Cent. Eur. J. Biol. • 8(1) • 2013 • 64-77 DOI: 10.2478/s11535-012-0104-6

Central European Journal of Biology

Postglacial vegetation history as recorded from the subalpine Lake Ribno (NW Rila Mts), Bulgaria

Research Article

Spassimir Tonkov1,*, Elissaveta Bozilova1, Goran Possnert2

1Laboratory of Palynology, Department of Botany, Faculty of Biology, Sofia University “St. Kliment Ohridski”, 1164 Sofia, Bulgaria

2Angstrom Laboratory, Division of Ion Physics, 14C-Lab, Uppsala University, SE-75120 Uppsala, Sweden Received 06 April 2012; Accepted 31 August 2012

Abstract: A pollen analysis conducted on a 600 cm core from Lake Ribno (2184 m) in the Northwestern Rila Mountains, supplemented by 13 radiocarbon dates, has revealed the basic stages in its postglacial vegetation dynamics. The lateglacial vegetation, composed of Artemisia, Chenopodiaceae and Poaceae, with stands of Pinus and Juniperus-Ephedra shrubland, dominated in the stadials and partly retreated during the Bølling/Allerød interstadial (14700-12900 cal. yrs. BP). The afforestation in the early Holocene (11600-7800 cal. yrs. BP) started with pioneer Betula forests, with groups of Pinus and Juniperus at mid-high altitudes, and Quercus forests with Tilia, Ulmus, Fraxinus, Corylus below the zone. A coniferous belt composed of Pinus sylvestris, P. peuce and Abies was shaped under the conditions of a more humid (7800-5800 cal. yrs. BP). The last trees that invaded the study area were Fagus after 4300 cal. yrs. BP and Picea abies after 3400 cal. yrs. BP. Evidence for destructive changes in the vegetation and indications of agricultural and stock-breeding activities (pollen of Triticum, Secale, Plantago lanceolata, Rumex, Juglans) was continuously recorded since the Late Bronze Age (3400-3200 cal. yrs. BP). The postglacial vegetation history in the Northwestern Rila Mountains demonstrated close similarities with that of the Northern Pirin Mountains.

Keywords: Pollen analysis • Radiocarbon dating • Vegetation history • Rila Mountains • Lake Ribno © Versita Sp. z o.o.

1. Introduction radiocarbon chronologies and local/regional correlations remain indispensable for more precise conclusions. The importance of the Balkan peninsula in relation to During the last decade, intensive palaeoecological European Holocene vegetation and flora history has research was conducted in the northwestern part of the been discussed in a number of publications [1-8]. In Rila Mountains – an area that became a key element this context, the geographical position of Bulgaria in the in resolving important issues, such as the origin and middle of the Balkans offers prospective opportunities evolution of glacial lakes and peat-bogs after the retreat for exploring and understanding the mechanisms of of the , the patterns of vegetation and climate postglacial vegetation development in Southeastern dynamics, the location of tree refugia and migration . An important prerequisite for such studies is the routes, and anthropogenic impact [11,12,17-20]. This availability of long pollen and plant macrofossil records paper presents the results of pollen analysis and supplemented by reliable chronologies. radiocarbon dating of a core obtained from the glacial Regarding the vegetation history of the highest Lake Ribno (2184 m). It is located in the cirque of the Bulgarian mountains, Rila and Pirin, which had Seven Rila Lakes and is linked with two other lakes been glaciated during the late Quaternary, the main (Sedmo Rilsko and Trilistnika) from which analysis of afforestation stages after the last deglaciation are pollen records have already been published [11,17,21]. comparatively well defined [9-16], although consistent The new information provides a more detailed picture

* E-mail: [email protected] 64 S. Tonkov et al.

