A 2.18 m sediment profile was collected in August 2015 from the deepest (14.8 m) part of Prášilské jezero (49° 04′ N, 13° 24′ E, 1,079 m a.s.l.) and is comprised of two parallel and overlapping cores (PRA 15-2-1 and PRA 15-2-2). The sediment profile was sampled from a floating platform using a hand-percussion Russian-style corer (1.5 × 0.075 m). The sediment–water interface was collected using a 0.1 m diameter gravity corer (PRA15-2GC) (Boyle, 1995). At Rachelsee (48° 58′ 29” N, 13° 24′ 7” E, 1,071 m a.s.l.), 11.8 m of sediment was collected in August 2012 at 12.5 m water depth using a Livingstone piston corer (Wright, 1967) (RAA1-4). Sediment was not recovered between depths 0–57 cm, therefore, the mud-water interface depth begins at 57 cm. Sediment age-depth relationships were established using ¹⁴C radiocarbon dating at Prášilské jezero (n = 10) and Rachelsee (n = 7), with an additional ²¹⁰Pb series (Appleby, 1978) at Prášilské jezero (Table 1). For all sites, the geochronological data including the sediment surface were compiled within the Bayesian routine “BACON” (Blaauw and Christen, 2011). This analysis partitioned both cores into 0.05 m thick sections and estimated the accumulation rates for each segment using a Markov Chain Monte Carlo (MCMC) approach (Christen and Pérez, 2009). The analyses were constrained by a prior model of sediment accumulation rate (a gamma distribution with mean 50-year cm⁻¹ and shape 1.5) and its variability (memory, a beta distribution with mean 0.5 and shape 16) for both sites. All ¹⁴C ages were calibrated and modeled in “BACON” using the IntCal13 curve (Reimer et al., 2013), with a Student-t distribution to account scatter in the ¹⁴C measurements and to allow for statistical outliers (Blaauw and Christen, 2011). The weighted mean modeled ages against depth were smoothed using a 21-point moving average (Figure 3).

At Prášilské jezero, pollen analysis was conducted at 1–2 cm resolution throughout the core by V. A. Carter. For each sample, 0.5 cm⁻³ was processed using standard pollen procedures (Faegri et al., 1989). At Rachelsee, pollen analysis was conducted at 1 cm resolution for 72–124 cm, 2 cm for 126–184 cm, and 4 cm for 188–540 cm by J. F. N. van Leeuwen. The sediment between depths 57–71 cm was too watery and could not be subsampled for pollen. Here, we present data from depths 72–368 cm. At Rachelsee, a minimum of 300 tree pollen grains, and at Prášilské jezero, a minimum of 500 pollen grains were counted in each sample. A Lycopodium tablet with a known amount of spores were added to each sample as an exotic tracer in order to calculate pollen concentration and influx rates (Stockmarr, 1972). Total pollen counts were converted into pollen percentages and plotted using Tilia software (Grimm, 1987) based on the abundance of each pollen type relative to the sum of all terrestrial identified pollen. The pollen profiles were divided into pollen assemblage zones based on optimal splitting using the broken-stick model (Bennett, 1996). Pollen counts were converted to concentrations (grains cm⁻³) in Tilia using the counts of Lycopodium tracers and sample sediment volumes (Grimm, 1987). The smoothed Bayesian age-depth model for both sites was used to calculate and plot pollen influx rates from the concentration data using Tilia software.

The REVEALS model (Sugita, 2007a) was applied to estimate pollen-derived regional vegetation cover during the Holocene. Previous studies from the region (e.g., Abraham et al., 2014) mainly used records from peat bogs to calculate a REVEALS output, with the model showing the robustness of peat sequences in portraying present-day vegetation in a realistic manner. Here, the REVEALS model was used to estimate land-cover abundance from both lakes and non-lakes (i.e., peat bogs and mires) using new (this paper) and previously published and unpublished pollen data obtained from the Czech Quaternary Pollen Database (Kuneš et al., 2009; Table 2). Pollen counts were aggregated into 500-year intervals for the entire Holocene, and then all available sites in each 500-year time window were used to estimate mean vegetation abundances within a 60 km radius. The following parameters were selected: 4 m s⁻¹ wind speed, different dispersal-deposition models for bogs and mires (Prentice, 1985) and lakes (Sugita, 1994), and taxon-specific relative pollen productivities for 28 selected pollen-equivalent taxa (see Supplementary Table 1; Abraham and Kozáková, 2012; Mazier et al., 2012; Abraham et al., 2014). For model calculations, we adjusted the basin sizes of non-lakes to 4.5 ha. Larger basin sizes produce unrealistic estimates for some taxa as some trees may still grow within the basin, violating the model's assumptions (Abraham et al., 2014). Therefore, assuming smaller non-lake basins was more appropriate for our calculations. Openness was calculated by the summation of all herbs included in the model. All calculations were conducted in R (R Core Team, 2016) using functions to calculate mean vegetation abundances and their error estimates based on bootstrap methods (https://github.com/petrkunes/LRA).

