Highlights

  • Disturbances create regeneration windows for better site-adapted species.

  • Post-disturbance forest dynamics reverse earlier economy-driven management.

  • Low elevation spruce forests partly transform to more broadleaeved forests.

Introduction

Climate change is altering species distributions (Lenoir and others 2020) with important consequences for human well-being (Pecl and others 2017), and the continued change is expected to amplify during the century (Pereira and others 2010; Lindner and others 2014; Dyderski and others 2018). Resulting changes in forest structures and species compositions may potentially alter ecosystem processes and related services, claiming for an adaptation of management practices (Millar and others 2007; Civantos and others 2012; Brang and others 2014). Many models predict dramatic changes in tree species distributions over the coming decades (for example, elevational or latitudinal shifts of species; Gehrig-Fasel and others 2007; Dyderski and others 2018; Jandl and others 2019), altered frequencies of disturbances (Seidl and others 2017) and post-disturbance changes in vegetation (for example, as a consequence of drought-induced mortality; Allen and others 2015; McDowell and others 2020). On the other hand, representative surveys have observed only marginal changes under ongoing climate warming during the last decades (for example, Lenoir and others 2008, 2010b; Bertrand and others 2011; Küchler and others 2015; Scherrer and others 2017; Etzold and others 2019). In European forests, shifts in distribution ranges of tree species have been mostly assessed at their present latitudinal or altitudinal limits (for example, Grundmann and others 2011; Bolte and others 2014; Hernández and others 2014), with larger effects in species restricted to mountain habitats or with faster population turnover (Lenoir and others 2008; Dyderski and others 2018). This discrepancy between model simulations and recent field observations of late-successional species could be explained by four main factors all related to the longevity of dominant tree species: (1) slow regeneration processes due to long persistence of the optimal phase of dominant trees, high plasticity and rare mast fructification (for example, Bertrand and others 2011; Ascoli and others 2017; Copenhaver-Parry and others 2020), (2) limited dispersal (for example, Svenning and Skov 2007; Lenoir and others 2010a), (3) limited establishment due to competition, fluctuating resource availability and forest management (for example, Bertrand and others 2016; Nikolova and others 2019; Shi and others 2020), and (4) lack of gap-creating disturbances generating opportunities for the recruitment of (new) species (Grubb 1977). All these factors hinder rapid climate-driven changes in tree species composition and thereby may contribute to an increasing amount of extinction and colonisation debt (Talluto and others 2017; Liang and others 2018). On the other hand, reports supporting rapid vegetation shifts due to climate warming often refer to opportunistic case studies and neglect the vast majority of other forests, where possible substantial changes may remain unobserved (Jump and others 2007; Brandl and others 2020).

Both simulation studies (for example, Liang and others 2018; Scherrer and others 2020) as well as forest surveys (for example, Peñuelas and Boada 2003; Rigling and others 2013; Brice and others 2019, 2020), suggest that disturbances and extreme events (natural or anthropogenic) are a key component in facilitating forest transitions under climate change by creating favourable conditions for opportunistic species better adapted to the novel (and presumed future) climatic conditions. Whether such events that lead to tree and stand mortality, either alter tree species compositions or simply result in direct ingrowth (that is, infilling by the already dominant species; for example, Kramer and others 2014) might be highly dependent on the interaction of environmental conditions, disturbance agents and their severity (Seidl and others 2017; Jentsch and von Heßberg 2019; Nikolova and others 2019). Whether post-disturbance tree composition will change or not also depends on stand age, development stage, dominant disturbance regimes, environmental constraints (for example, high vs low elevation) and understory dynamics (prevailing ground vegetation; Bolte and others 2014).

To find evidence for representative disturbance-induced forest changes, we used the unique nationwide data from the Swiss National Forest Inventory (NFI; Fischer and Traub 2019) covering several bioclimatic regions (that is, Jura, the Plateau, the Northern Alps, the Western and Eastern Central-Alps and the Southern Alps) and spanning an elevation range from 240 to 2400 m a.s.l. All main Swiss forest types are represented in this systematic inventory allowing us a statistically representative overview of disturbance events and tree species-specific post-disturbance dynamics, for example, changes in basal area and recruitment density over an average period of 25-years. We focused on the four prevalent natural disturbance agents wind, insect outbreaks, forest fires and prolonged drought events that likely increase in frequency with future environmental change (Seidl and others 2017) to investigate: (1) how do natural disturbance agents differ in frequency among different biogeographic regions, climatic conditions and forest types, (2) which species were most affected by different natural disturbance agents and (3) are there any differences between pre- and post-disturbance tree species composition (adult trees and recruitment) that may suggest potential vegetation shifts?

