Human-induced changes in fire regime and subsequent alteration of the sandstone landscape of Northern Bohemia (Czech Republic)

Abstract

Multiproxy palaeoecological evidence from a sandstone region in northern Czech Republic was collected to explore the impact of fire disturbances on the decline of the broadleaved forests during the Late Bronze Age (3250–3050 cal. BP). It has been hypothesized that human-accelerated soil leaching affected the nutrient availability in the sandstone area, thus promoting the expansion of oligotrophic-adapted plant communities in the late-Holocene. Little is known about the mechanisms which induced such large-scale vegetation transformation. We sought to determine which driving forces were involved using independent proxy records – soil and sedimentary charcoal, pollen and fungal spores. Local fire history was derived from the variation in charcoal accumulation rates (CHAR) preserved in Eustach peatbog. The fire frequency (FF) estimation over the past ~7500 years revealed distinct phases of increased burning between 3100 and 2120 cal. BP (3.0 fires 1000 yr⁻¹) and 1400–600 cal. BP (4.3 fires 1000 yr⁻¹). Rapid compositional changes in the pollen assemblage were documented during the Late Bronze Age period, suggesting vegetation responded to increased fire disturbances. The human influence on the fire regime is implied by the short-term increase in cereal pollen concurrent with a major fire event, indicating possible use of slash-and-burn cultivation by Late Bronze societies. This type of human subsistence strategy practised in the sandstone landscape further evolved to pastoralism as suggested by continuous presence of coprophilous fungi Sporormiella and Sordaria, which occurred since the Hallstatt/La Tène period (2750–1950 cal. BP). Our study documents, for the first time, the intentional, human-caused biomass burning from densely forested areas of Northern Bohemian sandstone region. Our results imply that increased rate of fire disturbances contributed to the Late Bronze Age transformation of broadleaved forests to oligotrophic forest communities of late-Holocene.

Keywords

Charcoal analysis fire history fungal spores Late Bronze Age sandstone slash-and-burn

Introduction

Holocene vegetation history of sandstone regions from Northern Bohemia, Czech Republic, has been shaped by various factors, including climate change, soil development, species migration legacies and human impact (e.g. Kuneš et al., 2008; Ložek, 1998; Novák et al., 2012, 2015; Pokorný and Kuneš, 2005). However, all these processes are widely modulated by the unique feature of this landscape; extraordinarily variable topography formed as a consequence of sandstone bedrock erosion (Bruthans et al., 2014; Turkington and Paradise, 2005; Young et al., 2009). The resulting landforms are responsible for most of the spatio-temporal vegetation patterns observed across various environmental gradients (Stein et al., 2014), thus making interpretations of Holocene development highly challenging from the region. This also applies for the recent fire dynamics, which is strongly related to relief characteristics (Adámek et al., 2015). As a consequence, multi-trajectory vegetation development has been postulated by previous palaeoecological studies (Bobek, 2013; Novák et al., 2015) which have attempted to bring together multiple lines of evidence, including pollen, macrofossils, soil and archaeological charcoal. Based on these proxies, the reconstructions from the Czech sandstone regions suggest that vegetation could behave in two contrasting modes: progressively changing, or persistently stable, even at the spatial scale of neighbouring forest stands. However, particular ecological mechanisms responsible for such divergent ecosystem response remain poorly understood. So far, the most accepted ecological mechanism suggests that sandy soil degradation leads to the substantial loss of Ca²⁺ and Mg²⁺ cations, resulting in the formation of oligotrophic-adapted species-poor plant communities in the Czech sandstone area (Pokorný and Kuneš, 2005). Such gradual soil impoverishment seems to be an integral part of the Quaternary climatic cycles, as proposed by Andersen (1969; see also Birks and Birks, 2004; Kuneš et al., 2011). Although the process itself is naturally driven by various external factors, such as precipitation changes or disturbance frequency, the rate of soil degradation could also be accelerated by increased anthropogenic impact. Such human-forced changes could lead to widespread landscape-scale vegetation transformations far exceeding the naturally driven processes.

One of the most striking examples of such vegetation change induced by soil depletion occurred during the Late and Final Bronze Age (c. 3000 cal. BP) in the central part of the sandstone region in northern Czech Republic. This event (called ‘Late Bronze Age environmental collapse’; Ložek, 1997, 1998) has originally been recognized as the sudden decline in species-rich snail faunas and subsequent expansion of species-poor oligotrophic snail communities, which are characteristic for present-day Pinus sylvestris–dominated forests. It has been suggested that human-induced woodland clearance and grazing pressure, coupled with late-Holocene climatic instability, were responsible for speeding up the process of soil leaching and irreversible ecosystem changes (Pokorný, 2005; Pokorný and Kuneš, 2005). However, pollen and microcharcoal data used in previous studies do not allow for joint assessment of various driving forces, particularly local fire disturbance history and grazing activity, which are often found to be responsible for pronounced changes in vegetation composition, especially in mountain areas (Feurdean et al., 2013b; Roepke and Krause, 2013). Furthermore, commonly used secondary anthropogenic pollen indicators (Behre, 1981; Brun, 2010), such as Urtica, Cirsium and Rumex acetosa-type, have limited applicability within the densely wooded sandstone landscape, because these plant species are frequently found also within present-day early successional forest stages (Adámek et al., 2016). Therefore, independent proxy, such as coprophilous fungal spores, is urgently needed to quantify the effect of grazing in the area. There is also a growing body of evidence that fire has been involved in landscape-scale vegetation transformations during the Late Bronze Age, as indicated by radiocarbon-dated fire events from charcoal embedded in soil (Novák et al., 2012). Also, numerous findings of charcoal-rich layers in peat sequences (Bobek, 2013) suggest widespread occurrence of forest fires in the Czech sandstone areas in the past. Consequently, a great amount of biomass had been repeatedly transformed into the volatile compounds through burning, mobilizing nutrients and making them vulnerable to leaching out of the ecosystem cycling (Leys et al., 2016). The occurrence of fire events coupled with subsequent nutrient loss and increased erosional processes could have accelerated degradation of sandy soils in the past.

This study aimed to address the effect of fire disturbances and grazing on the Late Bronze Age vegetation transformation within a small portion (c. 79 km²) of the sandstone region in northern Czech Republic. We have collected a comprehensive fire history record based on changes in charcoal accumulation rates (CHARs) and spatially extensive soil charcoal dating and identification. This is the first opportunity that estimates fire frequency (FF) from the sandstone region of Northern Bohemia since the beginning of middle-Holocene. Moreover, spatially precise charcoal data derived from soil enabled us to investigate fire history and forest composition at the stand scale. Simultaneous analysis of pollen and fungal spores from a small forest hollow allowed us to determine local changes in vegetation composition in relation to grazing activity, fires and human impact. By conducting this study, we aim to improve existing understanding of fire history of temperate forest ecosystems during the Holocene, which until recently was a limited scientific interest in Central Europe (Feurdean et al., 2013a; Marcisz et al., 2015; Niklasson et al., 2010; Robin et al., 2013a, 2013b). Specifically, we focus on (1) assessing the role of fire disturbances during the Late Bronze Age environmental decline in the sandstone region and (2) validating presumed grazing activity in the area using independent evidence.

