Mollusc succession of a prehistoric settlement area during the Holocene: A case study of the České středohoří Mountains (Czech Republic)

Abstract

Some Central European areas were attractive for the first agricultural settlements due to their suitable natural conditions. The Holocene development of such areas was thus under long-term human pressure, whose impact on the whole landscape is still poorly understood. One of such areas is the České středohoří Mountains. While pollen analyses can provide the general pattern of the landscape development, the analyses of mollusc succession provide landscape details, which are important primarily in landscapes with high habitat diversity. Based on the study of 11 mollusc successions situated at the České středohoří Mountains, we describe the postglacial development of the area and show the moderate fluctuation of woodland, wetland and open country habitats without any distinct succession peaks of particular habitat types during the whole Holocene. However, a detailed look of species exchange has provided additional information of succession pattern. The impoverishment of woodland communities is probably caused by human pressure, not natural processes, because fully developed woodland assemblages had occurred there during the past interglacials. It seems for now that humans had affected the whole landscape of the prehistoric settlement area, including hard-to-access sites, not only the nearby surroundings of their settlement.

Keywords

České středohoří Mountains Czech Republic Holocene mollusc succession prehistoric settlement radiocarbon AMS dating

Introduction

Postglacial mollusc successions in the zone of mid-European uplands situated north of the Alps show a general developmental pattern characterized by the accumulation of forest malacocoenoses in the middle and late climatic optimum (Late Atlantic and Epiatlantic sensu Jäger (1969)) (Alexandrowicz, 1987; Frank, 2006; Fuhrmann, 1973; Füköh, 1993; Füköh et al., 1995; Ložek, 1964, 1982a; Mania, 1972, 1973; Meyrick, 2001 etc.). However, in a number of sites situated in warm-dry lowlands and hill countries of middle and north-western Bohemia as well as Moravia, the above mentioned culmination is absent or at best indicated by temporal appearance of sparse euryoecious woodland taxa (Ložek, 1964; unpublished data). It may be stressed that these areas are characterized by chernozem soils, developed mostly on loess and concentrated prehistoric agricultural colonization (early Neolithic) during 6th millennium bc (Jiráň and Venclová, 2007–2008) that hindered the expansion of woodland, botanically, so-called thermophyticum (Skalický, 1988; Slavík, 1984). In German literature, they are called Altsiedlungslandschaft (e.g. Firbas, 1949, 1952) – primordial settlement landscape. One of such areas is the lower Ohře lowland, where the strong human impact to the recent alluvial and fossilized malacofauna was still shown (Juřičková et al., 2013). Of particular interest is the fact that these early cultivated areas are surrounded by a belt including more or less forested hill countries or lower mountain chains with a number of habitats that seem to be favourable for fully developed closed-forest malacocoenoses, but recently occupied only by snail communities with reduced number of species (e.g. Horáčková et al., in preparation). Therefore, the question arises whether this species paucity is due to natural processes or human impact from the adjacent cultural landscape. Ideal conditions to study this problem occur in the western section of the České středohoří Mountains (Bohemian Middle Mountains) in northwest Bohemia, which has also been continuously influenced by human activities up to the present (Jiráň and Venclová, 2007–2008).

Pollen data provide only scattered information from this area, which is too dry for peat formation and therefore has poor fossilization conditions. However, the vegetation development from the surrounding areas (Komořanské jezero – e.g. Jankovská (1988); the Elbe River valley – e.g. Břízová (1995, 1999) and Kneblová (1956) and other Central Bohemian sites – Albert and Pokorný (2012) and Pokorný (2005)) can provide the framework of the Central Bohemian Holocene vegetation development. Pokorný (2005) underlines the diachronous development of vegetation succession of the three sites at a wider area of Central Bohemia. While some parts were heavily impacted by humans, which made the spreading of open country vegetation and/or some trees (hornbeam – Carpinus betulus) easier, the development of other parts has remained for a long time undisturbed.

Unlike pollen, mollusc fossilization depends on the CaCO₃ content, and not humidity, so this group can be used as a suitable proxy capable of reflecting changes in this dry area and on finer spatial scales than pollen (Davies, 2008; Firbas, 1949, 1952; Ložek, 1964). Although particular sites with nine mollusc successions have been studied in this area for 50 years of the last century and published in regional literature (Horáčková et al., 2013; Ložek, 1962a, 1963a, 1967, 1976, 2005), the generalization of the Holocene development and absolute chronology are still lacking.

Therefore, we summarize the data from the whole area of the České středohoří Mountains where mollusc succession from 11 profiles were analysed, out of which 3 are from this study. We provide a new radiocarbon chronology for some key sites. Our aim is to describe the general course of mollusc succession in the classical prehistoric settlement area and show the landscape development under the continual human pressure.

