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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 three classes. The variable represents a simplification of the NaiS altititudinal vegetation belts in six classes (NAISHSTKOMB6KL) in which the class «hyperinsubric and colline» is mearged with «submontane» to form the class «hyperinsubric, colline, submontane», the class «lower and upper montane» with «high montane» to form the class «montane» and the class «subalpine» with «upper subalpine» to form the class «subalpine». 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.

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 classified: «no», «one», «two» or «more than two» red wood ant-heaps in the 500 m² circle. Reference: Field Survey (MID 837: Ameisenhaufennummer)

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

Aspect, classified into the following five classes: North, East, South, West and indeterminate. «Indeterminate» means that the slope is ≤10%. Reference: Field Survey (MID 191: Azimut der Exposition)

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

Stage of stand development, defined by the dominant diameter at breast height (dbhdom = dbh of the 100 largest [thickest] trees per hectare). Young growth/thicket: dbhdom <12 cm, pole timber: dbhdom 12-30 cm, young timber: dbhdom 31-40 cm, medium timber : dbhdom 41-50 cm, old timber: dbhdom >50 cm, mixed: trees of different development stages, no development stage predominant or groups of different development stages covering < 500 m². Reference: Field Survey (MID 261: Entwicklungsstufe)

Sample plots with/without dry stone walls and piles of stones on the interpretation area (50 × 50 m), classified according to the type of objects. Reference: Field Survey (MID 217: Trockenmauer und Steinhaufen)

Sample plots without/with damage gaps on the interpretation area, and the extent of damage classified into ten classes according to the proportion of the forested interpretation area that is damaged. Reference: Field Survey (MID 601: Ausmass der Schadenfläche)

Presence of the forest function 'recreation' according to forest plans or an assessment by the local forest service. Reference: Forest Service Survey (MID 327: Spezielle Waldfunktionen)

Classification of the forest according to the forest decision and accessibility into the three classes «accessible forest without shrub forest», «inaccessible forest without shrub forest» and «shrub forest».

Sample plots without/with geomorphological objects (e.g. bands of rock, boulders, limestone pavements or gullies) on the interpretation area (50 × 50 m), classified according to the type of the largest object. Reference: Field Survey (MID 218: Geomorphologische Objekte, Kleinrelief)

Sample plots with/without signs on the interpretation plot (50 × 50 m) that the forest stand or forest site is subject to very intense (excessive) recreational use or other strong human influence (e.g. landfill, land drainage, construction). Where there are such signs, indication of the type of overuse or disturbance. Reference: Field Survey (MID 219: Spuren von Überbelastungen und Störungen)

Sample plots without/with a distinct vertical stand edge («inner forest edge») on the interpretation area (50 × 50 m). If an inner edge is present, what the lower of the two stands consists of is also indicated. Reference: Field Survey (MID 281: Innenränder)

Sample plots with/without current recreational use within a radius of 100 m around the sample plot centre, classified according to visitor frequency as a measure of the intensity of recreational use. No recreational use: <10 persons/year; very low intensity: <1 person/day; low: 1-10 persons/day; moderate: 11-100 persons/day; high: 101-500 persons/day; very high: >500 persons/day based on the entire year or the relevant season, taking into account all types of recreational use. Reference: Forest Service Survey (MID 330: Intensität der aktuellen Erholungsnutzung)

Sample plots with/without tracks of timber-harvesting vehicles such as tractors, forwarders and harvesters on the forest floor within the 200 m² circle, classified according to where the tracks occur. Reference: Field Survey (MID 545: Schadenumgebung)

Sample plots with either no wood pile or with at least one compact wood pile ≥30 cm in height and 3 m² in area on the interpretation area (50 × 50 m) left in the stand after harvesting. Reference: Field Survey (MID 209: Asthaufen)

Sample plots with/without at least one still clearly recognisable root plate that protrudes at least 30 cm above the terrain. Reference: Field Survey (MID 227: Wurzeltellerklasse)