Material and Methods

Study Region and Climate Data

The study area is Switzerland, encompassing a territory of 41,285 km2 of which 31.9% (13,169 km2) is forested (Cioldi and others 2020). Forests range from about 200 to 2400 m a.s.l. with the lower elevations being naturally dominated by broadleaves and the higher elevations by conifers (Figure S1). Corresponding with regions and with steep elevation gradients around the Alpine arc, the climate strongly varies. In the north of the Alps, the climate is mainly influenced by the Atlantic Ocean, from where mild, humid winds blow. In Zurich-Affoltern at 443 m a.s.l., winters are mild (0.3 °C in January) and summers warm (18.8 °C in July), with annual precipitation of 1054 mm. In the south of the Alps, the climate is influenced by the Mediterranean system with temperatures 2–3 °C warmer than in the North, and annual precipitation usually exceeds 1600 mm (Locarno-Monti; 366 m a.s.l.). Central Alpine valleys are dry with nationwide lowest annual precipitation (603 mm, Sion; MeteoSwiss 2020). Following this climate heterogeneity, Switzerland is categorised into six biogeographic regions (Gonseth and others 2001; Figure 1).

Figure 1
figure 1

Biogeographic regions of Switzerland (Gonseth and others 2001) and geographic positions of NFI sample plots used in this study. Different point colours indicate the type of natural disturbance recorded in the period 1993–2017 and the shade colours different biogeographic regions. None = Not naturally disturbed.

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Forest Inventory Data

We used data of the Swiss National Forest Inventory (NFI), recorded on a 1.4 km × 1.4 km systematic permanent sample grid covering the whole country, during the second NFI (NFI2; 1993–1995) and the fourth NFI (NFI4; 2009–2017), respectively. Previous to analyses, we removed all plots classified as shrubland or not visited in both NFI2 and NFI4, eventually retaining the 5521 accessible forest plots. Passing from NFI2 to NFI4 the protocol of terrestrial data collection slightly changed as well as the survey period did, which switched from a three-year assessment window to a nine-year-window (continuous inventory since NFI4; Brändli and Hägeli 2019; Lanz and others 2019). The vast majority of Swiss forests were since centuries and still are managed following a ‘close-to-nature’ silvicultural approach (Spathelf and others 2015). The commandment of privileging natural tree recruitment or planting of native site-adapted species when natural regeneration is difficult is even embodied in the national forest law. Thus, about 81% of today’s Swiss Forests originate from natural tree regeneration (Brändli and others 2020b) and large monoculture plantations are very rare as are non-native tree species (Table S1), which are only present on 2.4% of NFI plots and contribute less than 0.6% to the basal area of Swiss forests (Brändli and others 2020a).

A terrestrial NFI plot consists of several plot elements. Standing and lying trees are measured using two concentric circular plots (500 m2 and 200 m2). On the large plot, all trees with a diameter at breast height (DBH) ≥ 36 cm are measured, while on the small plot also trees and shrubs with a DBH ≥ 12 cm and <  = 35.9 cm are measured (Lanz and others 2019). Tree recruitment is defined as individuals ≥ 10 cm in height and <  = 11.9 cm in DBH and was assessed using concentric circles with different sizes. In NFI2 two satellite plots (10 m West and East of the plot centre) with two concentric circles differing in radius were used (r = 1 m for heights 10–39 cm and r = 2.12 m for heights >  = 40 cm and DBH <  = 11.9 cm; Stierlin 1994) and in NFI4 one satellite plot (10 m West of the plot centre) with four concentric circles was used (r = 0.9 m for heights 10–39 cm, r = 1.5 m for heights of 40–129 cm, r = 2.5 m for heights >  = 130 cm and DBH <  = 3.9 cm and r = 4 m for DBH 4–11.9 cm; Düggelin and Keller 2017). We used the total available dataset, as preliminary analyses showed changes in sampling design between NFI2 and NFI4 to have no significant impact on recruitment density per hectare (Table S2). Different size of the satellite plots might impact analysis of species diversity, which, however, is not in focus of the present study. On the interpretation area of 50 × 50 m (0.25 ha) around the plot centre disturbances and the NaiS site type (NST; ARGE Frehner and others 2020; Frey and others 2021) were determined. After estimating the extension of existing damages in the field, the disturbance agent was evaluated by interviews with the local foresters (Stierlin 1994; Düggelin and Keller 2017). The NST was defined by experts that visited 25% of the NFI sample plots and concluded community identities by analogy (using factor maps and local knowledge) for the remaining 75% of the plots, according to the Swiss protection forest classification (NaiS; ARGE Frehner and others 2020). Based on floristic composition, site quality and structural characteristics, NST depicts the idealised tree species composition (including recruitment) in natural forests at the optimum developmental stage, that is in balance with the dominant environmental conditions (for example, climate, topography, soil; Frey and others 2021). In this study, we used 17 aggregated NST defined according to the main tree species (for example sub-montane beech forests or sub-alpine spruce forests). We used the NST to distinguish between NFI plots hosting tree species corresponding to the natural forest vegetation from NFI plots where the current dominance of a tree species results from anthropogenic influences (for example, management or land-use history). A forest stand is then defined as natural when the occurring dominant tree species based on NFI data (that is, based on basal area) is identical to the NST (for example, Picea abies has highest basal area per ha on NFI plots belonging to the sub-alpine spruce forest NST). However, due to present and/or past management decisions, differences may exist between the actual dominating species and the NST. To prevent confusion between dominant taxa and the NST of a NFI plot, we will use in this paper the scientific taxon name for the dominant species (for example, Picea abies, Fagus sylvatica) and the English names for the NST (for example, spruce forest and beech forest).