Regional setting

Our study area is located inside the Bohemian Switzerland National Park (hereafter BS area), which is situated at the margin of the Northern Bohemian sandstone region (Figure 1). The landscape is formed by Cretaceous sandstone bedrock, which later eroded into enormously diverse geomorphological features consisting of a network of deep gorges separated by rocky ridges (hereinafter referred to as ‘sandstone landscape’; Härtel et al., 2007). Moreover, the Tertiary volcanic activity induced penetration of basaltic rocks through sedimentary deposits resulting in the formation of isolated hills. The altitudinal gradient ranges from 115 to 556 m a.s.l. Soils that developed on sandstone bedrock are mainly dystric cambisols or acidic sandy podosols. However, remnants of glacial wind-blown deposits are relatively common on the plateaus, creating more favourable luvisols. The study area belongs to the temperate oceanic climate (Cfb according to Köppen) with yearly precipitation of 700 mm yr⁻¹ and a mean annual temperature of 8°C (Tolasz, 2007). The area is situated in the mid-altitudinal zone of beech (Fagus sylvatica) and mixed beech-fir (Fagus sylvatica–Abies alba) forest (Plíva, 1991). Nevertheless, enormous habitat diversity allows for coexistence of open-canopy pine (Pinus sylvestris) forests on rocky plateaus and mountain-like spruce (Picea abies) stands in cold valley bottoms. The recent vegetation composition is heavily affected by management practices, which have transformed large areas into spruce plantations. Nowadays, forest fires occur more frequently within the Northern Bohemian sandstone region in comparison with other parts of Czech Republic (Adámek et al., 2016; Kula and Jankovská, 2013).

Figure 1.

Location of the Bohemian Switzerland National Park (BS area) (a) in relation to the other protected sandstone regions of the Northern Bohemian sandstone area. An example location (b) of soil profile (site PRH) on exposed plateau showing typical position of sampling sites with respect to landscape geomorphology. (c) Shaded relief map indicating spatial distribution of the soil profiles and the Eustach peat core.

Human presence has been recorded since the Mesolithic by numerous archaeological excavations of rock shelters (Šída et al., 2011, 2014; Svoboda, 2003). Repeated findings of fireplaces, hazel (Corylus avellana) nutshells and stone tools provide evidence about hunter and gatherer occupation in the sandstone landscape. The area was most likely not affected by the Neolithic agricultural societies. Traces of Bronze Age human occupation are still missing in archaeological record coming from the studied area; however, several findings have been made in the neighbouring area of Lusatian Mountains (Kozáková et al., 2015) and České Středohoří (Jenč and Peša, 2013). The same situation applies to the early Iron Age, which is represented here by the occasional findings of artefacts. From the beginning of the late Iron Age until the Early Middle Ages, there is a substantial hiatus in archaeological evidence, indicating human abandonment of the area. However, the situation rapidly changed with respect to medieval colonization activities culminating during the High Medieval.

Material and methods

Chronologies

Chronologies were based on accelerator mass spectrometry (AMS) radiocarbon dating (CEDAD, University of Salento) and alpha spectrometry measurements of ²¹⁰Pb activity (Czech Geological Survey) in the topmost peat layers. For ²¹⁰Pb dating, bulk peat samples were extracted in regular 2-cm increments from core depths 0–36 cm. Sample age was estimated using a constant-rate-of-supply model adapted from Binford (1990). Terrestrial plant macroremains were used for ¹⁴C dating whenever possible. However, because of scarcity of suitable plant material within a peat profile, one sample of bulk sediment had to be used. Charred wood particles extracted from soil profiles were selected for ¹⁴C dating according to their weight and taxonomic identification. Sampling strategy aimed to date each soil layer whenever suitable fragment was present. All radiocarbon measurements were calibrated based on the IntCal13 curve (Reimer, 2013) provided by OxCal v4.2.24 (Ramsey, 2009) and reported as the median date before present (cal. BP) within the 2σ interval. Age–depth modelling was performed in Clam 2.2 R package (Blaauw, 2010) using a cubic spline interpolation between all ¹⁴C- and ²¹⁰Pb-dated layers. The confidence envelope was produced for sedimentation model on the basis of Monte Carlo permutation repeated 1000 times.

Pollen, microcharcoal analysis and fungal spores

A small mire (Eustach site, 50°53′26.4″N 14°25′42.33″E) located at the bottom of a sandstone valley was selected for the study site. The peat profile was extracted using a U-shaped corer (5 cm diameter, 1 m length). Samples for pollen analysis were taken in 5-cm intervals; however, high-resolution sampling was applied to the expected fire horizons determined by lithographic description in the field, thus resulting in 53 pollen samples analysed in total. Peat samples of 1 cm³ volume were prepared for pollen analysis according to standard methods (Berglund and Ralska-Jasiewiczowa, 2003). A Lycopodium marker (Stockmarr, 1971) was added to each sample prior to the treatment and soaked in 10% HCl solution to remove carbonates. The material was then carefully passed through a 500-µm sieve with aid of deionized water. Acetolysis was followed in a boiling water bath for 10 min. Final samples for pollen analysis were mounted in glycerine for storing. A minimum number of 200 pollen grains or 50 Lycopodium markers were counted in each sample. Pollen types were determined to the highest taxonomic resolution using the reference collection at the Institute of Botany (CAS, Průhonice), pollen atlases (Reille, 1992, 1995, 1998) and pollen keys (Beug, 2004; Punt 1976, 1980, 1981, 1984, 1988, 1991, 1996, 2003). Algal remains were determined using special keys (Jankovská and Komárek, 1982; Komárek and Jankovská, 2001). Non-pollen palynomorphs (NPPs) were determined according to available publications (Pals et al., 1980; Van Geel, 1978; Van Geel et al., 1983, 1986, 1989) and follow the terminology reviewed by Miola (2012). Fossil stomata were identified according to C. A. Sweeney (2004). Charcoal counting on pollen slides (microcharcoal) was performed in three size classes (largest axis: 10–50, 50–100 and ≥100 µm), focusing exclusively on completely black, opaque fragments with angular shape (Clark, 1988a). Microscopic charcoal particles were counted until the whole area of single pollen slide was inspected. The results of pollen analyses (selected taxa) and NPPs are presented as a percentage diagrams plotted on time. All percentage values were calculated proportionally to the total terrestrial pollen sum of arboreal pollen (AP) and non-arboreal pollen (NAP) types where aquatic plants, ferns and NPPs were excluded. Microcharcoal content is expressed as a number of particles of respective size class per one slide. Stratigraphic diagrams were plotted in Tilia v.1.7.16 (Grimm, 2011). Pollen zones were delimited using optimal splitting by sum of squares technique implemented in Psimpoll v. 4.27 (Bennett, 2009).

