Where possible, the NFI6 applied the same methods as its predecessor, the MFV. That way, results could be compared and developments could be monitored over time. The number of variables recorded for each sampling point was reduced; for example, in NFI6 there were no vegetation measurements, no questions were put to recreationists and no assessments were made of surrounding noise. On the other hand, a few new characteristics were added; namely, European Forest Type, which is used in international reports, and the thickness of the humus layer (only for mineral soil) to gain a better estimate of the carbon sequestration in Dutch woodland. Another addition to NF16 is the description of the regeneration, which refers to all individuals with a DBH < 5 cm and a height of 50 cm and above.
A forest inventory can be carried out in different ways, which depends on the objectives, the required accuracy, the available budget and specific site conditions. A common method is mapping of the forest (using existing (topographic) maps or remote sensing/aerial photo interpretations), distribution of sample points (systematic, random or stratified example forest type), visits and measurement of the sample points, and the processing of the data.
The maps used by NFI6 were the same maps that form the basis for the reports for the UN Climate Treaty and the land-use sector of the Kyoto Protocol (Land Use, Land Use Change and Forestry, aka LULUCF). The LULUCF maps are derived from the Basiskaart Natuur 2009 (BKN 2009). The selection consisted of forested areas greater than 0.5 hectares. The BKN is a ‘grid map’, which divides the surface area of the Netherlands into squares measuring 25x25 metres. Each square (cell) is assigned a type of land use. In compliance with the customary definition of a forest (see below) a distinction is drawn between areas that are larger or smaller than 0.5 hectares. Areas are designated as larger than 0.5 hectares if they consist of at least eight consecutive cells of forest on BKN 2009. This results in 373,480 hectares of aggregated large forest (more than 0.5 hectares) and 22,092 hectares of aggregated small forest (less than 0.5 hectares).
Definition of forested land
The sampling points must describe the Dutch forest and are therefore situated only in ‘forested terrain’. The definition of ‘forested terrain’ applied in NFI6 is borrowed from the ‘forest’ category’ in the LULUCF map (see Alterra report 1916) and has been derived from the FAO definition of forest:
Land with tree crown cover (or equivalent stocking level) of more than 10 percent and an area of more than 0.5 hectares. The trees should be able to reach a minimum height of 5 metres at maturity in situ. A forest may consist either of closed formations where trees of various heights and undergrowth cover a high proportion of the ground; or of open formations with a continuous vegetation cover in which tree crown cover exceeds 10 percent.
Young natural stands and all plantations established for forestry purposes which have yet to reach a crown density of 10 percent or tree height of 5 metres are included under forest, as are areas normally forming part of the forest area which are temporarily unstocked as a result of human intervention or natural causes but which are expected to revert to forest.
- Includes: Forest nurseries and seed orchards that constitute an integral part of the forest; forest roads, cleared tracts, firebreaks and other small open areas within the forest; forest in national parks, nature reserves and other protected areas such as those of special environmental, scientific, historical, cultural or spiritual interest; windbreaks and shelterbelts of trees with an area of more than 0.5 hectares and a width of more than 20 metres. Rubberwood plantations and cork oak stands are included.
- Excludes: land predominantly used for agriculture.
Selection of sampling points
Samples were taken at a density of one point per 100 hectares. A nationwide grid with potential sampling points was created in the MFV. A point was selected at random within cells of one square kilometre. The same grid was used for NFI6. The grid was placed on the LULUCF map, then all the points in the LULUCF forest were studied on recent aerial photos. Out of the 3,745 points in LULUCF forest, 198 were clearly not forest. As the LULUCF map is based on square cells (25 x 25 metres), and each cell has one land-use designation, it is possible for a small plot of land in a ‘forest’ – usually on the edge – to be used for other purposes. A sampling point might also lie, by sheer coincidence, on a built-up area inside forested land or in an area that has actually been deforested for a conservation project or to clear a temporary poplar site. The remaining 3,547 points were selected for a field visit. The owners were asked permission for the inventory activities on their land. Access was denied in only 87 cases. It then emerged in the field that 154 sampling points did not meet the definition of forest and another 116 were inaccessible for a variety of reasons. Eventually measurements were recorded at 3,190 of the 3,393 points; of these, 1,235 were measured for the second time (permanent points).
Procedures, sampling points
The MFV used permanent and temporary sampling points. The exact locations of the temporary sampling points were not determined, and the coordinates of the individual trees were not registered. Permanent sampling points were described in a way that would enable them to be exactly relocated in the field. Permanent points were used so that the development of individual trees could be monitored through time (growth, harvest, decomposition of dead trees).
General details were noted for each sampling point (soil, thickness of the litterfall, dominant species in the stand, even-aged/uneven-aged, etc.). Then a circular sample plot was laid out with a radius that was determined by the number of trees per hectare. At least 20 trees had to be inside the plot, but a maximum radius of 20 meters is used. All the trees inside the plot had to be measured (DBH, species). All the trees inside the circle had to be measured (DBH, species, trunk quality). In some cases additional measurements were taken, such as the height, and a visual assessment was made of the quality of the stem. Each tree at the permanent sampling points from the MFV had to reported back, indicating if the tree was still alive (by re-measuring the diameter), or if the tree had been felled, or had died or disappeared
After a quality check the field data were used to compile estimates on certain aspects of forested land in the Netherlands. The NFI6 measurements at all the sampling points were used – permanent and temporary. These estimates might relate to, for example, tree volume on the basis of diameter and height measurements, distribution according to diameter category, tree species distribution, etc. The data from the permanent plots were also used to develop new functions for increment and harvest, which enabled the increment and harvest to be estimated for all the plots (temporary and permanent). The NFI6 therefore went step farther than the MFV, which had no repeat measurements. The report appeared in 2014.