From roughly 300 different NFI attributes recorded on each plot, we only extracted the species-specific basal area of trees with DBH ≥ 12 cm (BA, m2 ha−1), the species-specific density of tree recruitment (stems ha−1), the damaged area per ha due to natural disturbance events (%), the natural disturbance agent, the date of the last timber harvest and the NST.

Data Pre-processing

Of the 56 tree species that were recorded in the NFI plots, only a few are very abundant and contributed to the vast bulk of biomass while the majority of species are rare or added little biomass. We, therefore, reclassified all species into twelve taxa: Abies alba (silver fir), Picea abies (spruce), Pinus cembra (Swiss stone pine), Larix spp. (larch), Pinus spp. (pine), other conifers, Castanea sativa (sweet chestnut), Fagus sylvatica (European beech), Acer spp. (Maple), Fraxinus spp. (ash), Quercus spp. (oak), other broadleaves (for details see Table S1). The dominant taxon for each plot was defined based on the BA represented. Further, we divided the tree recruitment into individuals with DBH <  = 3.9 cm (saplings) and individuals with DBH 4–11.9 cm (pole-stage) as these size groups represent very different age cohorts and amount of (self-)thinning. While the saplings recruitment provides qualitative and quantitative information about the available species pool, the pole-stage recruitment contains information about the species that successfully established.

Defining Disturbed Plots

To distinguish between naturally disturbed NFI sample plots (hereafter called ‘naturally disturbed’) and all others, we used the following criteria: ‘naturally disturbed’ if (a) the NFI field team reported damage affecting at least 10% of the interpretation area, and (b) the local forester reported a specific disturbance agent for the plot (for example, wind, fire). Plots with a contradiction between the foresters’ report and the NFI field team observations were removed from further analysis, leaving us with 5092 plots. Most of these mismatches were the result of minor disturbances already invisible at the time of the next NFI visit. In this study, we were interested in obvious, that is canopy disturbances with the potential to catalyse vegetation shifts, and consequently, these small-scale and very low-severity disturbances were not analysed. In the cases where more than a single disturbance event and/or disturbance agent affected a NFI sample plot, we determined the main disturbance agent by comparison of the area damage caused by each disturbance agent. For the later classification of plots into different disturbance agents, the main damage type (as determined by maximum area damaged) was used.

Plots without a natural disturbance event were further subdivided based on the information of the NFI into ‘undisturbed’ that is without any recorded natural disturbance and human intervention in the past 25 years, ‘treated’ that are plots where planned silvicultural treatments occurred, and ‘salvage logged’, which resulted from mortality/damage events related to smaller natural disturbances (that is, not recorded in the NFI). This classification allowed us to test if a larger-scale natural disturbance leads to a different vegetation trajectory compared to a human-induced disturbance (that is, ‘treated’ and ‘salvage logged’) or no intervention and no disturbance at all (that is, ‘undisturbed’).

Data Analysis

To test if certain site conditions (that is, biogeographic region, dominant taxa or NST) were linked to a specific natural disturbance agent, we used Chi-squared analysis. The effect of disturbance agents on the different considered taxa and the broader groups broadleaves versus conifers was analysed by comparing the absolute and relative BA changes (Wilcoxon-test). To test if and where disturbances may catalyse future vegetation shifts (that is, change in forest composition), we compared the species-specific recruitment density (for saplings and pole-stage trees) on ‘naturally disturbed’, ‘salvage logged’, ‘treated’ and, ‘undisturbed’ sites. Additionally, we used mixed-effects models to check for combined effects of site and species-specific factors on BA changes of the adult tree species and on the tree recruitment density. All statistical analyses were conducted in R 3.6.1 (R Core Team 2020) using the packages dplyr (Wickham and others 2020) and rstatix (Kassambara 2020).