Quantitative analyses of pollen data

We used Detrended Canonical Correspondence Analysis (DCCA; Hill and Gauch, 1980) to quantify compositional turnover of chronologically ordered pollen percentage data (Birks, 2007). The DCCA results are scaled in ordination space as standard deviation (SD) units, thus enabling direct assessment of compositional change between samples. The length of the first ordination axis gives a total dissimilarity (gradient) of the pollen sequence. A half-change in species composition is reached at 1 SD; however, a full turnover between neighbouring assemblages appears at 4 SD. Analysis was performed in CANOCO v.5 (Ter Braak and Šmilauer, 2012) with pollen percentages standardized using a square-root transformation and detrended by segments. Rare species down-weighting was not applied. Calculations were constrained by sample age estimate according to the age–depth model to take into account stratigraphical nature of the pollen sequence. An alternative approach to assess the speed of compositional changes is based on quantifying the inter-sample dissimilarity per equal time unit (Jacobson and Grimm, 1986). This method is critically dependent on reliable age estimation, which is used to standardize dissimilarity and may give false results during the ¹⁴C plateau (Lotter et al., 1992). Our age–depth model includes dates laying outside major radiocarbon plateaus (Guilderson, 2005), which we consider to be suitable. An index value was calculated by the squared chord distance (i.e. Euclidean distance of square-rooted pollen percentage data) between two adjacent pollen assemblages divided by its time span and expressed as dissimilarity change per year. Interpolation and resampling to equal time interval prior to a dissimilarity calculation was not applied. The direct effect of fire history and grazing pressure on vegetation composition has been assessed using redundancy analysis (RDA; Legendre and Legendre, 2012). In order to employ an appropriate response model (linear vs unimodal), we examined length of gradient using detrended correspondence analysis (DCA). Since we observed minor changes in species composition along main gradient (less than 3 SD), we preferentially applied linear techniques. We used FF estimates (number of fires per 1000 years based on CHAR analysis, see below) as a proxy for local fire history. Moreover, total influx of coprophilous fungal spores (Sporormiella, Sordaria, Podospora and Delitschia) was used to quantify the presence of browsing animals (Baker et al., 2013). The number of spores counted was converted into concentration values based on the Lycopodium marker added to the sample and counted in the pollen slide. Fungal spore influx was calculated by dividing these values by sediment accumulation rate. An estimated sample age has been used as a co-variable to partial out the variance originating from temporal trend. The significance of the first constrained axis has been tested using unrestricted Monte Carlo permutation test (999 permutations).

Soil charcoal sampling

The macroscopic fragments of charred wood (>0.5 mm) buried in soils are valuable markers of in situ fire occurrence (Carcaillet, 1998; Ohlson and Tryterud, 2000). As these particles are limited in aerial transport by their size, they do disperse only several tens of metres around the burned sites (Clark, 1988b). They are also taxonomically identifiable according to wood anatomy, so reconstruction of past vegetation composition can be achieved. In total, 11 sampling locations within the BS area covering the main types of site conditions (i.e. rocky ridges, elevated basaltic hills and large plateaus) were selected (for site abbreviation, see Figure 1c). Charcoal samples were excavated from soil trenches dug down to the parent material. We did not take samples from slopes steeper than 5° or the valley bottoms in order to minimize input of allochtonous charcoal particles via soil erosion. Precise trench location was conducted on the basis of micro-topographical features reconnaissance in the field, which allowed to avoid pit-and-mound relief caused by tree uprooting. Soil samples of volumes between 8 and 12 L were taken in non-continuous fashion with respect to visible soil layering. Profiles exhibiting texture and colour inhomogeneity within soil horizons were excluded from sampling because of possible mixed stratigraphy. The soil samples were then dried in the laboratory (60°C for 24 h) in order to improve charcoal flotation capability. Charred wood particles were extracted using a water flotation technique (Carcaillet and Thinon, 1996) followed by wet sieving with a 200-µm mesh size. Subsequent hand-sorting of the fraction greater than 2 mm under a stereomicroscope resulted in charcoal assemblages suitable for taxonomical identification. All recovered charcoal particles were analysed; however, several charcoal-rich samples were randomly sub-sampled to reduce the total sum. Taxonomical analysis was performed under a reflected light microscope (Jenatech 50–500×) by observing three anatomical planes (transversal, tangential and radial) created with the aid of razor. The determination was based on the presence of anatomical features, which were compared with wood anatomy atlases (Benkova and Schweingruber, 2004; Richter et al., 2004; Schweingruber, 1990, 2011; Wheeler et al., 1989), and charcoal reference collection material. Identification was generally feasible to the genus level; however, several anatomically well-defined taxa were determined to the species level. Distinguishing between genera Vaccinium and Calluna on small charcoal pieces was not always possible, so we aggregate the anatomical group Ericaceae, which includes Calluna vulgaris, Vaccinium vitis-idaea, Vaccinium myrtillus and Vaccinium uliginosum.

CHARs and fire event detection

Macroscopic charcoal (typical size 125–250 µm) records from lake and peatbog sedimentary sequences can be used to infer local fire history (Whitlock and Larsen, 2001). It has been theoretically demonstrated that this approach is reliable to detect fires on the spatial scale ranging between 0.5 and 1 km (Gavin et al., 2003a; Higuera et al., 2007). The charcoal series from small sedimentary basins (i.e. forest hollows) tends to consistently record high-severity fires, while low-severity fires are less well represented (Higuera et al., 2005). Therefore, the resulting FF reconstruction originating in such depositional context should be regarded as a conservative estimate. We contiguously sampled the peat core into 1-cm resolution, and a sample size of 2 cm³. The aim was to reveal local fire activity, so we extracted charcoal particles greater than 125 µm (Whitlock and Millspaugh, 1996). Peat samples were gently disaggregated using 10% KOH for 12 h and washed through a 125-µm sieve. The procedure resulted in a coarse fraction consisting mainly of plant tissues which was then bleached using 3% hydrogen peroxide to increase contrast between black charcoal and non-charred organic material. However, time-consuming hand cleaning had to be applied in order to remove larger plant debris making charcoal counting easier. We applied a semi-automated charcoal quantification method executed on uniformly dispersed particles in a thin water layer inside Petri-dishes. Sample images were captured using a high-resolution scanning device, and particle counting was processed by ImageJ (Schneider et al., 2012) capabilities. An obtained series of charcoal concentrations (pieces cm⁻³) were converted into CHAR (CHAR_raw, pieces cm⁻² yr⁻¹) by multiplying by sediment accumulation rate (cm yr⁻¹) inferred from the age–depth model. We adopted a widely used method for fire history reconstructions based on CHAR in lake and peat sequences (Clark, 1988b; Higuera et al., 2010). To derive a local fire event, we employed a peak detection procedure implemented in CharAnalysis 0.9 software (Higuera et al., 2009) (available at http://phiguera.github.io/CharAnalysis). The charcoal series was resampled into constant time intervals (CHAR_int) corresponding to the median sample resolution of 32 years. The CHAR time series was decomposed into a peak component (CHAR_peak) and a background component (CHAR_back). The CHAR_peak represents the sudden increase of charcoal input into the sedimentary record, which can be attributed to a local fire event. However, a slowly varying CHAR_back represents changes caused by regional shifts in fire activity or sediment mixing within the basin. The optimal sampling interval to detect a local fire followed a criterion of 0.12 times the known mean fire return interval (mFRI) for the studied ecosystem (Higuera et al., 2007), which equals 60 years per sample when assuming 500 years FRI reported from Central Europe (Robin and Nelle, 2014). We modelled CHAR_back using a robust locally weighted regression (LOWESS) within a 1000-year window. We applied non-transform-residual model (CHAR_peak = CHAR_int − CHAR_back) to remove a background trend from CHAR_int series resulting in high-frequency residual CHAR_peak (Higuera et al., 2010). This procedure did not include any variance-stabilizing methods (i.e. data transformation) and assumes that charcoal peaks are created via additive processes (i.e. charcoal input per fire is constant). The distinction between fire-related charcoal peaks (CHAR_fire) and random noise (CHAR_noise) was based on locally fitted Gaussian mixture model to 1000-year window (Gavin et al., 2006; Higuera et al., 2009). Charcoal peaks exceeding 99th percentile of the modelled noise distribution were considered as a local fire event. We assessed a separation of CHAR_fire and CHAR_noise populations using signal-to-noise index (SNI), which compares variability inherent to both distributions (Kelly et al., 2011). In order to test a reliability of each peak, CHAR_fire series was screened with the minimum-count test to account for statistically insignificant variation in charcoal counts (Higuera et al., 2010). Fire history was expressed as the number of fire events per 1000 years (i.e. FF, fires 1000 yr⁻¹) span which was smoothed afterwards using LOWESS filter.