Material and methods

České středohoří Mountains – characterization of the landscape

This mountain range extends from the Ohře River valley in the southwest towards the northeast, where it passes into the Lužické hory Mountains at the northern boundary of Bohemia. The 300- to 500-m-deep valley of the Elbe River divides the České středohoří Mountains into the western and eastern range. The western section towers above the adjacent lowlands forming an impressive landscape scenery, dominated by numerous steepened volcanic cones and dome-like features that consist of basaltic and trachytic rocks and that pierce or surmount the basement whose main body is built of Cretaceous marlstones and sandstones or volcanic–sedimentary complex. Their slopes are rich in rocky outcrops and huge open accumulation of screes without fine-grained matrix. Quaternary sediments are mostly represented by colluvial loams rich in stone fragments; loesses occur at lower elevations of the valley’s foothill zone and along the Elbe River where gravel terraces have also developed. Small tufa deposits are scattered in the western part.

Altitudes vary between 130 m at the Elbe bank and 836 m at the Mt Milešovka summit. The southwestern marginal hill country is extremely dry with 450 mm annual rainfall that increases to 500–550 mm in the central region and culminates at about 600 mm and more in the northeast. Mean temperature attained at the foot is 9°C and in the Elbe River valley and decreases to 7–6°C in the highest zone. However, both the temperatures and rainfall are considerably modified by local anemo-orographic factors. The whole western range lies in the rain shadow of the Krušné hory Mountains (Ore Mountains).

Steep-sided volcanic bodies and valley sides are covered by predominantly deciduous woodland with dominant oak (Quercus) and varying admixture of maples (Acer) and ash (Fraxinus). Lime tree–dominated (Tilia) scree forests are confined to abrupt slopes covered by block fields. At higher elevations in the central part, the beech (Fagus) covers north- and northeast-facing slopes with deeper soils, whereas the hairy oak (Quercus pubescens) occupies warm patches on south-facing slopes at lower elevations. The westernmost part is characterized by numerous isolated volcanic cones with extensive steppe grasslands that also form isolated rocky steppe patches within the woods of the central part. In general, the western range is characterized by a varied vegetation patchwork, since it is typical of the flora on these volcanic peaks that although each has rich vegetation, each equally has a distinct flora with a few unique species. Gentle slopes and flat areas in the foothill zone and broad valleys among the volcanic bodies include fields, hay meadows and, in particular, extensive orchards that create exceptional landscape sceneries.

Whereas the adjacent lowlands have been densely colonized since the Neolithic, the colonization of the mountain range itself has been rather sparse. However, scattered traces of prehistoric humans occur throughout the whole range. Two important hill forts are situated in the central part (Hradišťany and Štěpánovská hora). Scattered records of pottery fragments or other artefacts on a number of hard-to-access peaks, probably representing observation or ritual pasts, document prehistoric human activities even in localities that seemingly conserved their natural character up to the present (Jiráň and Venclová, 2007–2008).

Site characteristics and sampling

Three mollusc successions were newly analysed (Figure 1) in this study. Ostrý (N 50°33′14″, E 13°57′58″; 340 m a.s.l.) is the profile through the landslide on the northeast slope of the Ostrý Hill near Velemín, 60 m above the floodplain of the Milešovský Brook. A pastureland is situated at the site. The deposit of nodular calcareous tufa is situated at the southeast slope of the Paškapole Hill near Velemín (N 50°33′58″, E 13°56′59″; 450 m a.s.l.). The tufa with potato-like nodules is covered by black humic soil with coarse basalt scree and incrustations. The tufa deposit at Lhota (N 50°31′54″; E 13°55′40″; 300 m a.s.l.) is situated at the foothill of the Lhota Hill (Medvědí vrch) near Milešov, 2 m above the floodplain of the Pálečský Brook. The site has been covered by natural forest recently.

Figure 1.

Location of the study area in the Czech Republic, with published sampling sites (black dots) and newly studied successions (white dots): 1 – Řisuty; 2 – Dobroměřice; 3 – Kuzov; 4 – Mrsklesy; 5 – Pod Lhotou; 6 – Pod Ostrým; 7 – Paškapole; 8 – Richard; 9 – Úštěk; 10 – Pokratice and 11 – Kamýček.

Ecological groups were used sensu Ložek (1964, 1965) and Alexandrowicz (1987); the nomenclature follows Horsák et al. (2010). Mollusc diagrams expressed absolute and relative proportions of the total number of species (MSI – malacospectra of specimens and MSS – malacospectra of species) in separated layers. Only MSI of newly presented successions were shown. Subdivision of the Holocene was used sensu Jäger (1969) and Ložek (1982b).