Results

Distribution of Natural Disturbance Events

Of the 5092 NFI plots, 2024 (40%) were classified as ‘undisturbed’, 1600 (31%) as ‘treated’ and 778 (15%) as ‘salvage logged’. Subsequently, 690 (14%) were classified as ‘naturally disturbed’ of which 411 (59%) were disturbed by wind, 110 (16%) by insects (predominantly bark beetle), 101 (15%) by snow/avalanches, 8 (1.2%) by fire, 11 (1.6%) by drought and 49 (7.2%) by other agents including mass movements such as landslides (39), flooding (4) and rockfall (3), phytopathogens (2) and cattle (1). Out of the 690 ‘naturally disturbed’ plots, 163 (24%) were affected by several disturbance events during the observation period (Table S3). The majority of these multiple disturbance events were combinations of wind and insects (72%) in which usually a wind disturbance favoured an insect outbreak a few years later. Another notable source of multiple disturbances was snow and drought, which both reoccurred in at same plots over the years.

Natural disturbance events were significantly more frequent in the ‘Plateau’ and the ‘Northern Alps’ and less frequent than expected by chance in the ‘Jura’ (Figure 2). This finding is, however, confounded with the extensive storm damage area produced by storm Lothar (winter 1999) that mainly affected the ‘Plateau’ and ‘Northern Alps’ and resulted to be by far the most impactful natural disturbance event during the observation period (8% of all NFI plots). The ‘Northern Alps’ were significantly more affected by bark beetles, while forest fires occurred significantly more often in the ‘Southern Alps’ (Figure 2). Storm Lothar also led to a strong synchronisation of wind and insect disturbed plots as over 85% of the wind affected sites were recorded in 1999 and 85% of the insect affected sites during the 5 years following the storm (2000–2005, Figure S2). As a side effect of this ‘synchronisation’ of wind and insect damage, our analysis becomes much more consistent as the time interval between disturbance event and NFI observation showed low variability and therefore regrowth and tree recruitment stages are comparable. NFI plots in the ‘Jura’ and ‘Plateau’ were significantly more often ‘treated’ and ‘salvage logged’ than the regions of the Alps (Figure 2).

Figure 2
figure 2

Matrix of different disturbances agents and biogeographic regions (left), dominant tree species (middle) and NaiS site types (right). The colours indicate more frequent (orange) or less frequent (blue) interactions than statistically expected. The asterisks indicated significant differences (p < 0.05; Chi2-test) between expectations and observations. The numbers indicate the number of observations. ‘Naturally disturbed’ reflects the pooled data of the six different natural disturbance agents.

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Sample plots dominated by P. abies were overproportionally affected by natural disturbances (63% of all disturbed plots), which especially holds for wind and insects (> 80%; Figure 2) primarily in the ‘Plateau’ and ‘Northern Alps’. Plots dominated by C. sativa more often suffered from drought (Figure 2). In contrast, plots dominated by F. sylvatica or Larix spp. were significantly less often affected by any disturbances. Additionally, stands dominated by Pinus spp. (mostly Pinus sylvestris) were over-proportionally affected by drought in the dry valleys of the ‘Western Central-Alps’ but not nationwide (X2 = 7.3898, df = 1, p-value = 0.00656). Sample plots dominated by A. alba were over proportionally often ‘salvage logged’, and plots dominated by F. sylvatica and P. abies were significantly more, respectively less often ‘treated’.

Plots with a NST of silver fir-Norway spruce forests were significantly more often affected by disturbances than any other community, and in particular by wind, insects and snow (Figure 2). Wind also over-proportionally affected NSTs of beech and silver fir-beech forests, while drought over-proportionally disturbed plots with a NST of hornbeam and oak forests (Figure 2). More than 75% of the wind disturbed plots with a NST of beech forest were dominated by another tree species (mainly P. abies and, to a lesser degree, A. alba). In fact, P. abies dominated stands were significantly more often affected by wind disturbance when found on sites with NSTs dominated by broadleaves (for example, potential natural beech forests; X2 = 8.7242, df = 1, p-value = 0.00314). Beech forests and silver fir-beech forests were over proportionally ‘treated’ and ‘salvage logged’ and subsequently significantly less often ‘undisturbed’, while the opposite was true for Norway spruce forests (Figure 2).

Disturbance Effects on Species Composition

Between 1995 and 2017, the total BA of Swiss forests, as well as the BA per ha, increased from 34.8 M m2 and 31.0 m2 ha−1 to 37.5 M m2 and 32.7 m2 ha−1, respectively. However, the subset of NFI plots used in our study showed a more pronounced BA increase of + 2.4 m2 ha−1. This difference is explained by the fact that the NFI plots omitted in our analysis showed a decrease in BA (− 7.4 m2 ha−1) and represent most likely disturbed/damaged plots, but were removed from the dataset due to missing information on disturbance agents. The observed BA change mostly represented an increase of broadleaf trees (+ 1.5 m2 ha−1, + 14.8%), while coniferous trees remained almost constant (+ 0.9 m2 ha−1, + 4.6%; Table S4) increasing the BA contribution of broadleaves by 2%. If merely ‘undisturbed’ plots were considered, BA of both conifers and broadleaf species increased proportionally (+ 31.4% for conifers and + 29.7% for broadleaves), but conifers were much more affected by natural disturbances than broadleaves (− 39.4% for conifers and − 4.9% for broadleaves; Table S4). This shift from conifers towards broadleaves after natural disturbance events was particularly strong in the lowlands and decreased with elevation (Figure S3).