Results

Peat profile chronology and sample resolution

The Eustach peat profile continuously covers the last 7532 years. A sedimentary age–depth model (Figure 2) was developed using 18 ²¹⁰Pb dates and 4 AMS ¹⁴C dates, respectively (Table 1). However, one radiocarbon date was excluded from the model, as it yielded an unexpectedly old date. This was probably caused by selecting reworked material originated from a Picea abies trunk embedded in the peat sequence. The uppermost core section (0–75 cm) revealed rapid accumulation of unconsolidated Sphagnum peat at the mean rate of 0.196 cm yr⁻¹. The following part (76–154 cm) is characterized by a gradual transition to woody peat and a decrease mean in sedimentation rate to 0.023 cm yr⁻¹, giving the median time resolution of 43 yr cm⁻¹ (Figure 2). The bottom core section (155–200 cm) consisted of well-humified dark peat, which decreased in median resolution to 75 yr cm⁻¹. Because of the absence of any sand or clay layers, we can reasonably assume continuous sedimentation without any major erosional events.

Figure 2.

Age–depth model for the Eustach peatbog based on ²¹⁰Pb and ¹⁴C dating. Calculations were performed in Clam 2.2 (Blaauw, 2010) using a cubic spline interpolation between dated layers. 95% Confidence interval (dashed lines) were constructed based on Monte Carlo permutation test (n = 1000). One outlying date was excluded from the model.

Table 1.

Lab code	Site	Context	Depth (cm)	Material dated	¹⁴C yr BP (±σ)	Calibrated age range (cal. yr BP; 2σ)	Median age (cal. BP)
LTL12358A	MLY	Soil horizon	2–10	Quercus charcoal	1399 ± 40	1379–1270	1313
LTL8202A	MLY	Soil horizon	15–20	Quercus charcoal	1449 ± 45	1476–1285	1344
LTL8203A	EUS	Soil horizon	12–16	Pinus charcoal	3174 ± 50	3555–3249	3399
LTL8204A	EUS	Soil horizon	19–30	Pinus charcoal	6625 ± 45	7575–7437	7513
LTL12357A	PRU	Soil horizon	7–13	Pinus charcoal	938 ± 45	932–746	851
LTL8205A	PRU	Soil horizon	21–30	Picea charcoal	4546 ± 45	5435–5046	5167
LTL8206A	PRH	Soil horizon	3–10	Larix charcoal	236 ± 45	437–0	275
LTL8207A	PRH	Soil horizon	14–26	Pinus charcoal	350 ± 45	496–311	400
LTL8208A	PRY	Soil horizon	4–8	Pinus charcoal	368 ± 30	503–316	433
LTL8209A	PRY	Soil horizon	10–16	Pinus charcoal	2626 ± 40	2845–2716	2755
LTL8210A	PRY	Soil horizon	19–26	Picea charcoal	7132 ± 50	8029–7848	7959
LTL8211A	JED	Soil horizon	9–12	Pinus charcoal	340 ± 45	494–307	395
LTL8212A	JED	Soil horizon	15–20	Pinus charcoal	1074 ± 40	1061–927	983
LTL8213A	JED	Soil horizon	23–30	Quercus charcoal	6356 ± 50	7418–7175	7293
LTL8214A	PON	Soil horizon	0–6	Fagus charcoal	after AD 1950
LTL8215A	PON	Soil horizon	6–10	Fagus charcoal	1650 ± 45	1693–1413	1552
LTL8216A	PON	Soil horizon	37–50	Fagus charcoal	1667 ± 45	17,011,417	1574
LTL12347A	CES	Soil horizon	11–5	Pinus charcoal	1351 ± 45	1342–1182	1280
LTL12348A	CES	Soil horizon	28–34	Quercus charcoal	178 ± 45	302–0	173
LTL12349A	CES	Soil horizon	34–51	Pinus charcoal	9727 ± 65	11,254–10,793	11,151
LTL12350A	SUC	Soil horizon	40–50	Pinus charcoal	961 ± 45	960–782	859
LTL12351A	PIK	Soil horizon	60–75	Pinus charcoal	5844 ± 45	6776–6509	6660
LTL12352A	PIK	Soil horizon	32–49	Pinus charcoal	9528 ± 55	11,099–10,664	10,868
LTL12353A	PIK	Soil horizon	10–15	Pinus charcoal	818 ± 35	789–680	727
LTL12354A	JET	Soil horizon	39–50	Quercus charcoal	617 ± 45	665–541	601
LTL12355A	JET	Soil horizon	25–36	Fagus charcoal	4225 ± 45	4864–4616	4745
LTL12356A	JET	Soil horizon	8–10	Quercus charcoal	370 ± 35	505–315	431
LTL13405A	Eustach	Peat profile	195	Picea needle	6344 ± 45	7170–7415	7279
LTL13406A	Eustach	Peat profile	142	Bulk peat	3010 ± 35	3076–3337	3200
DeA-6116	Eustach	Peat profile	112	Picea wood	3630 ± 26	3865–4071	3940
DeA-6117	Eustach	Peat profile	168	Poaceae tissue	3870 ± 45	4418–4155	4301

