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Altitude above sea level in classes of 400 m. Reference: Digital height model DHM 25 from Swisstopo

Altitudinal vegetation belts in the system used in the guidelines for monitoring the sustainability and performance of protection forests (NaiS; Frehner et al. 2005), reduced to six classes. The variable represents a simplification of the NaiS altitudinal vegetation belts with ten classes (NAISHSTKOMB) in which the classes «hyperinsubric» and «colline» are merged with «colline with beech» to form the class «hyperinsubric and colline» and the class «lower montane» with «upper montane» and «lower/upper montane» to form the class «lower and upper montane». The information is based on the altitudinal vegetation belts determined by experts (accessible forest sample plots of NFI4 on the 1.4-km network; Arge Frehner et al. 2020), as well as on the altitudinal vegetation belts modelled for the period 1981-2010 (other sample plots; Zischg et al. 2021). n/a: located above the forest boundary modelled by Zischg et al. and sometimes in the area of lakes, which were relatively roughly cut out by Zischg et al.

Sample plots without/with gaps ≥100 m² from canopy edge to canopy edge that intersect with the interpretation area (50 × 50 m), classified according to the area of the largest gap into five classes. Reference: aerial photo interpretation

Measure of inter-tree crowding of the tree crowns in a stand. Reference: Field Survey (MID 266: Schlussgrad)

Proportion of the ground area covered by tree crowns according to the aerial photo interpretation – in five 20-percent classes. The percentage is calculated using the 25 grid points on the interpretation area (50 × 50 m). Reference: aerial photo interpretation

Proportion of the ground area that is covered by established regeneration, i.e. by living coniferous and broadleaved trees ≥1.3 m in height and 11.9 cm in diameter at breast height (dbh). Reference: Field Survey (MID 273: Deckungsgrad der gesicherten Verjüngung)

Proportions of conifers and broadleaves making up the stand structure, classified according to their proportional basal areas into four classes: pure conifer forest: 91-100 % conifers, mixed conifer forest: 51-90 % conifers, mixed broadleaved forest: 11-50 % conifers and pure broadleaved forest: 0-10 % conifers. Reference: Field Survey (MID 265: Mischungsgrad)

Proportion of the area covered by regeneration, i.e. broadleaves and conifers ≥10 cm in height and ≤11.9 cm in diameter at breast height (dbh) - in seven classes. The variable has been in use since NFI3 (2004-2006). For comparisons going back to NFI2 (1993-1995), the variable «degree of regeneration cover (6 classes)» can be used. Reference: Field Survey (MID 270: Verjüngungs-Deckungsgrad)

Combination of the 18 groups of NaiS site types, each with a similar objective for the main tree species (NAISGGROB20), into 10 large associations known as «forest formations». *As the characterisation of the site types in the NaiS-NFI project is on a small scale, it is possible that non-forest site types such as meadow, pasture and rock may be present in sample plots that are classified as «forest» in NFI. Similarly, «forest without shrub forest“ may also contain «shrub forest» site types.

Classification of forests into 12 classes («types») according to their development, structure and management. Unlike in the 17-class forest-type classification, in this 12-class classification all development stages (from young growth to timber) are subsumed in the class «uniform high forest». Reference: Field Survey (MID 255: Nutzungskategorie, MID 257: Waldtyp nach Aufnahmeanleitung LFI, MID 260: Waldform, MID 267: Bestandesstruktur, MID 261: Entwicklungsstufe)

Type of trees and shrubs ≥12 cm in diameter at breast height (dbh) with the ten most common species or species groups in Switzerland ('main tree species') categorised, and the classes 'other conifers' and 'other broadleaves' for the remaining species. The main tree species are: spruce (Picea spp.), fir (Abies spp.), pine (Pinus sylvestris, P. nigra, P. strobus, P. mugo subsp. uncinata), larch (Larix spp.), Arolla pine (Pinus cembra), beech (Fagus sylvatica), maple (Acer spp.), ash (Fraxinus spp.), oak (Quercus spp.) and chestnut (Castanea sativa). Reference: Field Survey (MID 50: Baumart)