The two dominant natural disturbance agents (wind and insect outbreaks) mainly affected dense conifer stands dominated by P. abies (63% of disturbed plots), particularly in the ‘Plateau’ and the ‘Northern Alps’ (Figure 2), leading to a drastic reduction in BA of P. abies (− 44%; Table S5, Figure 3). Pinus spp. was the only taxon that decreased in overall BA and was especially affected by wind and drought disturbances (Table S5, Figure 3). The only broadleaf species showing a significant decrease in BA was F. sylvatica after windthrow. Additionally, C. sativa showed a tentative disturbance effect on BA after drought in the ‘Southern Alps’ (Table S6, Figure 3).

Figure 3
figure 3

Change in basal area (m2 ha−1; top), sapling recruitment (stems ha−1 with height > 10 cm and DBH < 4 cm; middle) and pole-stage recruitment (stems ha−1 with DBH 4–11.9 cm; bottom) between NFI2 and NFI4 records on sample plots affected by different disturbances agents. Number is bracket indicate the number of NFI plots per disturbance category. Coloured bars indicate the mean, black bars the standard errors. Asterisks mark significant changes (pairwise-Wilcoxon with Holm correction, p < 0.05). Aa = Abies alba, Pa = Picea abies, Pc = Pinus cembra, La = Larix spp., Pi = Pinus spp., Oc = other conifers, Cs = Castanea sativa, Fs = Fagus sylvatica, Ac = Acer spp., Fr = Fraxinus spp., Qu = Quercus spp., Ob = other broadleaves.

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A detailed analysis of sites affected by wind or insect disturbances revealed that both, plots naturally dominated by P. abies (that is, NST of spruce forests) as well as plots naturally dominated by broadleaves showed a dramatic decrease of P. abies BA. However, the effect on P. abies was significantly stronger on sites naturally dominated by broadleaves (− 56 ± 4% for wind,− 37 ± 9% for insect) than on sites with NST of spruce forest (− 21 ± 7% for wind,− 24 ± 14% for insects; Wilcoxon rank-sum test with continuity correction W = 19,375, p < 0.001 for wind, W = 1719, p = 0.04 for insects; Figure 4).

Figure 4
figure 4

Change in basal area (m2 ha−1; a,d), sapling recruitment (stems ha−1 with height > 10 cm and DBH <  = 3.9 cm; b,e) and pole-stage recruitment (stems ha−1 with DBH 4–11.9 cm; c,f) between NFI2 and NFI4 records on NFI sample plots affected by wind (a,b,c) and insect disturbance (d,e,f) grouped into plots naturally dominated by spruce (blue, montane/sub-alpine Norway spruce forests and Silver fir-Norway spruce forests) and naturally dominated by broadleaves (red). Aa = Abies alba, Pa = Picea abies, Pc = Pinus cembra, La = Larix spp., Pi = Pinus spp., Oc = other conifers, Cs = Castanea sativa, Fs = Fagus sylvatica, Ac = Acer spp., Fr = Fraxinus spp., Qu = Quercus spp., Ob = other broadleaves.

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In contrast to the ‘naturally disturbed’ plots, the ‘undisturbed’ plots (that is, no human intervention or natural disturbance) showed a significant increase in BA of all taxa except Pinus spp. (Figure 3). ‘Treated’ plots showed an increase in BA of A. alba, P. abies, Larix spp., other conifers, Acer spp. and Fraxinus spp. and a decrease for F. sylvatica (Figure 3). On ‘salvage logged’ plots, the BA of F. sylvatica increased and Pinus spp. decreased.

Post-disturbance Vegetation Trajectories

Sapling density was higher in NFI4 than in NFI2 (+ 29.7%, Table S7) while the density of pole-stage trees was lower (− 5.6%, Table S8). Surprisingly, the number of plots without any saplings increased from 20 to 34% while the number of plots without pole-stage individuals decreased from 57 to 47% (Tables S7–S8). On ‘naturally disturbed’ plots mean sapling-density was reduced, and pole-stage-density increased compared to ‘undisturbed’ plots (Tables S7–S8). Sapling density was dominated by broadleaves (87%) while the pole-stage density was evenly distributed between conifers and broadleaves (Figures S7–8). ‘Undisturbed’ plots showed a slight decrease in saplings recruitment of conifers (− 2.5%) and an increase for broadleaves (+ 10.7%), but conifers were much more successful in prevailing into the pole-stage recruitment (+ 16.3%) than broadleaves (− 18.6%, Tables S7–S8). Both ‘salvage logged’ and ‘treated’ plots showed an increase in sapling recruitment for both conifers and broadleaves but broadleaves were more successful in the pole-stage recruitment than conifers, especially on ‘salvage logged’ plots (Tables S7–S8). ‘Naturally disturbed’ plots showed an increase of sapling recruitment in conifers (+ 60%) while the pole-stage recruitment was dominated by broadleaves (+ 81.6%).