Soil charcoal data and age

We have collected 11 soil profiles distributed within 79 km² of the BS area. In total, c. 500 L of soil were sampled and processed by flotation and wet sieving which resulted in 37 charcoal assemblages from different soil horizons. The anatomical identification was successful for 3024 charcoal fragments belonging to 12 taxa at the species or genera level (Figure 3). The overall species composition of the charcoal assemblages were dominated mainly by Pinus spp. (64%; consider that Pinus sylvestris prevails; however, this could potentially include other native species Pinus mugo and Pinus rotundata, which are not present within the area at present) followed by Quercus spp. (9%; includes species Q. robur, Q. petraea and Q. pubescens) and Fagus sylvatica (7%). A minor proportion of the charcoal spectra was formed by other tree or shrub taxa such as Picea abies (5%), Abies alba (4%) and Calluna vulgaris (4%), respectively. The charcoal spectra with Pinus spp. proportion exceeding 50% of total sum were frequently accompanied by acidophilus understorey shrub Calluna vulgaris and Vaccinium spp. (expressed as the summary of the Ericaceae group). Other light-demanding tree taxa characteristic for early successional stages are not frequent (<1%), except for Betula spp. (2%). The main differences in site species composition were driven by higher abundance of deciduous trees Fagus sylvatica and Quercus spp. Such charcoal spectra have occurred more frequently at the sites located on basaltic bedrock or near alluvium of Křinice River (sites MLY, JET, SUC and PON). The quantity of Fagus sylvatica increased up to 10% at such sites, and rare findings of nutrient-demanding taxa such as Fraxinus spp., Acer spp. and Tilia spp. were also recorded. Radiocarbon dating of 27 charcoal particles extracted from soil horizons shows wide range of dates covering the whole Holocene (Figure 3 and Table 1). The oldest Pinus sylvestris charcoal preserved in the soil profile revealed a date of 11,151 cal. BP showing the presence of this tree within the sandstone area at the Pleistocene/Holocene transition. Based on radiocarbon dating, several fire episodes have been detected during the middle-Holocene; however, the majority of ¹⁴C dates belongs to the Early/High Middle Ages.

Figure 3.

The taxonomic composition of charcoal assemblages acquired from 11 soil profiles which were placed within the BS area.

Vegetation development inferred from the Eustach profile

The Eustach peat profile records both the vegetation and fungal spore changes from the BS sandstone area since the onset of the middle-Holocene (Figures 4 and 5). Considering the small size of the mire and the valley bottom location within the sandstone landscape, observed vegetation changes mirror local dynamics in plant communities. The pollen diagram is divided into the six local pollen assemblage zones (LPAZs).

Figure 4.

Simplified pollen percentage diagram of the Eustach peatbog based on the total pollen sum of all terrestrial taxa excluding wetland pollen taxa. Charcoal values are given as a proportion to total pollen sum. Local pollen assemblage zones (LPAZs) are delimited according to optimal splitting by sum of squares.

Figure 5.

Simplified non-pollen palynomorph (NPP) percentage diagram of the Eustach peatbog. Percentage values are proportional to total pollen sum.

In LPAZ EU 1 (7532–6207 cal. BP), circumjacent valley floor habitats were overgrown by mixed oak forests, as suggested by the high pollen percentages of Quercus, Ulmus, Fraxinus and Tilia. Basal sections show maximum values of Corylus (40%), which demonstrate its important admixture in forest vegetation. The limited spatial extent of coniferous forests is indicated by low Pinus sylvestris values (10%) and the occurrence of Vaccinium-type, Calluna vulgaris and Vaccinium vitis-idaea pollen. Pinus sylvestris stands were located on dry sandstone plateaus and ridges probably since the early Holocene. Picea stomata indicate its local occurrence on waterlogged sites during the initial stage of the peat layer formation. Sporadic finds of Rumex acetosa-type (including species Rumex acetosella) pollen suggest the occurrence of early successional stages (Figure 4). Fire events are indicated by microcharcoal and spores of Gelasinospora and Neurospora (Figure 5).

During LPAZ EU 2 (6207–3313 cal. BP), a gradual change of forest vegetation towards coniferous types was observed. Picea expanded outside of the waterlogged sites, as indicated by pieces of Picea charcoal from the soil layer on the plateau (site JET). This occurrence could be attributed to spruce’s ability to colonize disturbed sites (Engelmark, 1993). Also, Pinus sylvestris pollen increased suggesting the spread of oligotrophic communities on poor sandy soils. Quercus pollen suggests the population sustained ongoing changes in a stable state; however, Corylus pollen decreased during this zone. This process of hazel retreat was accelerated during the rapid expansion of Fagus sylvatica and Abies alba, as the light-demanding hazel could not sustain the competitive pressure of beech. Low pollen percentages of both the secondary anthropogenic indicator Plantago lanceolata and the single find of Triticum-type probably reflect human occupation located outside BS area. Short-term increases of coprophilous fungal spores (Podospora sp., Sporormiella-type and Sordaria-type) represent grazing by animals.

LPAZ EU 3 (3313–2252 cal. BP) is characterized by an abrupt increase of microcharcoal and the retreat of broadleaved trees such as Tilia, Ulmus and Fraxinus. A pronounced increase in fire activity is also documented by the abundant findings of fire-demanding fungus Gelasinospora sp. The delayed expansion of the pioneering tree, Betula, suggests the presence of early successional forest stages. The same applies for the peaks in Abies alba and Picea abies, which are able to easily colonize disturbed soils affected by fire or abandoned agricultural land (Engelmark, 1993; Volařík and Hédl, 2013). Also, the presence of herb species Senecio-type, Asteraceae subfam. Cichorioideae and fern Pteridium aquilinum supports the gap occurrence within the tree canopy. The marked increase in Secale cereale and Rumex acetosa-type was related to samples with elevated microcharcoal content. A continuous presence of dung fungus Sordaria-type, Sporormiella-type as well as the presence of Juniperus communis pollen confirms grazing by animals in the surrounding area.

In LPAZ EU 4 (2252–945 cal. BP), forest recovery following the previous disturbance phase has occurred. Fagus sylvatica further expanded reaching maximum pollen percentage values. The formation of extensive beech-fir forest is documented by the increase in Abies alba pollen. The human impact substantially decreased, as reported by rare findings of cereal pollen.

LPAZ EU 5 (945–423 cal. BP) shows an expansion of poor pine-dominated forests with Calluna vulgaris, Vaccinium-type and Vaccinium vitis-idaea undergrowth. Microcharcoal size classes 51–100 and >100 µm are moderately abundant indicating biomass burning in the wider area.

LPAZ EU 6 (423–(-62) cal. BP) denotes High Medieval colonization activity in the surroundings of the BS sandstone area. Medieval farming is represented by Fagopyrum together with abundant Secale cereale, Triticum-type and Hordeum-type pollen. Microcharcoal input could be partly attributed to the presence of kiln sites in the vicinity.

Quantitative assessment of vegetation changes

In order to quantify compositional turnover throughout the whole pollen record, we used DCCA ordination constrained by sample age (included in Figure 7). The first canonical axis accounts for 13.1% of the total variability. The differences in sample scores on the first axis show a period of great compositional change during the Middle and Late Bronze Age, which coincided with an increase in FF. The total change over this period accounts for 0.7 SD, indicating exchange of nearly half of the available species pool. When compositional change induced by modern forestry practices is neglected (~last 250 years), Middle to Late Bronze Age species turnover represents the most significant plant community transformation during the last seven millennia.

When including both FF and influx of selected coprophilous fungi in RDA as explanatory variables, first two ordination axes account for 8.9% of the total variability. The analysis is statistically significant as assessed by Monte Carlo permutation test (pseudo-F = 2.4, p = 0.002). RDA revealed a close association of pollen taxa Calluna vulgaris, Vaccinium vitis-idaea, Plantago lanceolata-type and Pteridium aquilinum spores with increasing influx of coprophilous fungi (Figure 6). Furthermore, several tree taxa (Pinus, Fagus, Abies and Carpinus) were assigned to the periods of higher FF. A minor correlation to fire has been observed for Secale cereale, Betula, Asteraceae subfam. Cichorioideae and Rumex acetosa-type.