Sample plots with/without at least one pile of soil or mulch, created by the decay of a root plate and protruding at least 30 cm above the terrain. Reference: Field Survey (MID 227: Wurzeltellerklasse)

Sample plots with/without tracks of timber-harvesting vehicles such as tractors, forwarders and harvesters on the forest floor within the 200 m² circle. If there are tracks, indication of the track type. In the case of track type 3 (severe disturbance), it can be assumed that the damage to the soil fertility will be significant and long term (Lüscher et al. 2016). Reference: Field Survey (MID 544: Fahrzeugspuren, MID 496: Bodenschadentyp)

Presence of the primary function: «nature conservation» according to forest plans or an assessment by the local forest service during NFI5 (2018-2026). Reference: Forest Service Survey (MID 911: Vorrangfunktion 2)

Area in/outside a forest that provides protection against avalanches which the cantons designated «protective forest» according to the harmonised criteria of SilvaProtect-CH in 2022. Reference: GIS data from FOEN, 2022

Area in/outside a forest that provides protection against channel processes which the cantons designated «protective forest» according to the harmonised criteria of SilvaProtect-CH in 2022. Reference: GIS data from FOEN, 2022

Area in/outside a forest that provides protection against hillslope debris flows/landslides which the cantons designated «protective forest» according to the harmonised criteria of SilvaProtect-CH in 2022. Reference: GIS data from FOEN, 2022

Area in/outside a forest that provides protection against rockfall processes which the cantons designated «protective forest» according to the harmonised criteria of SilvaProtect-CH in 2022. Reference: GIS data from FOEN, 2022

Area inside/outside the protective forest that the cantons had designated in 2022 according to the harmonised criteria of SilvaProtect-CH (Losey & Wehrli 2013). In 2022, Canton Aargau had not yet designated its protective forest. Reference: GIS data from FOEN, 2022

Sample plots without/with recreation infrastructure (paths and specific recreation facilities) on the interpretation area (50 × 50 m). Reference: Field Survey (MID 220: Erholungseinrichtungen)

Sample plots with/without current recreational use 'camping' within a radius of 100 m around the sample plot centre. The type of recreation is recorded with a visitor frequency ≥10 persons per year. Reference: Forest Service Survey (MID 329: Art der aktuellen Erholungsnutzung)

Sample plots with/without current recreational use 'cross-country skiing' within a radius of 100 m around the sample plot centre. The type of recreation is recorded with a visitor frequency ≥10 persons per year. Reference: Forest Service Survey (MID 329: Art der aktuellen Erholungsnutzung)

Sample plots with/without current recreational use 'cycling' within a radius of 100 m around the sample plot centre. The type of recreation is recorded with a visitor frequency ≥10 persons per year. Reference: Forest Service Survey (MID 329: Art der aktuellen Erholungsnutzung)

Sample plots with/without current recreational use 'hiking' within a radius of 100 m around the sample plot centre. The type of recreation is recorded with a visitor frequency ≥10 persons per year. Reference: Forest Service Survey (MID 329: Art der aktuellen Erholungsnutzung)

Sample plots with/without current recreational use 'horse-riding' within a radius of 100 m around the sample plot centre. The type of recreation is recorded with a visitor frequency ≥10 persons per year. Reference: Forest Service Survey (MID 329: Art der aktuellen Erholungsnutzung)

Sample plots with/without current recreational use 'jogging' within a radius of 100 m around the sample plot centre. The type of recreation is recorded with a visitor frequency ≥10 persons per year. Reference: Forest Service Survey (MID 329: Art der aktuellen Erholungsnutzung)

Sample plots with/without current recreational use 'mountain biking' within a radius of 100 m around the sample plot centre. The type of recreation is recorded with a visitor frequency ≥10 persons per year. Reference: Forest Service Survey (MID 329: Art der aktuellen Erholungsnutzung)