All mollusc successions were sampled by standard methods (Ložek, 1964) – 8 dm³ of space-discrete samples of the sediment were taken from the central part of each macroscopically distinguishable layer (Figures 2 –5) within 80-cm-wide excavation pits. Mollusc shells were extracted from the sediments by a combination of floating and sieving. After careful drying, each sample was disaggregated in water and then in hydrogen peroxide. Floating snails were repeatedly decanted into a 0.5-mm sieve and dried under laboratory conditions. Afterwards, the sediment was dried and sorted by sieving. Shells were systematically removed from the sediment and examined under a binocular microscope.

Figure 2.

Lithology of the profile Pod Lhotou and MSI histogram, ecological groups sensu Ložek (1964). Layers: 1 – black humus–rich calcareous crumbly loam (topsoil) without tufa particles and sparse basalt fragments; 2 – basalt block scree with blackish brown humus–rich loamy matrix and sparse corroded carbonate aggregations; 3 – dark greyish brown humic loam and rather rich in tufa particles; 4 – light greyish to whitish pale yellow fine-granular pure tufa; 5 – whitish grey fine-granular pure tufa with weak diffuse iron staining; 6 – grey somewhat marly very fine-grained tufa and 7 – light olive grey mouldered Cretaceous marlstone.

Figure 3.

Lithology of the profile Paškapole and MSI histogram, ecological groups sensu Ložek (1964). Layers: 1 – dark brownish grey humus–rich loam (rhizosphere) with scattered small tufa nodules; 2 – dark brownish grey humic loam with numerous potato-shaped tufa nodules; 3 – loamy granular pale grey tufa with numerous potato-shaped nodules; 4 – dark grey humus–rich clayey loam with charcoals and potato-shaped tufa nodules; 5 – dark brown iron-stained humus–rich clayey loam with scattered tufa nodules and 6 – greyish brown loam rich in basalt detritus. Basalt rubble to blocks occurs throughout the section.

Figure 4.

Lithology of the profile Ostrý and MSI histogram, ecological groups sensu Ložek (1964). Layers: 1 – dark brownish grey humus–rich loam, fragments of indurated tufa and bricks, rhizosphere; 2 – brownish black very humus-rich clayey loam, coarse crumb structure, sparse nodules of indurated rufa and basalt fragments; 3 – brown to yellowish tufaceous loam with diffuse lentils of impure tufa; 4 – black clayey loam, very high in humus; scattered fragments of Mediaeval pottery at the surface; 5 – brownish black calcareous clay with numerous shells, visible in the section face; and 6 – very coarse to bouldery basalt scree with dark brown yellowish grey veined clayey matrix, weathering product of upper Cretaceous marlstones.

Figure 5.

Comparison of MSI of all 11 mollusc successions from the České středohoří Mountains. Chronozones were used sensu Jäger (1969): SR – sub-Recent; SA – sub-Atlantic; SB – sub-Boreal; EA – Epiatlantic; A – Atlantic; B – Boreal; PB – pre-Boreal and G – last glacial. On the left side of spectra are numbers indicating radiocarbon-dated layers.

Radiocarbon dating

If possible, two samples were radiocarbon-dated per profile, one close to the bottom and the second in the middle of each profile. The ages of other non-dated layers were estimated based on a depth–age model constructed for each succession using method and script by Blaauw (2010). Radiocarbon analyses were performed on mollusc shells in the Center for Applied Isotope Studies of the University of Georgia, United States, by the accelerator mass spectrometry (AMS) method and calibrated for variable initial ¹⁴C concentration using the OxCal v4.1 calibration program (Bronk Ramsey, 2009). Unfortunately, no types of fossil remains are at our disposal other than mollusc shells. Shells can contain so-called dead carbon, which can lead to an over-estimation of their age (Goodfriend and Stipp, 1983). To minimize the dating error, we used amalgam of small shell species for the radiocarbon dating because 78% of them did not contain any dead carbon (Pigati et al., 2010). The lithology was used as another proxy to control the undisturbed development of particular sites.

Results

The 11 compared successions cover the Lateglacial and the whole Holocene, but none of them showed a complete succession. The majority of profiles contain the late Holocene succession (since Atlantic or Epiatlantic), and the Mrsklesy profile contains the early Holocene mollusc succession only. Figure 1 shows situation and radiocarbon dates for selected layers using mollusc shells obtained from these layers. The nearly complete sequence with the oldest sample that dated back to 7937–7608 bc is the Kamýček site; however, the Dobroměřice profile is Lateglacial in age with index glacial species Columella columella, which later has survived only in the alpine zone of mid-European high mountains (Alps and Carpathians). Numbers of layers, lithology, MSS and MSI, and species composition are given in Figures 2 –4 and Tables 1 –3, and further compared in Figure 5.