Figure 6.

Redundancy analysis (RDA) ordination triplot of pollen record from Eustach peatbog. Explanatory variables (coprophilous fungal spores, FF) and co-variable (age) expressed as arrows. Only selected taxa are shown. The total variance in pollen composition explained by ordination axes equals 8.9%.

Figure 7.

Summary diagram linking diverse fossil evidence from the Eustach peat profile, and soil charcoal recovered within the sandstone area. (1) Charcoal accumulation rates (CHARraw) with estimated background charcoal deposition (CHARback), crosses denote a timing of a fire event; (2) dating and taxonomical identification of selected soil charcoal (early-Holocene dates are not shown), calibration 2σ range given by IntCal13, median calendar age is denoted by dots; (3) presence of fungal spores (Neurospora and Gelasinospora) indicating fire occurrence; (4) FF (fires 1000 yr⁻¹, solid line); (5) presence of dung fungal spores (Sporormiella, Sordaria and Podospora) linked to large herbivore activity (Baker et al., 2013); (6) percentage of Betula pollen, vegetation openness expressed as AP/NAP ratio; (7) compositional change revealed by DCCA ordination (units in SD) and rate-of-change analysis (units in chord distance dissimilarity per 1 year); (8) Mid-Europe lake-level fluctuation (Magny, 2004); (9) archaeological periods for Bohemia (Kuna and Danielisová, 2014).

CHARs and fire history reconstruction

Charcoal concentration in peat samples range from 3 up to 919 pieces cm⁻³. When considering changes in sedimentation rate, CHARs (CHAR_raw) vary between 0.233 and 24.06 pieces cm⁻² yr⁻¹ (Figure 7). An observed median time between continuous charcoal samples was 32 years (mean = 39 years, SD = 29 years, range = 1–87 years). This value was used for the interpolation of the CHAR_raw series. Low CHAR_raw occurred from the basal part of the record to 6868 cal. BP. Thereafter, a pronounced increase in CHAR_raw values resulted in two charcoal peaks between 6838 and 6238 cal. BP, afterwards it decreased to the mean value of 1.571 pieces cm⁻² yr⁻¹ lasting for three millennia. A calculated FF peaked at 2.2 fires 1000 yr⁻¹ during the period 7528–6148 cal. BP and then dropped to the lowest recorded value of 0.6 fires 1000 yr⁻¹ between 6148 and 3118 cal. BP. Significant changes occurred during the Bronze/Iron Age period between 3118 and 2098 cal. BP when several distinctive peaks indicate instantaneous charcoal input into the sedimentary basin. This period is characterized by rapid increase in FF up to 3.0 fires 1000 yr⁻¹. CHAR_raw decreased again to low values during the period between 2098 and 1468 cal. BP, which corresponds to minimum of 1.9 fires 1000 yr⁻¹. Since the Early/High Medieval Times (1408–628 cal. BP), CHAR_raw suddenly increased and peaked at 24.06 pieces cm⁻³. The FF reconstruction showed the maximum value of 4.3 fires 1000 yr⁻¹. A gradual decrease in CHAR_raw and FF denotes the youngest part of the record (628–0 cal. BP). Altogether, the decomposition of charcoal series revealed 15 fire events (Figure 7). Mean SNI (SNI = 4.9, not shown) was constantly above the threshold (SNI > 3) recommended by empirical studies (Kelly et al., 2011) which indicated sufficient separation of charcoal peak component from inherent noise of the series.

Discussion

Early-Holocene fire disturbance regime

Soil charcoal data revealed at least two fire disturbance episodes at the beginning of the Holocene. These fires primarily affected Pinus sylvestris stands, because charcoal of this species prevailed within soil assemblages. An understorey dwarf shrub belonging to the Ericaceae family has also been found, indicating poor acidic soils, while Betula sp. further suggest post-disturbance successional phases or presence of mixed pine–birch woodlands. However, pedoanthracological results should be interpreted with caution because of biases introduced by soil mixing processes, which could result in an unclear age chronology (Carcaillet, 2001; Šamonil et al., 2013). A combination of multiple-site sampling approach and extensive radiocarbon dating could partly overcome such limitations. Thus, negative evidence of other tree taxa within the soil layers containing the early-Holocene charcoal lead to the conclusion that Pinus sylvestris–dominated forests grew on the sandstone plateaus during the early-Holocene. This is consistent with anthracological analysis carried out on charcoal from archaeological layers buried under rock shelters, which also show the prevalence of Scots pine (Novák et al., 2015). Since there is a lack of sedimentary charcoal within the BS area dating back to the early-Holocene period, FF could not be reconstructed. However, a general increase in biomass burning has been observed across Central Europe, suggesting climate change as the main driving force (Marlon et al., 2013). Regional lake records (Hošek et al., 2014) document climatic warming at the transition from the Younger Dryas to the early Holocene, which was followed by rapid afforestation. Such a re-colonization process may have led an increase in biomass accumulation, which would have created an essential prerequisite for fire activity. Therefore, fire disturbances may have influenced the postglacial forest re-establishment in Central Europe and should be integrated in forest dynamics.

Since the Northern Bohemian sandstone landscapes have been widely occupied by Mesolithic hunter–gatherer communities (Svoboda, 2003), there is a possibility that fire has been used by humans (Innes et al., 2013). Intentional burning aimed to stimulate early successional phases within closed-canopy woodlands was part of the subsistence strategy of the Mesolithic people (Zvelebil, 1994), although its long-term effect on forest ecosystems seems to be weak and hardly detectable in pollen records (Kuneš et al., 2008). It has also been hypothesized that Mesolithic communities may have promoted Corylus avellana for dietary reasons (Kuneš et al., 2008; Regnell, 2012). Frequent findings of carbonized nutshells and hazel wood within Mesolithic archaeological sites around the entire area of the Northern Bohemian sandstone region (Novák et al., 2015; Opravil, 2003; Šída et al., 2011) support this hypothesis. While its intentional spreading by humans is questionable (Huntley, 1993), hazel benefits from increased FF (Clark et al., 1989), as it has an ability to re-sprout easily from the stem basis, which facilitates the rapid regeneration after a fire disturbance (Delarze et al., 1992). A common coincidence of high proportions of hazel pollen and elevated charcoal concentrations evokes a positive mutual relationship (Finsinger et al., 2006). Higher charcoal values found in the basal layers of the Eustach peatbog resemble such a pattern of high hazel pollen and charcoal concentrations. However, there are two problematic points within our record. First, the CHARs revealed FF of two fires per millennium which is insufficiently low to maintain early successional forest stages at the broader spatial scale. In the case of deliberate human biomass burning, we have to assume low population density or extensive migration between camp sites in order to explain infrequent fire occurrence. Low FF at the Upper Mesolithic/Neolithic transition could also be attributed to the gradual retreat of hunter–gatherer societies, which is consistent with diminishing traces of Mesolithic inhabitation around 7500 cal. BP (Šída et al., 2011). Second, we did not find any trace of Corylus avellana from the soil charcoal assemblages investigated throughout the region (sites CES, PRH, EUS, PIK, PRY and JED). This contradicts its ubiquitous presence in our pollen record (up to 40%). Fires that burned in Corylus avellana–dominated bushes most likely would have burned the entire stand due to its dense stem structure, thus leaving vast quantities of charcoal on the soil surface. The overall absence of Corylus avellana charcoal during the early and mid Holocene on plateaus leads to the conclusion that vegetation must have had distinct spatial structure which could not be captured by our soil sampling design. Thus, forest stands with strong admixture of Corylus avellana were probably limited to the steep slopes along the ravine hillside (i.e. sites not sampled here due to input of allochtonous soil material).