Due to the relatively small sampling area and the associated extrapolation to the hectare, data on tree recruitment were highly stochastic and variable. As a result, only very few significant disturbance-induced and species-specific changes in vegetation trajectories could be detected. On PNCs of broadleaves that were mostly stocked with conifers, wind disturbance led to a shift of the vegetation trajectory towards more broad leaf recruitment over the last 20 years (Figure 4). No such shift in tree recruitment was observed on wind-disturbed, and insect-attacked sites naturally dominated by spruce (Figure 4). Additionally, P. abies was the only species showing a significant negative effect of silvicultural treatment on pole-stage recruitment. In sites affected by drought, recruitment by Pinus ssp. decreased in favour of Quercus spp., especially in the ‘Western Central-Alps’ while in the ‘Southern Alps’, the recruitment of C. sativa decreased on drought-prone sites (Tables S9–S12; Figure 3). Castanea sativa was also the only species showing a consistent decrease in tree recruitment (saplings and pole-stage trees) across all plots with the notable exception of post-fire tree recruitment (Tables S11–S12; Figure 3).

Discussion

Canopy Disturbances as Catalysts of Forest Vegetation Shifts

Over the last 25 years, the Swiss forests showed a shift from conifer towards a higher proportion of broadleaf species especially at lower elevations (Figure S3). This life form shift was mainly driven by extensive natural canopy disturbances (> 0.1 ha), or a combination of the natural and anthropogenic disturbance (‘salvage logged’), as the conifers-broadleaves ratio did not change in ‘treated’ and ‘undisturbed’ plots. Additionally, in the warmest and driest regions of Switzerland, comparisons indicate that extreme droughts led to diverging tree species compositions affecting mostly chestnut groves and oak-pine forests.

Although part of this general shift in taxa composition of the tree recruitment might reflect successional dynamics indicated by an increase in fast-growing and more site-specific species (for example, Acer spp., Fraxinus spp. and other broadleaves). our data indicate that this also reflects a lasting transition towards a new climax state dominated by broadleaves (that is, mostly beech).

Anthropogenic Disturbances (Forest Management)

Anthropogenic disturbances (‘salvage logged’ and ‘treated’) are unevenly distributed across regions and taxa but seem mostly correspondent to two factors: Accessibility and phytosanitary instances due to insect outbreaks. Both ‘salvage logged’ and ‘treated’ plots are over-proportionally located in the ‘Jura’ and ‘Plateau’ as well as in NST of beech forests and plots dominated by F. sylvatica, which are mainly located in these two biogeographic regions. Forest in these regions is highly accessibility due to a dense system of forest roads at widely unproblematic terrain and stand productivity favouring more intensive silvicultural interventions. ‘Salvage logged’ plots were also over proportionally practiced in stands dominated by A. alba and consequently in NST of Silver fir-Beech forest and likely related to the containment and prevention of insect outbreaks as well as good accessibility.

Fatal Attraction: P. abies Outside of Their Natural Range

Although the overall contribution to BA of most species only slightly changed (± 0.7%) between 1995 and 2017, a considerable if not dramatic change was observed in P. abies (− 2.9%), and stands dominated by P. abies were confirmed to be especially susceptible to certain natural disturbances (62% of all natural disturbance events), which resulted in a shift towards more broadleaf species. This is not surprising when considering the general susceptibility of P. abies to both windthrows (for example, Hanewinkel and others 2011) and the European spruce bark beetle (Ips typographus; for example, Wermelinger 2004; Faccoli and Bernardinelli 2014), including the interaction of beetle attacks in spruce forests weakened by wind disturbance and/or prolonged drought (Stadelmann and others 2013; De Groot and others 2018; Dobor and others 2020).

In our study large-scale wind damage in P. abies stands occurred predominantly during the Lothar storm (1999), which mostly affected forests at low elevations within the range of the natural beech and silver fir-beech communities. No major storm occurred during the observation period in montane and subalpine Norway spruce forests. In mountain spruce forests affected by Vivian (storm of February 1990 at higher elevations, predominantly in the Central Alpine valleys) the post-disturbance regeneration trajectories were dominated by P. abies (Schönenberger 2002). In contrast, in the Lothar-affected lowlands and foothills of the Pre-Alps, where P. abies is mostly out of the natural range as it was frequently planted in the past and promoted in the frame of adopted German silvicultural systems (Bürgi and Schuler 2003), the post-disturbance trajectories indicate a decrease in P. abies. This pattern was additionally reinforced by the abundant bark beetle outbreaks that occurred on plots previously affected by wind disturbance during the heatwave in summer 2003 (Forster and Meier 2010; Stadelmann and others 2014).