Table 1.

Mollusc assemblages of separate layers of Pod Lhotou near Milešov in the České strˇedohorˇí Mountains, layer number related to Figure 3. Ecological characteristics – general ecological groups: A – woodland (in general); B – open country; C – woodland/open country; D – water, wetland. Ecological groups: 1 – woodland (sensu stricto); 2 – woodland, partly semi-opened habitats; 3 – damp woodland; 4 – xeric open habitat; 5 – open habitats in general (moist meadows to steppes). Woodland/open country: 6 – predominantly dry; 7 – mesic or various; 8 – predominantly damp; 9 – wetlands, banks; 10 – aquatic habitats. Biostratigraphic characteristics: (+) – local or occasional loess species; ! – species of warm phases; (!) – eurythermic species of warm phases; !! – index species of warm phases; G – species surviving glacial out of loess zone; (G) – ditto as relics. Presence in layers: 1 – number of individuals; ?1 – only an approximate determination.

Ecology group		Biostratum index	Depth–age model (cal. yr BP)	Layer/depth (cm)
				1	2	3	4	5	6a	6b
			Mollusc species	806	2453	3614	4660	5800	6670	8321
				0–19	19–38	38–45	45–58	58–66	66–74	74–97
A	1	!	Acanthinula aculeata	97	31	47	44	87	2	125
		!	Aegopinella pura	269	117	204	93	146	12	151
		!	Cochlodina laminata	10	14	9	38	12	5	13
		!!	Discus perspectivus	6	15	23	1	1
		(G)	Discus ruderatus					15	6	19
		!	Ena montana	9	3	24	9	11	4	24
		!	Isognomostoma isognomostomos		1
		!	Macrogastra plicatula		2	6	6
		!	Merdigera obscura			1
		!	Monachoides incarnatus	33	17	30	20	10	4	22
		!	Platyla polita	28	18	29	12
			Semilimax semilimax	30	9	3	3	1	1	5
		!	Sphyradium doliolum	7	11	13	37	8		1
		(!)	Vertigo pusilla	3	5	8	26	11	1	13
	2	!	Aegopinella minor	206	70	64	46	44	16	105
		!	Alinda biplicata		25	2	54	1		72
			Alinda biplicata/Laciniaria plicata	101	29	159		87
		(+)	Arianta arbustorum	21	15	17	40	21	7	21
		!	Cepaea hortensis	1	1	1	3	1	1	1
		!	Discus rotundatus	40	96	166	273	256	1	177
		(!)	Fruticicola fruticum	6	3	9	11	6	4	2
		!	Helix pomatia	3	3	1	1	2	1	1
		!	Oxychilus glaber			1
		(+)	Vitrea crystallina	8
	3	(G)	Clausilia pumila	9	21	8	62
		!	Macrogastra ventricosa	4	5	15	46	14	25	26
		G	Perforatella bidentata	30	8
		!	Urticicola umbrosus	13	3	3			2
B	4	M	Oxychilus inopinatus			3
		(+)	Pupilla triplicata							3
	5	(!)	Euomphalia strigella	17	16	6	10		?1
		+	Pupilla muscorum	121	79	17		1		8
		(!)	Truncatellina cf. cylindrica			1
		(+)	Vallonia costata	934	530	203	43	425	26	546
		G	Vallonia pulchella	188	131	22
		(G)	Vertigo pygmaea	133	77	36	1
C	6	(!)	Cochlicopa lubricella			4	11	14	2	7
	7	!	cf. Helicigona lapicida	1
		(+)	Clausilia cf. dubia	2	9				1	8
		(+)	Clausilia parvula				14	4
		(+)	Cochlicopa lubrica	96	30	22	119	170	17	115
		(+)	Euconulus fulvus	13	8	8	28	27	2	18
		!	Laciniaria plicata			3			8
			Limacidae/Agriolimacidae	5	1	2
		M	Oxychilus cellarius	22	25	6
		M	Oxychilus sp.		1		1	9	1	31
		(+)	Perpolita hammonis	86	50	114	393	391	30	263
		(+)	Punctum pygmaeum	100	59	80	111	108	2	121
		+	Trochulus hispidus	195	43	36	65	111	22	116
			Vitrea contracta		12	29	78	118	13	176
		(G)	Vitrina pellucida	116	20	9
	8	!	Carychium tridentatum	902	352	695	1424	2062	41	1812
		(!)	Columella edentula	41	9	5	57	88	2	73
		(G)	Perpolita petronella			2	14	66	6	?5
		+	Succinella oblonga	33	11	5	1
		(G)	Vertigo angustior	153	112	48	1	5		3
		(G)	Vertigo substriata	10	6		21	82	6	67
D	9	G	Carychium minimum	26	49	16	119	44	3	35
			Oxyloma elegans		9
		(+)	Succinea putris	18	9	1
		(!)	Vallonia enniensis	23	31	23
		(G)	Vertigo antivertigo	33	36	24	1	2		2
		G	Vertigo genesii					2	1	94
		G	Vertigo geyeri							8
		(+)	Zonitoides nitidus	1	7	26	233	182	12	61
	10	(+)	Aplexa hypnorum				1
		(+)	Galba truncatula	13	30	25	18	36	2	46
		(+)	Pisidium casertanum	2	3	12	1	22	1	6
			Pisidium personatum			2	47	9	6
			Radix peregra			1
			Sphaerium cf. nucleus				1
			Valvata piscinalis				1	1	2	4
Number of specimens				4218	2277	2329	3520	4713	299	4406
Number of species				48	52	55	47	43	39	42

