured sample trees efficiently, and scales up at area-level to the species-specific totals based on the sample trees. The calculation included the following steps:

  1. The NFI sample trees were allocated to the strata measured on the stand by utilizing information about tree storeys, tree species and diameter of the trees and strata. A tree could be allocated only in a stratum that belongs to the same storey. If no stratum was defined with the species of the tree as the main tree species, the tree was allocated to a stratum of other species. However, a conifer stratum was priorized for conifer trees, a birch stratum for birch trees and other broadleaved stratum for other broadleaved trees. Retention trees could be allocated only to a retention tree stratum and retention tree stratum could include only retention trees. The allocations were utilized in (1) estimating the diameter distributions of the stratum, (2) estimatimg the height-diameter curve of the stratum and (3) in estimating the tree species proportions of the stratums by main tree species, as described below. However, when estimating the diameter distribution and height-diameter curve, only conifer trees were used for conifer strata, only birches for birch strata, and only other-broadleaved trees for other broadleaved strata.

  2. Average species proportions were estimated for classes determined by region and stratum main species based on how much trees of different species were allocated to strata of different main tree species. This way we took into account that forests usually have small proportion of such species that seldom are described as main species of a stratum, and ensured that the generated trees of the input data have similar species distribution than the NFI sample trees have at regional level.

  3. Correction factors of mean diameter and mean height were defined by regions and tree species so that the mean diameters and height at regional level matched with values based on NFI sample trees by species and regions.

  4. A two-parameter Weibull distribution was fitted for every stratum so that the mean diameter matched by the field-measured mean diameter of the stratum after adjustment using the factor determined in step 3 above, the distribution fits to the sample trees of the stratum, and that the diameter distribution at regional level matches well to the diameter distribution of the NFI sample trees. Trees were generated from the estimated diameter distribution according to the tree species proportions determined in step 2. The total number of trees was proportional to the basal area of the stratum so that the mean number of generated trees was 33 trees per calculation unit.

  5. Such retention trees that were not allocated to any retention tree stratum were added to the data as such and the basal areas of trees of that stand were reduced correspondingly.

  6. The estimated number of stems for every generated tree was adjusted so that the total basal areas matched to the estimates based on all NFI sample trees (i.e. also those from stands without NFI plot center). The adjustment was done by regions for retention trees and by regions and species for other trees.

  7. The Height-Diameter models of Siipilehto and Kangas (2015) were re-estimated to a dataset where the measured height sample trees of NFI13 were added to the original model fitting data. The model was used to predict heights to the generated trees of the MELA input data. If there was one or more height sample trees measured in a stratum, the model was calibrated for the stratum using a random-effect calibration approach, otherwise the fixed part of the model was used in prediction.

A detailed description of the method is given in Mehtätalo et al. (2026).

For strata with mean height below 1.3. meters, an approach based on height distribution was used.

Forest management categories

The management units were classified into three categories according to sample plot restrictions concerning wood production (see Table):

  1. no restrictions for wood production
  2. restrictions for wood production
  3. no wood production allowed

Table. Land areas and growing stock volumes (over bark) by management categories of MELA data which were formed according to the NFI field measurements for the whole country.

Management category Area,
million ha		 Proportion,
% of total area
  Forest land Scrub land Total  
No restrictions for wood production 17.14 - * 17.14 74.9
Restrictions for wood production 1.16 0.98 2.14 9.3
No wood production allowed 2.01 1.61 3.62 15.8
Total 20.31 2.59 22.89 100.0
  Volume,
million m3		 Proportion,
% of total volume
No restrictions for wood production 2 059 - * 2 059 80,7
Restrictions for wood production 165 29 194 7,6
No wood production allowed 255 45 299 11,7
Total 2 478 74 2 552 100,0
* Scrub land (poorly productive forest land) belongs either in the category "restrictions for wood production" or in the category "no wood production allowed". However, in the calculation there were no logging events on scrub land.