Our data on pole-stage recruitment suggest that on low elevation sites, where forests are naturally dominated by broadleaves, disturbances significantly altered tree species compositions away from P. abies (and conifers in general) towards more broadleaf species. The shift towards more broadleaves was stronger after wind than insect disturbance. Bark beetles usually mainly attack trees older than 50 years, leaving younger trees healthy (Wermelinger 2004) and therefore limiting the potential for a dramatic shift in vegetation trajectories. Nevertheless, salvage logging to control bark beetle infestations and intensified harvests of P. abies to avoid financial loss due to expected further lowering of wood prices likely amplified this trend (Kläy 2015). As the most influential disturbance events happened around 15–20 years ago the shift in vegetation trajectories was mostly visible in the well-established pole-stage that is nowadays dominated by broadleaves. This indicates that, at low elevations, broadleaf species are competitively superior to P. abies, as had been demonstrated in post-windthrow patches (Kramer and others 2014). The increased reliance on natural regeneration in the second half of the twentieth century (Burgi 1999) has also contributed to their recent re-expansion. Additionally, at lower elevations foresters started to transition forests away from P. abies by species regulation in the juvenile phase (up to thickets) and through targeted thinning, explaining the observed negative effect of silvicultural treatment on pole-stage recruitment of P. abies. Because most future climate change scenarios predict an increased frequency of extreme weather events such as large storm systems, prolonged droughts and heatwaves, linked disturbances (for example, Seidl and others 2017) need to be considered. Insect attacks likely increase, as wind-disturbed and climatically stressed trees, become more susceptible (Brandl and others 2020) and warmer temperatures stimulate bark beetle population dynamics resulting in more generations and higher densities during one season (Stadelmann and others 2013; Jakoby and others 2016). At higher elevations, disturbances did not seem to negatively affect the recruitment of conifers both after wind and insect disturbances. In these cooler forest habitats, the tree regeneration is dominated by direct ingrowth of the stand-forming coniferous species. This is in line with findings of Kramer and others (2014) on regeneration in gaps produced by windthrow across Switzerland. Therefore, while P. abies is still increasing in BA and pole-stage recruitment on ‘undisturbed’ sites, the expected increase in natural disturbances in the frame of the ongoing climate change might accelerate the tree species shift of these stands in the future.

Impressive Though Rare: Fire and Drought

In contrast to damage by wind and insect outbreaks, other disturbance agents or extreme events such as fire and drought were rarely observed during the study period and affected only a small proportion of the Swiss forest area (< 0.2% of plots) mostly located in the warmest (‘Southern Alps’) and driest biogeographic Swiss regions (‘Western Central-Alps’). In the dry valleys of the ‘Western Central-Alps’ drought has negatively affected P. sylvestris that has for long dominated slopes at low elevation due to traditional land-use practices (pasture and litter collection; Gimmi and others 2010) towards the dry limits of forest growth (Etzold and others 2019). Land-use change in the past decades in combination with repeated extreme drought events increased tree mortality (for example, Rigling and others 2013) and benefit the more drought-resistant and shade-tolerant downy oak (Q. pubescens). Combined drought-shade effects to drought-tolerant P. sylvestris (Niinemets and Valladares 2006), hampers successful recruitment (Bachofen and others 2019). Similar transitions from pines to oaks are reported for dry regions in Italy (Vacchiano and Motta 2015) and Spain (for example, Zavala and others 2000; Galiano and others 2010; Carnicer and others 2014; Morán-López and others 2014).

In the ‘Southern Alps’ drought mostly affected forests dominated by C. sativa. The chestnut groves in southern Switzerland are the result of a long-term land use practice aimed at producing both fruits for the self-sustainment and wood (Krebs and others 2012), which encouraged the chestnut cultivation at the limits of the tree and in some cases even at the edge of its fundamental niche (Muster and others 2007). Similar to the pine-dominated forests in the Valais, the suspension of any management input made the chestnut groves particularly susceptible to the colonisation by other more site-specific tree species, exposing the light-demanding C. sativa trees to competition, climatic stress and specific diseases (Conedera and others 2001). On the driest and most exposed sites, this resulted also in an enhanced summer drought susceptibility as revealed by the 2003 summer drought and heatwave (Conedera and others 2009). All this induces a shift towards the NST, that is, corresponding to beech on rich sites at higher elevations and broadleaf mixed forests dominated by lime (Tilia spp.) as well as maple trees (Acer spp.) in the lowlands (Conedera and others 2000; Muster and others 2007). Fire displays, on contrary, the opposite effect by enhancing the recruitment of the disturbance-adapted C. sativa that profits from its extreme resprouting capacity and outcompetes other fire-sensitive broadleaves (Delarze and others 1992), including shade tolerant and slow growing invasive evergreen species such as the native Ilex aquifolium or the neophytitic Trachycarpus fortunei and Prunus laurocerasus (Grund and others 2005), but not fast growing and invasive neophytic pioneer tree species such as Ailanthus altissima or Paulownia tomentosa (Maringer and others 2012).