Müller (1927) originally presented data for 12 tree taxa (Figure 2), however, the discussion of this research will focus on the three dominant canopy species growing in the Bohemian/Bavarian Forests at modern times; spruce (Picea abies), beech (Fagus sylvatica), and fir (Abies alba). Based on the broken-stick model (Bennett, 1996), the pollen percentages profiles from Prášilské jezero and Rachelsee were divided into seven and eight pollen assemblage zones, but for simplicity, the pollen reconstructions were grouped into three main periods; (1) Early- to Mid-Holocene, (2) Mid- to Late-Holocene, and (3) Last-Millennium.

Spruce first arrived at the high-elevation lakes Prášilské jezero and Rachelsee between 10,500 and 10,000 years ago, but was present at mid-elevation peat bogs/mires by ~11,500 cal. years BP (Figure 4). Spruce rises to dominance between 10,000–8,500 cal. years BP across all elevations (Figure 4) replacing pine (Pinus) and hazel (Corylus) as the dominant canopy species (see Supplementary Figures 3–5). Thereafter, spruce percentages fluctuated between 35% and >20% pollen at both Prášilské jezero and Rachelsee, and around 30% at mid-elevation peat bogs/mires, respectively. Pollen influx data for spruce conversely differ between the two sites with a peak of 4,000 grains cm⁻² yr⁻¹ at Prášilské jezero, whereas the increases at Rachelsee are more subdued at <500 grains cm⁻² yr⁻¹ (Figure 5). Quantitative land-cover reconstructions of spruce percentages calculated using REVEALS (Sugita, 2007a) rise around 10,000 cal. years BP from ~5% to >60% calculated on a local (using peat bogs/mire records) basis (Figure 6). On a regional basis (using lake records), spruce comprised >50% total land-cover. The REVEALS land-cover reconstructions suggest that spruce forms close to double the land-cover percentages compared to pollen percentages. Beech begins to expand at both sites from 7,000 cal. years BP onwards, but is at low percentages in the Early- to Mid-Holocene. The dynamics of fir are restricted to after 5,000 cal. years BP. Total herb percentages decrease from ~20% and oscillate around 10% at both Prášilské jezero and Rachelsee. Pollen influx data for total herbs decrease ~10,500 cal. years BP from values >2,000 grains cm⁻² yr⁻¹. The REVEALS reconstruction also suggests that the percent of landscape openness is close to double that of pollen percentage data at both the regional and local-scale (Figure 6).

Between 6,800 and 6,000 cal. years BP beech increases gradually in pollen percentages at both Prášilské jezero and Rachelsee, and then oscillates around 25% (Figure 4). Beech percentages from mid-elevation peat bogs/mires begin to increase ~500 years earlier than in the high-elevation lakes. At the local-scale (peat bogs/mires), beech percentages gradually increase throughout the Mid- to Late-Holocene peaking at ~25% roughly 1,000 cal. years BP. Fir expansion commences 5,500–5,000 cal. years BP to around 20% at both Prášilské jezero and Rachelsee. These increases in pollen percentages are associated with no substantial reduction in spruce pollen percentages at Prášilské jezero and Rachelsee, but spruce percentages decrease from ~40% to ~20% <900 m a.s.l. (Figure 4). Viewed from a perspective of pollen influx, the down core pattern of increase and stability in species percentage data are reflected in the influx data (Figure 5). Though as with spruce, influx rates for beech and fir are much lower at Rachelsee. The REVEALS land-cover reconstructions for spruce and fir are lower at the regional scale (i.e., lake sites) relative to the more local-scale (i.e., peat bog/mire sequences), whereas beech is an equally abundant component in both reconstructions comprising ~25% land-cover (Figure 6). The land-cover contribution of spruce remains double (>40%) the values indicated by pollen percentage data (>15%), but these differences are not present for beech and fir. Land-cover percentages of spruce remained relatively stable at the lake sites however, spruce decreases from >60% to ~40% at mid-elevation peat bog/mire sequences during this period. The REVEALS reconstruction also suggests that the percent of openness is <5% at both the regional and local-scales (Figure 6).

