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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

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

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)

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

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)

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.

Division of Switzerland into 14 regions (2 in the Jura, 3 on the Plateau, 3 in the Pre-Alps, 5 in the Alps and 1 in the Southern Alps). The economic regions are a subdivision of the production regions according to economic-geographical 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.

Area that meets the forest definition of the NFI, i.e. is «forest without shrub forest» or «shrub forest», and can be reached on foot.

Area accessible on foot that meets the NFI's definition of forest in both NFI4 (2009-2017) and NFI5 (2018-2026), i.e. was either «forest without shrub forest» or «shrub forest».

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 and accessible on foot in both NFI1 (1983-1985) and NFI2 (1993-1995).

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 and accessible on foot in both NFI2 (1993-1995) and NFI3 (2004-2006).

Forest that was less than two-thirds covered with shrubs in both NFI3 (2004-2006) and NFI4 (2009-2017) and could be reached 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.

Area that meets the NFI's definition of 'forest', i.e. is «a forest without shrub forest» or «a shrub forest».

Area that meets the NFI's definition of forest in both NFI4 (2009-2017) and NFI5 (2018-2026), i.e. was either «forest without shrub forest» or «shrub forest».

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).

Accessible forest, i.e. «forest without shrub forest» or «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).

Area designated in both the NFI4 (2009-2017) and the NFI5 (2018-2026) as «accessible forest», i.e. «forest without shrub forest» or «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).

Area designated in the five Inventories NFI1 (1983-1985), NFI2 (1993-1995), NFI3 (2004-2006), 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).

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).

Area that, in both the NFI4 (2009-2017) and the NFI5 (2018-2026), conforms with the NFI's definition of «forest», i.e. «forest without shrub forest» or «shrub forest», 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).