Table 2.

Mollusc assemblages of separate layers of Paškapole near Velemín in the České středohoří Mountains, layer number related to Figure 4. Ecological characteristics: general ecological groups: A – woodland (in general); B – open country; C – woodland/open country; D – water, wetland. Ecological groups: 1 – woodland (sensu stricto); 2 – woodland, partly semi-opened habitats; 3 – damp woodland; 4 – xeric open habitat; 5 – open habitats in general (moist meadows to steppes). Woodland/open country: 6 – predominantly dry; 7 – mesic or various; 8 – predominantly damp; 9 – wetlands, banks; 10 – aquatic habitats. Biostratigraphic characteristics: (+) – local or occasional loess species; ! – species of warm phases; (!) – eurythermic species of warm phases; !! – index species of warm phases; G – species surviving glacial out of loess zone; (G) – ditto as relics. Presence in layers: 1 – number of individuals; ?1 – only an approximate determination.

Ecological group		Biostratum index	Depth–age model (cal. yr BP)	Layer/depth (cm)
				1	2	3	4	5
			Mollusc species	568	1680	3195	5032	6793
				0–12	12–22	22–43	43–62	62–80
A	1	!	Acanthinula aculeata	4	2	1	1	1
		!	Aegopinella pura	21	12	12	3	4
		!	Cochlodina laminata	1	8	8	3	4
		!	Ena montana	2	11	3		?1
		!	Monachoides incarnatus	12	21	35	12	9
		!	Sphyradium doliolum	81	5		1
		!	Platyla polita		5	1		1
			Semilimax semilimax		1
		(!)	Vertigo pusilla		1		1
		!!	Discus perspectivus			5
	2	!	Alinda biplicata	12	61	114	20	16
		(+)	Arianta arbustorum	4	12	8	17	14
		!	Cepaea hortensis	2	3	4	1	1
		!	Discus rotundatus	19	59	112	37	29
		(!)	Fruticicola fruticum	1	3	2	3	4
		!	Helix pomatia	5	6	?2	5	8
		!	Aegopinella minor		5	10	7	8
	3	(G)	Clausilia pumila	10	20	15	10	8
		!	Macrogastra ventricosa	2	12	9	?1
B	4	M	Oxychilus inopinatus	3	10	5	8	9
	5	+	Pupilla muscorum	70	2
		(!)	Truncatellina cylindrica	259	13	4
			Vallonia excentrica	149	4	2	3
		G	Vallonia pulchella	255	48	41		6
		(G)	Vertigo pygmaea	110	8	1	1
		(+)	Vallonia costata	682	138	57	13	1
		(!)	Euomphalia strigella	17	4	1	?1
C	6	(!)	Cochlicopa lubricella	45	10	2
	7	(+)	Perpolita hammonis	4	7	2	3	1
		(+)	Punctum pygmaeum	34	2	1	2
		(+)	Cochlicopa lubrica		9	4	3	3
		(+)	Euconulus fulvus		1	2
		(+)	Limacidae/Agriolimacidae		6	15	5	1
		!	Vitrea contracta		2	7	5	3
		!	Laciniaria plicata				3
	8	!	Carychium tridentatum	51	18	20	17	15
		+	Succinella oblonga	4	4	10	2
		(G)	Vertigo angustior	72	17	7	3
D	9	G	Carychium minimum	4	4		1
		(!)	Vallonia enniensis	10	16	34	3
		(G)	Vertigo antivertigo	1	5	3
		(+)	Zonitoides nitidus	1	4	1
	10	(+)	Pisidium casertanum	1	7	?3	?1
			Pisidium personatum		2	6	?1	?2
		(+)	Galba truncatula		4	2		2
			Radix peregra			3
Number of specimens				1948	592	574	197	151
Number of species				34	42	39	32	23

Table 3.