Diverse vegetation structure of the mid-Holocene sandstone landscape

During the transition from the Upper Mesolithic to the Early Neolithic (LPAZ EU1, 7532–6207 cal. BP), charcoal influx was low, indicating little local fire activity. When comparing with sedimentary charcoal records from temperate Europe, the inferred FF of 2.2 fires 1000 yr⁻¹ is two times lower than what others have reported from the north Alpine foreland (Clark et al., 1989). Nevertheless, a corresponding FF (~2 fires 1000 yr⁻¹) has been detected within mid-Holocene broadleaved forests in the foothills of the Pyrenees (Rius et al., 2009). The overall temperature during the mid-Holocene warmed by ~1–2.5°C from the mean of the past 200 years (Renssen et al., 2009), which should stimulate vegetation burning (Daniau et al., 2012). Moreover, considering current microclimate variability which is highly modified by the landform characteristics (Wild et al., 2013), an increase in temperatures would lead to changes in the distribution of soil moisture, resulting in pronounced periods of drought on sun-exposed sites. This topography-driven mechanism could amplify the precipitation changes during the mid Holocene, when several low lake-level phases indicate the occurrence of dryer climatic conditions (Magny, 2004). As a result, higher temperatures, along with more pronounced droughts would have created favourable conditions for fire activity (Gavin et al., 2003b; Kane et al., 2015). However, we have observed low level of FF during the mid Holocene implying other driving forces than climate. Changes in vegetation composition could possibly explain the decline in FF, as the biomass changed to a temperate deciduous forest (i.e. ‘Quercetum mixtum’) during the Holocene thermal maximum (c. 8000–5000 cal. BP), which may have reduced the susceptibility to fire activity. While there is a general consensus about the widespread occurrence of temperate deciduous forests in the Central European lowlands (Firbas, 1949; Jamrichová et al., 2014; Kalis et al., 2003; Novák et al., 2017), prevalence on poor sandy soils within mid-altitudes regions (200–500 m a.s.l.) is questionable (Szabó et al., 2016). We detected rather low pollen percentages of temperate deciduous taxa, such as Quercus, Tilia, Ulmus and Fraxinus, in the Eustach record. This can be explained by the reduced spatial extent of such communities as a result of poor soil conditions on slopes. Nevertheless, there is also indirect evidence for the occurrence of deciduous oak forests, as demonstrated by the high abundance of Ustulina deusta (HdV-44 and HdV-117), a parasitic fungi on various broadleaved trees excluding Corylus avellana (van Geel and Andersen, 1988). Since this fungus is especially common on Fagus sylvatica, it may also indicate beech forest in the vicinity (Van Geel et al., 2013). This is consistent with finding of beech pollen and charcoal dated to this period. Moreover, our soil charcoal record revealed only minor occurrence of broadleaved taxa, thus implying a rather limited spatial extent of such vegetation type. Additionally, soil charcoal data contradict to extensive coverage of closed-canopy deciduous forests, because of the high proportion of light-demanding Pinus sylvestris. This suggests that seedling recruitment of Pinus sylvestris preferably takes place under high light conditions (Adámek et al., 2016). Moreover, a continuous pollen curve of the heliophilous dwarf shrub Empetrum confirms the persistence of open heath-like habitats, as this species hardly survives light depletion under the canopy of broadleaved trees (Svenning, 2002). Also, a pollen threshold value of local presence for Picea (1%) and Pinus (10%; Lisitsyna et al., 2011) has been far more exceeded which implies a considerable admixture of conifers in forest community or distinct vegetation pattern. However, pollen composition resembling oak-dominated deciduous forests has been discovered in other parts of the BS sandstone area (Pokorný and Kuneš, 2005), thus suggesting possible local occurrence of this vegetation type. All the above-mentioned evidence points to the mid-Holocene coexistence of light-demanding vegetation types (e.g. Pinus sylvestris) and broadleaved forests formed by Quercus and later on during the Sub-Boreal by Fagus. Since the fire occurrence was low during this period, vegetation structure was controlled by the heterogeneity in site conditions, rather than FF.