At both lakes, spruce percentages remain unchanged initially and then increase ~5% in the last 300 years (Figure 4). Spruce percentages remain unchanged at mid-elevation peat bogs/mires. Both beech and fir pollen percentages decline at both lakes and mires sequences through the last 1,000 years (Figure 4). Openness percentages increased dramatically over the past 1,000 years to ~12% (Figure 4). Pollen influx rates for all three taxa increase during the last 1,000 years at both lakes, and sharply in the last 300 years which likely reflects focusing of pollen toward the lake center (i.e., in-lake lateral redistribution of materials toward the deeper waters) rather than any change in species abundance (Figure 5). The REVEALS reconstructions broadly confirm the indications seen in the percentage pollen data, with declines in beech and fir occupied by open-ground indicators rather than any expansion in spruce. This is also shown in the pollen influx data, showing a dramatic rise in the influx of open-ground taxa (Figure 5).

Over the past 100 years, methodological improvements in the field of paleoecology such as radiocarbon dating and pollen-based quantitative land-cover modeling (i.e., REVEALS; Sugita, 2007a) have led to more realistic reconstructions of past vegetation change across temporal and spatial scales. Applying these approaches to the palaeoenvironmental records from Prášilské jezero and Rachelsee illustrates the long-term presence and dominance of spruce in the Bohemian/Bavarian Forests. First appearing in the pollen record ~10,000 cal. years BP, the local presence of spruce in the region is supported by Picea stomata at Prášilské jezero and Rachelsee, which agrees with previous paleoecological research in the region (Müller, 1927; Svobodová et al., 2001, 2002; Jankovská, 2006; Abraham et al., 2016). At the regional scale (i.e., lake sites), the REVEALS model suggests that spruce has comprised an average of ~50% of the total forest canopy, whereas at the more local scale (i.e., peat bogs/mires) spruce has comprised ~59%. Thus, spruce has been a dominant component of the forest canopy across a range of elevations >700 m a.s.l. in the Bohemian/Bavarian Forests.

Modern quantitative approaches also illustrate a unique history of forest composition throughout the region with beech and fir contributing as secondary canopy species relative to spruce at mid-elevations found between 700 and 900 m a.s.l. Beech established around 7,000 cal yr BP and became an abundant canopy species around 6,000 cal. years BP, with the REVEALS model indicating that it contributed ~20% of the total land-cover at the regional scale. However, at the more local-scale between 700 and 900 m a.s.l. beech comprised just 16% of the total land-cover during the Mid-Holocene. After the establishment of beech, Müller (1927) found an increase in fir percentages. Svobodová et al. (2001, 2002) suggested that fir had spread to elevations >700 m a.s.l. across the entire region between 6,300 and 3,400 cal. years BP. However, these records were constrained by few radiocarbon dates. At Prášilské jezero and Rachelsee, fir first appeared between 7,000–6,300 cal. years BP but did not become an important component of the canopy until 5,000–4,500 cal. years BP. The REVEALS model output indicates that for the last 4,000 years, fir has comprised an average of ~7% of the total land-cover at the regional-scale, but slightly higher (10%) at the more local-scale between 700 and 900 m a.s.l. As beech and fir expanded around 4,000 cal. years BP, spruce percentages declined to some extent but still comprised ~40% of the regional total land-cover, and >60% of the local-scale (i.e., mid-elevation) forest composition in the Bohemian/Bavarian Forests (Figure 6). This further demonstrates that beech has been a secondary canopy species relative to spruce at mid-elevations, specifically between 700 and 900 m a.s.l. Total land-cover of beech and fir populations found at mid-elevations began to decrease from their maximum values of 24% and 13% total land-cover ~1,000 years ago at both the regional and more local-scale (Figure 6). The decrease in beech and fir forests are likely the result of increasing anthropogenic landscape modifications. However, climate could also explain the decline in beech and fir as temperatures were likely unfavorable for these species during the Little Ice Age (Grove, 2001; Brázdil et al., 2017).