Mollusc assemblages of separate layers of Pod Ostrým near Velemín in the České středohoří Mountains, layer numbers related to Figure 2. Ecological characteristics: General ecological groups: A – woodland (in general); B – open country; C – woodland/open country; D – water, wetland. Ecological groups: 1 – woodland (sensu stricto); 2 – woodland, partly semi-opened habitats; 3 – damp woodland; 4 – xeric open habitat; 5 – open habitats in general (moist meadows to steppes). Woodland/open country: 6 – predominantly dry; 7 – mesic or various; 8 – predominantly damp; 9 – wetlands, banks; 10 – aquatic habitats. Biostratigraphic characteristics: (+) – local or occasional loess species; ! – species of warm phases; (!) – eurythermic species of warm phases; !! – index species of warm phases; G – species surviving glacial out of loess zone; (G) – ditto as relics. Presence in layers: 1 – number of individuals.

Ecological group		Biostratum index	Depth–age model (cal. yr BP)	Layer/depth (cm)
				1	2	3	4	5
			Mollusc species	1102	2914	4081	5647	7629
				0–20	20–36	36–44	44–67	67–84
A	1	!	Acanthinula aculeata			1	5	101
		!	Aegopinella pura	9	1	2	86	496
		!	Cochlodina laminata					31
		!	Cochlodina orthostoma	2	1		8
		!!	Discus perspectivus					170
		!	Macrogastra plicatula					4
		!	Monachoides incarnatus	1	1		2	76
		!	Platyla polita					29
			Semilimax semilimax				2	8
		!	Sphyradium doliolum				1	18
		(!)	Vertigo pusilla					1
	2	!	Aegopinella minor	7				236
		!	Alinda biplicata					224
		(+)	Arianta arbustorum					5
		!	Cepaea hortensis					4
		!	Discus rotundatus		2		9	140
		(!)	Fruticicola fruticum		1	3	2	2
		!	Helix pomatia		1			1
	3	(G)	Clausilia pumila	2	3	2	38	34
		!	Macrogastra ventricosa					10
		!	Urticicola umbrosus					1
B	4	!	Cecilioides acicula	2	9	11	9
		(+)	Chondrula tridens		1	1	1	16
		M	Helicella itala	6	4	1
	5	(!)	Euomphalia strigella	3	1	7	8
		+	Pupilla muscorum	26	132	117	56	32
		(!)	Truncatellina cylindrica	2		7	15	19
		(+)	Vallonia costata	260	300	1850	1180	384
		G	Vallonia pulchella	96	600	1750	266	344
		(G)	Vertigo pygmaea	15	116	292	45	21
C	6	(!)	Cochlicopa lubricella			4		2
		!	Tandonia cf. rustica				1
		(+)	Cochlicopa lubrica	15	30	550	6	6
		(+)	Euconulus fulvus			1	1	3
		!	Helicigona lapicida				2	1
			Limacidae/Agriolimacidae	2		2	3	4
		(+)	Perpolita hammonis				2	10
		(+)	Punctum pygmaeum	22	40	358	12	64
		+	cf. Trochulus hispidus				1	1
			Vitrea contracta					21
	8	!	Carychium tridentatum	3	73	603	183	1289
		(!)	Columella edentula					4
		+	Succinella oblonga	50	97	240	9	4
		(G)	Vertigo angustior	44	162	1058	177	30
D	9	(G)	Vertigo antivertigo	18	46	399	31	5
		(!)	Vallonia enniensis	6	10	292	27	3
		G	Carychium minimum	84	25	238	11	14
	10	(+)	Galba truncatula	40	45	78	12	75
		(+)	Pisidium casertanum			2	3
			Pisidium personatum	1				47
			Valvata cristata					1
Number of specimens				716	1701	7869	2214	3991
Number of species				23	24	25	32	44