Natural hazard processes that can occur in forests designated as protection forests by the cantons according to the harmonised criteria of SilvaProtect. Reference: Losey & Wehrli (2013)

Proportion of the area covered by regeneration, i.e. broadleaved and coniferous trees ≥10 cm in height and ≤11.9 cm in diameter at breast height (dbh), in six classes. The feature is available as of NFI2 (1993-1995). Reference: Field Survey (MID 270: Verjüngungs-Deckungsgrad)

Slope in classes of 20%. Reference: Digital height model DHM 25 from Swisstopo

Mechanical resistence of a stand to abiotic and biotic stresses within the next 10 to 20 years – in three classes. Reference: Field Survey (MID 310: Gesamtstabilität)

Vertical structure (stratification) of a stand, defined on the basis of the degrees of cover of the upper, middle and lower layers. Reference: Field Survey (MID 267: Bestandesstruktur)

Number of years since the last silvicultural treatment – in classes of ten years. Silvicultural treatments include harvesting and tending interventions, planting and afforestation, as well as sanitary and safety interventions. Reference: Forest Service Survey (MID: 607: Anzahl Jahre seit dem letzten Eingriff)

Number of years until the next silvicultural treatment according to the forest planning schedule. Silvicultural treatments include harvesting and tending interventions, planting and afforestation, as well as sanitary and safety interventions. Reference: Forest Service Survey (MID 337: Zeitpunkt des nächsten Eingriffs)

Demarcation of Switzerland into five regions (Jura, Plateau, Pre-Alps, Alps and Southern Alps) with relatively uniform growth and timber production conditions. The production regions were established by the Federal Office of Forestry long before the first National Forest Inventory (NFI1, 1983-1985). With a small exception along Lake Geneva, the boundaries of the production regions still follow the municipal boundaries of the time. Unlike the NFI, the Forestry Statistics of the Federal Statistical Office don't use the production regions as demarcation but rather the forestry zones, whose boundaries are somewhat different.

Demarcation of Switzerland used in NFI for protective forest analyses. The six protective forest regions were derived from the economic regions by combining individual regions according to natural and statistical criteria.

Regional demarcation with the cantons as a unit, with the two half-cantons, Basel-Land and Basel-Stadt, combined into one canton for statistical reasons.

Demarcation of Switzerland into six regions with similar flora and fauna. The six regions correspond to the basic categories in the publication «The Biogeographical Regions of Switzerland», which was published by FOEN in 2022.

Regional demarcation with the forest districts as a unit. This variable is based on a survey of the cantonal forest services in winter 2022/2023.

Forest of which less than two-thirds is covered with shrubs that can be accessed on foot.

Forest that was less than two-thirds covered with shrubs in the five inventories NFI1 (1983-1985), NFI2 (1993-1995), NFI3 (2004-2006), NFI4 (2009-2017) and NFI5 (2018-2026) and was accessible on foot.

Forest that was less than two-thirds covered with shrubs in both NFI4 (2009-2017) and NFI5 (2018-2026) and is accessible on foot.

Accessible forest without shrub forest («a.f.w.s.f.»), i.e. forest that is less than two-thirds covered with shrubs and can be accessed on foot, which is situated in a forest that the cantons designated «protective forest» in 2022 according to the harmonised criteria of SilvaProtect-CH (Losey & Wehrli 2013).

Area designated in both NFI4 (2009-2017) and NFI5 (2018-2026) as «accessible forest without shrub forest» (a.f.w.s.f.), i.e. «forest without shrub forest» that can be accessed on foot and is situated in a forest that the cantons designated «protective forest» in 2022 according to the harmonised criteria of SilvaProtect-CH (Losey & Wehrli 2013).