Sample plots with/without current recreational use 'other activities' within a radius of 100 m around the sample plot centre. The type of recreation is recorded with a visitor frequency ≥10 persons per year. Reference: Forest Service Survey (MID 329: Art der aktuellen Erholungsnutzung)

Sample plots with/without current recreational use 'ski- and snowboarding' within a radius of 100 m around the sample plot centre. The type of recreation is recorded with a visitor frequency ≥10 persons per year. Reference: Forest Service Survey (MID 329: Art der aktuellen Erholungsnutzung)

Sample plots with/without current recreational use 'snowshoeing' within a radius of 100 m around the sample plot centre. The type of recreation is recorded with a visitor frequency ≥10 persons per year. Reference: Forest Service Survey (MID 329: Art der aktuellen Erholungsnutzung)

Sample plots with/without current recreational use 'walking' within a radius of 100 m around the sample plot centre. The type of recreation is recorded with a visitor frequency ≥10 persons per year. Reference: Forest Service Survey (MID 329: Art der aktuellen Erholungsnutzung)

Sample plots with/without current recreational use within a radius of 100 m around the sample plot centre, classified according to seasonality of recreational use (all year round, almost only during the growing season, almost only in winter). Reference: Forest Service Survey (MID 403: Saisonalität der aktuellen Erholungsnutzung)

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

Sample plots with/without standing dead trees («snags») ≥20 cm in diameter at breast height (dbh) on the interpretation area (50 × 50 m), which together reach a volume of at least 1 m³. Reference: Field Survey (MID 211: Dürrständer)

Density of a stand – in four classes. The Stand Density Index (SDI) is calculated on the basis of the number of stems per hectare of trees and shrubs ≥12 cm in diameter at breast height (dbh) and the mean diameter. Reference: Field Survey (MID 60: Brusthöhendurchmesser, MID 62: Umfang, MID 1018: Baumzustand)

Sample plots with/without stumps and lying deadwood on the interpretation area. «With» applies if there are at least five tree stumps ≥30 cm in diameter and 20 cm in height or a lying tree ≥30 cm in diameter at breast height (dbh) on the interpretation area. Reference: Field Survey (MID 210: Stöcke)

Sample plots with/without traces of erosion due to water on at least 100 m² (sum of eroded parts) of the interpretation area (50 × 50 m). Reference: Field Survey (MID 194: Erosion durch Wasser) or – if the forest is inaccessible – aerial photo interpretation

Sample plots with/without extensive traces of forest fire (e.g. burnt ground vegetation, charred parts of tree or soil layers) on the interpretation plot (50 × 50 m). Reference: Field Survey (MID 203: Spuren von Waldbrand)

Sample plots with/without traces of landslides (e.g. a toe or crack, and trees leaning uphill) on at least 100 m² of the interpretation area (50 × 50 m). Reference: Field Survey (MID 193: Rutschung)

Sample plots with/without current traces of rockfall on the interpretation plot (50 × 50 m). Reference: Field Survey (MID 195: Steinschlag)

Sample plots with/without traces of snow movement, i.e. snow creep, snow sliding or avalanches on the interpretation area (50 × 50 m). Reference: Field Survey (MID 202: langsame Schneebewegung und MID 400: Lawinenspuren)

Sample plots with/without gaps ≥10 × 10 m from crown edge to crown edge and cover with woody plants of maximum 20% that intersect with the interpretation area. Classified into different gap types according to the predominant surface of the largest gap. Reference: Field Survey (MID 221: Lückentyp)

Sample plots with/without tracks of timber-harvesting vehicles such as tractors, forwarders and harvesters on the forest floor within the 200 m² circle. Reference: Field Survey (MID 544: Fahrzeugspuren)

Sample plots without/with water bodies on the interpretation area (50 × 50 m), classified according to the type of the largest water body. Pond: 1-200 m², small lake: >200 m², creek: channel width <2 m, stream: 2-5 m, river: > 5 m. Reference: Field Survey (MID 401: Gewässer)

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.

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.

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.