The interpretation of three newly discovered successions

The succession from the site of Pod Lhotou (Table 1, Figure 2) is divided into two phases. The layers 5–6b contain the impoverished woodland fauna and wetland species, including some relic elements, which survive in Central European lowlands only in sparse relic habitats. Discus ruderatus, which inhabited mountain forests, survives here in talus scree with special microclimate, while Vertigo genesii, Vertigo geyeri and Perpolita petronella survive at relic wetlands. These two habitats predominated here during the Epiatlantic (6461–6372 BP). The increase in common woodland species started from layer 4 respectively 3 (Platyla polita and Macrogastra plicatula). The important woodland species Isognomostoma isognomostomos and Merdigera obscura appeared and disappeared subrecently. Why the three last mentioned species became extinct here is not clear. The subrecent increase in humidity was probably related to the overgrowing of forest at the site and appearance of damp forest species like Vitrea crystallina, Clausilia pumila and Perforatella bidentata. The increase in abundance of common open country species started at the same time, including the terricolous Oxychilus inopinatus. The mosaic of impoverished woodlands, open country and wetlands composes the picture of the site from sub-Boreal. However, before the relic wetland species had become extinct, some other wetland species appeared. Occurrence of Vallonia enniensis indicates a salty open wetland. The total abundance of ecologically indifferent species has been constant during the whole succession, but with some interesting details. The first occurrence of modern immigrant Oxychilus cellarius was recorded during the sub-Atlantic together with Vitrina pellucida, while species of dryer forests or shrubs Vitrea contracta became extinct subrecently probably due to the above-mentioned increase in humidity. Some aquatic species were deposited here by a nearby brook. Figures 2 and 5 show succession of this site without distinct fluctuations, which were visible only at the above-mentioned details.

The fauna of common woodland and few open country and wetland species have been recorded at Paškapole in the Epiatlantic (4944–4780 BP) (Table 2, Figure 3). Only few sensitive woodland species appear later. The most interesting is the subrecent increase in abundance of Sphyradium doliolum. The abundance of open country and wetland species gradually increases to maximum in the sub-Recent. While the mosaic of woodland and open habitats has been recorded here since the Epiatlantic, the wetlands occurred later (probably since sub-Boreal). Species of dry woodland or semi-open landscape V. contracta declined here subrecently again. Occurrence of aquatic species corresponds with the spring at the site.

The site of Pod Ostrým (Table 3, Figure 4) provides a relatively rich mollusc fauna. Nevertheless, it is situated at the slope 60 m above the alluvium; the wetland and aquatic species were found there, which supports the existence of calcareous fens and periodic pools in the landslides. The wetland gradually changed to subthermophilous meadow, which was drained recently. Succession split in the older phase (5721–5636 bc) in layer 5 with predominant communities of lush forest with scattered humid parts suitable for demanding woodland species such as P. polita, Discus perspectivus and S. doliolum. However, together with woodland fauna, the open country species occurred there – both Vallonia species in high abundances. It gives the evidence of the mosaic of open country wetland and woodland. In comparison with other dated profiles, it corresponds with Epiatlantic phase of the Holocene. Majority of the woodland species disappeared in younger layers (1–4), while open country species reached very high abundances. The Mediaeval pottery was found at the boundary of layers 3 and 4. The first fossil occurrence of Helicella itala in the Czech Republic in layer 3 confirms relatively recent spreading of this southwest-European species.

Discussion

The full establishment of the woodland communities of the České středohoří Mountains was hindered

The most important characteristic of the mollusc succession of the České středohoří Mountains is the general impoverishment of woodland communities even in hard-to-access sites. Some species characteristic of Atlantic and Epiatlantic climatic optimum in central Europe (Ložek, 1964, 1982b) are completely absent here (Causa holosericea, Helicodonta obvoluta, Petasina unidentata and Vitrea diaphana) or are very rare (Cochlodina orthostoma, Daudebardia rufa, Ruthenica filograna, V. crystallina and Isognomostoma isognomostomos). The appearance and disappearance of Laciniaria plicata in some specific mollusc successions of this area (Pokratice, Kuzov, Pod Lhotou and Paškapole) are of particular interest, because this species is nearly extinct in a number of the Bohemian areas at present. Predominant oak communities (Quercetum mixtum) were reconstructed here from climatic optimum based on pollen records from sediments from nearby the Komořanské Lake with admixture of ash, lime tree, maple and elm (Ulmus) (Jankovská, 1988). The drier climatic condition and/or human impact probably caused general impoverishment of woodland mollusc communities. Some sites on the chernozem soils, for example, Richard (Ložek, 1967), even lack complete woodland fauna since Epiatlantic. Fragmented native forests have been hardly affected by man since sub-Boreal. Human activities are also reconstructed by Jankovská (1988) simultaneously with hornbeam spreading, which is considered to be human affected (e.g. Küster, 1997). Fast entry of fir (Abies alba), beech and pine (Pinus sylvestris) together with the decline of ash and maple during this period are reconstructed by Jankovská (1988). Considerably deforested landscape with remaining woodland islands Jankovská (1988) and local pine expansion (Kneblová, 1956) are reconstructed from sub-Atlantic. Some minor environmental changes have been evidenced during sub-Recent. The species of drier woodlands or shrubs V. contracta became extinct, and abundances of woodland mollusc communities increased in some sites (Paškapole and Pod Lhotou). This may be caused by overgrowing in some parts of the landscape (Kubát and Machová, 2010). The decrease in wetland species during sub-Recent together with increase in open country species (Řisuty, Kuzov and Paškapole) indicate draining of some sites. Pokorný (2005) and Albert and Pokorný (2012) show diachronous course of above-mentioned changes in forest composition within the broader area of central Bohemia depending on the human impact. Similar changes probably occurred in the area under study. While continuous decline of woodland species since Epiatlantic has been recorded at some sites (Pod Ostrým and Kuzov), at other sites (Paškapole and Pod Lhotou), such a trend is not visible, while woodland assemblages remain impoverished. Figure 5 shows locally different successions with moderate fluctuation of forest at particular sites.