Hidden Late Bronze Age land use in sandstone areas

An important environmental change has occurred during the Late Bronze Age, which is indicated by increased compositional changes in plant communities. Species turnover and rate-of-change analysis of pollen data exhibit substantial vegetation transformation exceeding the magnitude mid Holocene values (Figure 7). Because rate-of-change calculations depend on a precise chronology (Lotter et al., 1992), a simultaneous response of multivariate DCCA method can provide a more robust estimate of compositional shifts in community assembly. The overall trend shows a rapid retreat of broadleaved taxa such as Tilia, Ulmus, Fraxinus and Corylus and lagged expansion of Picea, Abies and Betula. Concurrent with this change are numerous independent evidences that document an increase in biomass burning. First, the CHARs showed large fluctuations which indicate a series of local fire episodes. The estimated FF increased to 3 fires 1000 yr⁻¹. Furthermore, fire-related fungal spores emerged, such as Gelasinospora, an ascospore fungus that is often found within charcoal-rich layers (Innes et al., 2013; Van Geel, 1978; Van Geel and Aptroot, 2006). Moreover, we found Pinus sylvestris charcoal particles in the soil profile EUS, located on a rock plateau <100 m away from the Eustach peat core, with an age that corresponds to the beginning of the Late Bronze Age (Figure 3). Woodland communities were subjected to moderate canopy opening because of fire disturbance as AP values decreased to 85%. The abrupt increase in charcoal influx was followed by a short-term occurrence of Secale cereale and Rumex acetosa-type pollen suggesting cereal cultivation. All above-mentioned indices suggest a linkage between fire occurrence and human activity in the sandstone area during the Late Bronze Age. A variety of agro-pastoral practices involved fire for human subsistence, as demonstrated by the simultaneity of clearance phase and cereal pollen occurrence in relation to slash-and-burn cultivation (Pitkänen and Huttunen, 1999). The spread of human settlements into less favourable regions outside of the lowlands is known for the Bronze Age period (Dreslerová et al., 2013), which makes such agricultural practices even more probable within the BS area. However, an overall absence of permanent settlements in a 16 km radius of the study site dating back to the Late Bronze Age and Hallstatt/La Tène period (Archaeological Database of Bohemia maintained by Institute of Archaeology, Prague) does not support such agricultural activity. Thus, the character of human landscape utilization must have been based on short-term exploitation events, without establishing a permanent settlement. This agrees with the requirements of slash-and-burn cultivation, which requires extensive forested areas in order to permit frequent shifts of burned/sown sites (Rösch, 2013). Long-term cultivation experiments from mixed-deciduous forests in Germany (Ehrmann et al., 2014; Rösch et al., 2002) proved this technique as a reliable tool for food production in a densely forested landscape, even on poor soils. The short-term effectiveness of this agricultural practice is illustrated by the higher crop yields during the first year after burning in comparison with the medieval three-field crop rotation system (Ehrmann et al., 2014), even without need of additional manure. Nutrients are primarily released from burned topsoil organic matter, which temporarily increases soil fertility. However, the speed of subsequent soil organic matter build-up is a factor limiting the frequency of slash-and-burn cycles. This is consistent with short-term occurrence of Secale cereale in our pollen record. In this regard, there was no need for weed suppression on burned sites because of their absence in the soil seed bank. Thus, common field weeds were substituted by native forest species making the detection of cultivated plots using pollen indicators difficult (Behre, 1981). This is in accordance with the Eustach pollen record which does not show any distinct increase in secondary anthropogenic indicators, except for Rumex acetosa-type. Even though this pollen type is routinely used for indication of man-made habitats, recent observation of secondary forest succession on burned sites suggests that Rumex species are also frequent within pioneering plant communities following fire disturbance (Adámek et al., 2016). Furthermore, it must be mentioned that intentional use of Secale cereale as a crop during the Bronze Age is rather questionable (Behre, 1992). Individual archaeobotanical finds of Secale cereale grains dated back to the Early Bronze Age are not considered as a reliable evidence (Hajnalová, 1990), and the beginning of intentional cultivation started during the La Tène period in the Czech Republic (Dreslerová and Kočár, 2013). However, also other pollen records originated from uninhabited forested regions show sporadic occurrence of Secale cereale throughout the Bronze Age (Kozáková et al., 2015), indicating that human agro-pastoral activity, likely associated with slash-and-burn practices, was probably more common within these areas. This could partly be explained by the previous status of Secale cereale which grew among other crop as a weed and has been recorded due to high pollen productivity. Nevertheless, the synchronous increase in Secale cereale and FF indirectly supports the use of slash-and-burn for cereal cultivation within a BS area. This fire-based subsistence strategy further developed since the beginning of La Tène period when hay making was introduced to provide winter feed (Hejcman et al., 2013). Since meadows were probably incorporated into non-forested areas surrounding villages, distant parts of the landscape become more attractive for pastoralism. Such change is visible in our fungal spores record as the frequent occurrence of both Sporormiella-type and Sordaria-type, a coprophilous taxa, indicate local presence of herbivores (Baker et al., 2013; Davis and Shafer, 2006). The BS sandstone landscape has probably been favourable for grazing because of ongoing fire activity. The presence of early successional forest stages since the Hallstatt period is in agreement with the recorded increase in Betula pollen which is able to readily colonize burned sites. During the Roman Times and Migration Period, such pastoral utilization of the landscape was interrupted as the evidence for grazing and biomass burning diminished.

Disentangling fire influence on the vegetation change

The originally proposed ‘Late Bronze Age environmental collapse’ (Ložek, 1998) revealed unprecedented landscape-scale vegetation transformation within the Czech lowland sandstone regions and outlined possible linkages to human influence on this change. Subsequent research, however, demonstrated that the environmental collapse event was a part of a climatically driven soil acidification process during the interglacial cycle, which were locally accelerated by human impact (Pokorný and Kuneš, 2005). Our data provide clear evidence that fire was involved in this vegetation transformation but had also been influencing vegetation dynamics in the BS area long before this event occurred. Soil charcoal indicates that fires continuously maintained Pinus sylvestris–dominated communities during the maximal expansion of mid-Holocene broadleaved forests. Because of decreased FF during that time, however, it must have acted synergically with poor soil conditions in order to sustain such forest type. Therefore, the climatic control over the fire regime seems to be the major factor operating during the early- to mid-Holocene period which is consistent with observations from other part of Central Europe (Robin et al., 2013a). Since the Late Bronze age, fire regime has undergone a change to a predominantly human-driven process, as indicated by the simultaneous occurrence of cereal pollen and various fire proxies, such as CHARs and Gelasinospora. We attributed this change to the expansion of Late Bronze Age human societies (i.e. Lusatian culture) which practised slash-and-burn agriculture in the forested landscape. The increased FF probably disrupted effective nutrient cycling within ecosystem pathways and may have triggered the soil leaching process. While we did not observe an abrupt change in the dominant tree species in the pollen data, distinct compositional changes in plant communities were identified using a DCCA and rate-of-change analysis, which illustrated immediate effect of fire disturbances on species composition. This differs from findings reported from densely settled lowland sandstone areas of Central Czech Republic where human intervention using fire and grazing induced rapid expansion of Pinus sylvestris stands (Pokorný, 2005). Therefore, the magnitude of Late Bronze Age environmental change differed between particular sandstone regions and seemed to be less pronounced in mid-altitude areas where oligotrophic forest types were already established since the early Holocene.

Conclusion

Our reconstruction of fire regime provided new insight into vegetation dynamics within the sandstone landscape of the Northern Czech Republic. The fire proxies revealed a continuous record of fire disturbances spanning the entire Holocene period. The general pattern of FF not only corresponds to climate trends but also reflects changes in human land-use practices during the late-Holocene. Radiocarbon dating of soil charcoal demonstrated that fire occurred early during postglacial forest expansion as a function of biomass accumulation. Possible influence of Mesolithic societies on the fire regime has been discussed, with special regard to intentional spreading of Corylus avellana for dietary purposes. We did not find any soil charcoal evidence for using the fire as a tool within human subsistence strategy; however, high pollen proportion of Corylus avellana in Upper Mesolithic section of Eustach peatbog confirmed its important role in the vegetation. FF decreased during the Holocene climatic optimum as a consequence of spreading of less flammable broadleaved forests. However, patches of Pinus sylvestris–dominated forests maintained by both recurrent fire disturbances and nutrient-poor soils continuously persisted throughout bottleneck of shade-giving mid-Holocene vegetation types. The major shift in fire regime occurred during the Late Bronze Age and Iron Age, when human activities increased FF as a consequence of landscape exploitation. Our pollen data suggest that the possible use of slash-and-burn practices may have been linked to short-term cereal cultivation by people of the Lusatian culture. The subsequent increase in coprophilous fungal spores since the Hallstatt/La Tène period further indicated the development of human land-use strategy to the pastoralism. This late-Holocene transition to the human-driven fire regime was followed by fluctuation in species composition which had a rather gradual character. Accordingly, we confirmed the anthropogenic origin of ‘Late Bronze Age environmental collapse’ (Ložek, 1998) within a Bohemian Switzerland sandstone area. Furthermore, we conclude that observed vegetation transformation was triggered by fire disturbances related to slash-and-burn cultivation.

Footnotes

Acknowledgements

We thank Tereza Klinerová, Zdeňka Konopová and Irena Krausová for the support in the laboratory and Martin Kačmar for providing ²¹⁰Pb measurement. Improvements to the manuscript by Dagmar Dreslerová and Vachel A. Carter are gratefully acknowledged. We also thank the Authority of the Bohemian Switzerland National Park for its general support.

Funding

This research was supported by the Czech Science Foundation (grant GA14-22658S) and The Czech Academy of Sciences (long-term research development project RVO67985939).

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