Declines in beech and fir characterize the last millennium with an expansion in open ground taxa in both the percentage pollen data and the quantitative REVEALS land-cover reconstructions, though spruce is slightly more abundant in terms of total land-cover (Figure 6). Svobodová et al. (2002) also indicate that beech-spruce-fir forests have decreased over the past three centuries, and are being replaced by spruce and Pinus rotundata in forest bogs at lower elevations. The regional reconstructions presented here point to a change in forest composition toward more open ground within the forest, and slightly increased dominance by spruce. Over the past 500 years spruce has comprised ~47% land-cover, while beech and fir has comprised ~8% and <5% at mid-elevations. The pollen influx values show a radically different story with the records from Prášilské jezero and Rachelsee illustrating an increase in all pollen taxa through the last 500 years. This increase in pollen influx likely reflects increased in-lake focusing of pollen to the lake center assisted by wind currents from a more open landscape (see Figure 5 for percent openness), rather than increased pollen rain from spruce plantations found at low-elevations. Openings within high-elevation forests are generally attributed to human clearance, fire, windthrow, or bark beetle outbreaks (Ips typographus) which are key disturbance drivers influencing Norway spruce forest dynamics (Holeksa and Cybulski, 2001; Fischer et al., 2002; Holeksa et al., 2007; Svoboda et al., 2012, 2014; Čada et al., 2016; Janda et al., 2017; Kulakowski, 2017). During the 1990s, an I. typographus bark beetle outbreak occurred in the region creating gaps (i.e., openings) in the canopy. Severe windstorms occurred in the region in 1986, 1999, 2007, and 2008, which also caused widespread forest damage (Svoboda et al., 2010). Yet, Zeppenfeld et al. (2015) found that spruce seedlings and young spruce trees already occur in areas affected by these large-scale disturbances, encouraging rapid recovery of spruce and not limiting flux of pollen to the lakes in the region. Therefore, increase in pollen fluxes especially at Prášilské jezero is likely a result of natural processes and not a result of spruce plantations found at lower elevations.

While the REVEALS model demonstrates that spruce has been the dominant canopy cover at both high- and mid-elevations, the model only provides an average percentage of land-cover for the entire Bohemian/Bavarian Forest. Local site conditions such as slope and aspect affect forest composition at the more local-scale (i.e., stand-scale). For example, spruce generally dominates on north-facing mesic sites with poorly-drained soils, whereas beech typically dominates on south-facing xeric sites where soils drain well. These local conditions can lead to stands of monospecific spruce or beech at mid-elevations, specifically in the Bavarian Forest National Park. However, the REVEALS model used in this study was used to inform managers of the natural average forest composition for elevations >700 m a.sl. for the area through time.

The overall management goal of the parks is to restore the original forest composition (IUCN, 1994). Within the Šumava National Park forest management has likely based their target “natural” forest composition from Geobotanical Maps (Mikyška et al., 1968–1972), and the Map of Potential Natural Vegetation (Neuhäuslová et al., 1998), which suggest that beech dominate over spruce at mid-elevations in central Europe (Ellenberg and Leuschner, 1996). More recently, Vacek and Mayová, 2000, suggest that beech is dominant over spruce at mid-elevations between 650 and 900 m. Together, these previous maps and research have influenced the management strategy of removing spruce from mid-elevation forests within Šumava National Park so that spruce will comprise 30–40% instead of its current representation of “84%” (Šumava National Park Authority²). However, these maps and previous research do not consider paleoecological studies which provide long-term dynamics of vegetation abundance through time. Paleoecological studies have utility in conservation and restoration in that they can provide baseline targets (i.e., more accurate representations of forest composition), as well as provide the natural range of variability within vegetation composition through time. While the REVEALS model shows that beech and fir forests reached a combined maximum of 37% at mid-elevations (between 700 and 900m a.s.l.) ~2,000 years ago, spruce forests still comprised 45% of the total forest composition at these elevations (Figure 6). A regional paleoecological perspective of the past 500 years shows that the forest composition at mid-elevations has been a mixed-forest cover of spruce (~50%), beech (~22%), and fir (~10%), which counters the findings of Vacek and Mayová (2000) Our findings support previous investigations that highlight that mixed-forests consisting of spruce-beech-fir typically occur between 650 and 1,150 m in the Šumava National Park (Röder et al., 2010), as well as showing the long-term dominance of coniferous forests in the Bohemian-Moravian highlands over the past 9,000 years (Szabó et al., 2017). The long-term presence of spruce throughout the Holocene illustrates that forest management practices involving the formation of spruce plantations at mid-elevations have not suppressed beech (Szabó et al., 2017). During the late Holocene the results from the regional-scale sites show some suppression of beech (~15% land-cover) that is not related to spruce dominance, but rather to the extent of open ground. This supports previous research from Šumava National Park with beech rarely reaching >20% from mid-elevation sites (Hrubý et al., 2014).