Mosaic landscape of the České středohoří Mountains during the whole Holocene

Besides woodlands, the other habitats have been composed of varied mosaic at the České středohoří Mountains landscape during the Holocene. While the diversity and abundance of open country mollusc species in general is rather high, the true steppe species, thus the inhabitants of most natural open habitats, are relatively rare, probably due to human impact. Some of them prefer deep soil horizons, but differ in their origin. Chondrula tridens and Helicopsis striata are glacial relics, while O. inopinatus and Helicella itala are late Holocene immigrants. The species indicative for rocky steppe became usually extinct after Neolithic colonization or survived very rarely (Pupilla triplicata, Pupilla sterrii and Granaria frumentum). Sub-Recent first occurrence of west-European species Helicella itala and sub-Atlantic first occurrence of southeast-European species Cepaea vindobonensis indicated the late spreading of these steppe species on the very border of their area. It seems that occurrence of suitable open habitats were not a limit for spreading of these species, because open habitats persisted there for a long time. The climatic change may be regarded as the reason of their late expansion, while the human impact (grazing) and/or climatic change can cause the change of occurrence of another open country species. Some wetland glacial relics survived here to Epiatlantic (V. geyeri) or later (P. petronella). On the other hand, the late appearance of V. enniensis indicates the enrichment of wetlands by mineral salts. Figure 5 shows regular occurrence of open country and wetland habitats during the whole Holocene and gives the evidence of the high habitat diversity of this area. The early Holocene species (D. ruderatus), which is characteristic of mountain forests recently, survive here at lower situated sites at specific talus scree microhabitats with hypothermal regime to recent (Horáčková et al., forthcoming). The moderate fluctuation of woodland, wetland and open country habitat without any distinct succession peaks of particular habitats types is the general character of the Holocene succession of the České středohoří Mountains However, the data from the early Holocene period are still insufficient.

Most mollusc successions were interrupted

Most local mollusc successions (except of Pod Kamýčkem site) were interrupted by the erosion phase at the boundary of Boreal–Atlantic. Thus, either the middle and late (majority of sites) or early Holocene (Dobroměřice, Mrsklesy – Ložek, 1962a, 1963a) successions were preserved. This interruption approximately related to massive erosion of travertine, which was already observed in a number of sites in central Europe (e.g. Jäger and Ložek, 1983), and to interruption of the Elbe palaeomeanders sedimentation (Břízová, 1999). It might relate to the 8.2 event (Alley et al., 1997; Kobashi et al., 2007). Unfortunately, it prevents conservation of more continual successions in the area under study.

The main factor affecting the specific Holocene succession of the České středohoří Mountains is probably the human impact

Several sites at the area under study contain remains of the mollusc fauna of previous two interglacials (Riss-Würm, Mindel-Riss), which enable comparison with Holocene succession (Ložek, 1962b, 1963b). Fully developed woodland assemblages were recorded here without any indication of impoverishment. Strictly forest species had occurred here (Aegopis verticillus, Bulgarica cana, H. obvoluta, P. unidentata, R. filograna), some of which even became extinct in central Europe later (Drobatia banatica, Soosia diodonta, Gastrocopta thelii and Azeca goodalli). As natural processes have been in charge of Holocene impoverishment, probably, a similar process would have been active during the previous interglacials too. Based on these indications and quite different Holocene succession trend, we compare that the spreading of woodland species to the hilly area of the České středohoří Mountains has been inhibited probably by early landscape change caused by humans, which seems to be more important than direct human impact to woodland assemblages (e.g. forest grazing), because we did not observe any important extinction of woodland species. A similar trend of succession was shown in the area of the lower Ohře River alluvium (Juřičková et al., 2013). Thus, the question remains whether man is the main stressor for the woodland development of other areas of prehistoric settlement in Central Europe and therefore whether a double-track development of landscape caused by man can be considered a general trend.

Footnotes

Acknowledgements

We are grateful to Vojtěch Abrahám for depth–age modelling, Mária Fapšová for technical collaboration and David Hardekopf for the English revision.

Funding

The research reported here was supported by the Grant Agency of the Czech Republic P504/10/0688 and 13–08169S.

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