So far, both drought and fire affected the forest dynamics trajectory only in NFI plots in the warmest and driest parts of the country that are dominated by certain species as a result of century-long management effects (that is, C. sativa and P. sylvestris) that partially grow at their ecological limits. The question arises how disturbances may affect the most widespread forest types such as spruce and beech forests in the future. Conspicuous drought damage in both beech and spruce forests during the recent summer drought periods (2018, 2019; Schuldt and others 2020) suggests that drought may assume a central role in shaping future tree species compositions in Swiss Forests. Evidence of forest fires impacting usually fire-avoiding forest types such as beech stands is also increasing in recent decades (Maringer and others 2016).

Limits of the Study

In this study, we used the data collected on revisited plots of the Swiss forest inventory. Besides the great advantage of the long time span of the observations (up to 25 years) and the permanent character of the plots, the used data present also some limits. The shift of the survey from a periodic three-years lasting survey (NFI2) to a continuous, nine-years lasting field activity (NFI4) implies that the period of observation may vary by up to eight years among plots. In addition, the survey protocol of the recruitment slightly changed between the two inventories and the retrospective approach in reconstructing and assessing the disturbances that occurred on each plot may represent a source of imprecision.

Systematic samples such as the NFI produce representative results in terms of quantitative rankings. However, important small-scale phenomenon such as the local spread of pioneer species (Maringer and others 2012; Conedera and others 2018), the dynamic of rare species or the study of detailed post-disturbance ecological processes require targeted and species-specific approaches (for example, Wohlgemuth and others 2002, 2018; Allen and others 2010; Moser and others 2010; Rigling and others 2013; Maringer and others 2020; Schuldt and others 2020).

Conclusions

Most future climate scenarios predict warmer and drier conditions across Switzerland and Europe in general (Zubler and others 2014) and an increased risk of prolonged droughts (Spinoni and others 2018), forest fires (Pezzatti and others 2016) and large storms (Collins and others 2019). However, while models often predict dramatic changes in forests with ongoing climate change (for example, Lindner and others 2014; Dyderski and others 2018), representative studies observe no or only slow transitions (for example, Gehrig-Fasel and others 2007; Lenoir and others 2010b; Küchler and others 2015). Despite the limits of the present approach, we detected natural and anthropogenic disturbance as a major trigger of tree species shifts in Swiss forests in the current global change context. At present, post-disturbance tree species shifts predominantly occur where dominant species have been cultivated out of its natural range due to economic reasons (that is, P. abies in the lowland of the Plateau). Additionally, forests seem increasingly at risk in hitherto traditional ranges of management (that is, widespread pure pine stands at low elevations in the Valais or the chestnut groves in the Southern Alps). Overall, our data suggest that the forest trajectory after a disturbance favours recruitment of species of the natural forest vegetation. In particular, in disturbed P. abies stands at the low-lands, broadleaf species regenerate most successfully, while in P. abies stands at higher elevations, direct regrowth of this and other coniferous species prevails. Undisturbed forest stands, on contrary, persisted in their species composition, and the constrained tree recruitment under the canopy was mostly comprised of the species dominating the stand. Tree regeneration under dense canopies is always sparse, and only a few species can build up an abundant ‘seedling bank’ in a closed and shady stand (Grubb 1977; Savage and others 1996). Most species rather regenerate in pulses realised in canopy gaps (Zackrisson and others 1995; Jentsch and White 2019). In a small gap, however, the chance that seeds from new species arrive during the window of opportunity for the seedling establishment is a stochastic event, and most gaps will be filled with seedlings from the already present dominant species (Moser and others 2010). Therefore, in undisturbed forests, the dominating tree species rather than the abiotic conditions are driving species selection. Larger-scale disturbances, on the other hand, seem to level the playing field allowing more species to establish and favour those (among which many opportunistic ones) that are adapted to the current environmental conditions and can increase resistance against future disturbances (for example, repeated insect outbreaks; Sommerfeld and others 2020). Larger-scale disturbances might therefore act as essential catalysts to enable/accelerate the transition of forest ecosystems in adaptation to changing environmental conditions (Brice and others 2019, 2020).