Within the Bavarian Forest National Park, soil conditions and south-facing slopes create more favorable conditions for beech and fir forests. While spruce has doubled its range since the nineteenth century in the region, foresters have been more concerned with the recent decline of fir. At the time of establishment of the Bavarian Forest National Park, fir comprised only 3.2% of the total forest composition (Heurich and Englmaier, 2010). This is supported by the regional-scale REVEALS reconstruction which shows the decline of fir over the past 500 years to 3.8%. However, at the more local-scale (i.e., peat bogs/mires) at mid-elevations fir has declined to just 2.9% (Figure 6). Stalling (1987) found that fir pollen frequencies reached a peak of ~32% fir roughly 3,000 years ago on the basis of 15 sites with a mean elevation of 837 m a.s.l. in the Bavarian Forest. However, quantitative land-cover reconstructions presented here indicate that fir reached a maximum regional land-cover of just 13% between 3,500 and 1,000 cal. years BP, and a maximum of ~20% at the more local-scale at mid-elevations (Figure 6). Methodological improvements such as quantitative land-cover modeling offer a more robust reconstruction of vegetation abundance from the Bohemian/Bavarian Forests of central Europe. Thus, the results presented here provide a more accurate and updated description of fir abundances than the previous work by Stalling (1987). However, the REVEALS model is limited in that all of the mid-elevation sites used in the model are located within the park boundaries of Šumava National Park. Mid-elevation paleoecological reconstructions do exist from the Bavarian Forest National Park, yet, the sites were not incorporated into the model because they were either not available publicly or they lacked radiocarbon dating. Therefore, the results interpreted here may be more reflective of vegetation abundances from mid-elevation in Šumava National Park. Regardless, the REVEALS model reconstructed the regional decline of fir, and it is therefore a concern that should be addressed by foresters in the Bavarian Forests. Heurich and Englmaier (2010) suggest that the trend of declining fir populations relative to increasing spruce populations in the Bavarian Forests is likely not solely related to human activity but also to natural causes such as the Little Ice Age and bark beetle outbreaks. Because spruce is a cold adapted species, conditions during the Little Ice Age would have favored spruce over fir. Additionally, downed logs from windthrow and bark beetle outbreaks are the preferred natural substrate that gives spruce seedlings a competitive advantage over other tree species (Svoboda et al., 2010). While several studies suggest that fir is sensitive to human activities and anthropogenically caused air pollution (Tinner et al., 1999; van der Knaap et al., 2004; Tinner and Lotter, 2005; Feurdean and Willis, 2008), human impacts in the form of pasturing, selective logging and litter raking have been shown to be beneficial for fir growth (Nožička, 1957; Málek, 1981; Kozáková et al., 2011). Tinner et al. (2013) suggest that the range of fir has the potential to increase with climate change as long as precipitation does not decrease below 700–800 mm/yr, or anthropogenic disturbances such as fire and grazing do not become excessive.

One of the benefits of the Šumava and Bavarian Forest National Parks is that it serves to protect montane spruce forests (Svoboda et al., 2010). Spruce is likely to be the species most impacted by climate change, and so is under threat in a warmer world (Bolte et al., 2009). Norway spruce is considered a cold-adapted species growing on mesic sites. Therefore, expected changes in temperature and precipitation in the future specifically the increased likelihood of more frequent and intense summer droughts across central and southern Europe (Gao and Giorgi, 2008; Feyen and Dankers, 2009), will likely result in the loss of available habitat for spruce (Hanewinkel et al., 2012). In the Bohemian/Bavarian Forests, Norway spruce has already occupied the highest possible altitudes and ostensibly the species has nowhere to go with climate change (Spathef et al., 2014). Conversely, beech and fir forests, the main competitors of spruce in the region, could continue to impinge on and expand in the lower altitude habitat of spruce. However, areas where beech predominates likely will also be threatened with increasing temperatures (Cheddadi et al., 2016). Migration of the optimum climate windows for these dominant forest taxa is a concern for forest managers, yet our paleoecological data show these plant communities have comprised a mixed-forest for >5,000 years in the region surviving previous climatic fluctuations during the Holocene. Mixed-forests with multiple tree species provide higher levels of ecosystem services (Gamfeldt et al., 2013) and should therefore be incorporated into forest management in the Bohemian Forests. We suggest that forest managers in the Šumava National Park consider utilizing information gained by paleoecological reconstructions that use pollen-based land-cover models that more accurately represent the natural vegetation abundances found at mid-elevations.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.