of the postglacial environmental changes in this area, The climate below 1000 m is transitional between and is compared with the palaeoecological evidence continental and sub-mediterranean. Above this altitude (pollen, plant macrofossils, radiocarbon ages) from it changes into montane and the mean temperatures other investigated sites in the Rila and Northern Pirin decreases by 0.5°C with each 100-m increase in altitude. Mountains (Figure 1). At 1800-2000 m, where the present tree line runs, the mean temperature is -6°C in January and 11.4°C in August. The mean annual precipitation above 1000 m 2. Experimental Procedures is 800–1000 mm, with a maximum in June. The highest precipitation, much of it snow, reaches 2000 mm in the 2.1 Study area 1300–2400 m zone. The southerly and southwesterly The Rila Mountains (2925 m) are the highest massif winds are the strongest and in winter time, with speeds on the Balkan peninsula, located between 41o52‘30“- reaching 35-40 m s-1 [19,23]. 42o21‘40“N and 23o01‘22“-24o01‘E in Southwestern The modern vegetation of the Rila Mountains and Bulgaria. These mountains, 2396 km2 in total area, are its altitudinal distribution [24,25] has been presented in bordered on the south and southeast by the Pirin (2914 m) detail in earlier publications [11,17,19]. A short summary and the Western Rhodopes (2191 m) mountains, and on of the vegetation zones reveals that the mountain the west and north by the valleys of the rivers Struma, slopes up to 1000 m are now covered by oak/hornbeam Iskar and Maritza (Figure 1). The Rila Mountains were forests dominated by Quercus cerris, Q. frainetto, glaciated during the late Quaternary, and currently two Q. dalechampii, Carpinus orientalis, and C. betulus, with types of glaciation – mountain valley and ice cap – from an admixture of Ulmus glabra, Acer platanoides, and the last three glacial epochs are accepted. The activity A. pseudoplatanus. Above this zone the beech forests of the glaciers resulted in the formation of diverse relief (1000–1600 m) are comparatively well represented, forms such as cirques, numerous lakes (about 140), occupying areas in the northern and western parts of different types of moraines, and trough valleys. The snow the mountains. Besides single-dominant communities, line during the last glaciation was passing at 2200 m on Fagus sylvatica forms mixed woodstands with Abies northern slopes and at 2300 m on southern slopes, or alba and Picea abies at higher altitudes. The coniferous 800-900 m lower compared with the contemporary one belt (1600–2000 (2200)) m is the best developed, [11,17,22]. composed mainly of forests of Pinus sylvestris and

Figure 1. A. Location of the Rila Mountains (closed circle) in Bulgaria on the Balkan peninsula. B. Site of investigation () - Lake Ribno. Location of sites in the Rila (1-5) and Pirin (6-11) mountains mentioned in text (): 1. Lake Sedmo Rilsko [17,20], 2. Lake Trilistnika [11,12], 3. Lake Panichishte [18,56], 4. Lake Suho Ezero [40,41], 5. Lakes Ostrezki [15], 6. Peat-bog Mozgovitsa [42,43], 7. Lake Ribno Banderishko [10], 8. Lake Bezbog [16], 9. Lake Kremensko-5 [16], 10 Peat-bog Praso [48], 11 Lake Popovo Ezero-6 [47].

65 Postglacial vegetation history as recorded from the subalpine Lake Ribno (NW Rila Mts), Bulgaria

P. abies. Pure or mixed communities of A. alba and 2.2 Core and lithology Betula pendula are also found within this belt at lower A sediment core 640 cm long and 5 cm in diameter elevations. The Balkan endemic Pinus peuce forms was obtained from a platform at the deepest part of the a sub-belt and shapes the tree line at many places, lake (2.5 m) with a square rod piston sampler [28]. The particularly in the study area ca. 2000 m, partly together lithology was as follows: 0–165 cm olive-brown gyttja, with P. sylvestris and P. abies. The subalpine belt (2000 165–425 cm dark brown gyttja, 425–505 cm olive/clay (2200)–2500 m) is dominated by thick stands of Pinus gyttja, 505–540 cm silty clay, and 540–640 cm grey silt. mugo with Juniperus sibirica, Vaccinium myrtillus and herb communities on open areas. The alpine zone 2.3 Pollen analysis (2500–2925 m) is occupied by herb communities of Pollen analysis was carried out at 5 cm intervals Carex curvula, Festuca airoides, F. riloensis, Dryas following the standard acetolysis procedure [29]. The octopetala, S. herbacea, and many other characteristic interval 600-640 cm contained no pollen. The pollen species. Traces of human interference are present in all sum (PS) used for percentage calculations was based vegetation belts, and at many places the tree line was artificially lowered in past times to acquire new pasture Altitude (m) 2184 land [26]. Length (m) 250 The site of the present study is Lake Ribno, situated at 2184 m (42o12’27.10”N; 23o19’26.31”E) in the cirque Maximum width (m) 210 of the Seven Rila Lakes, Northwestern Rila Mountains Mean width (m) 140 (Figure 2). The cirque itself is the largest in the mountain, elongated and extending in a northerly direction. The Area (ha) 3.5 lakes are located in tiers on the slope faults between Maximum depth (m) 2.5 2535 and 2095 m. The shores of Lake Ribno are flat. Mean depth (m) 1.1 The inlet brings water from the upper Lake Trilistnika (2216 m) and the outlet runs into Lake Sedmo Rilsko Volume (m3) 38000

(2095 m) [19]. The general characteristics of the lake Catchment area (km2) 2.73 basin are shown in Table 1 [27]. The lake is surrounded by sparse groups of P. mugo and J. sibirica among herb Table 1. General characteristics of Lake Ribno [27]. vegetation.

Figure 2. Map of the cirque of the Seven Rila Lakes in the Northwestern Rila Mountains with the lakes studied ( Ribno,  Trilistnika,  Sedmo Rilsko).

66 S. Tonkov et al.

on AP (arboreal pollen) + NAP (non-arboreal pollen). In the Laboratory of Palynology and special pollen keys most instances a PS of 400–550 grains was achieved, [29-31]. A percentage pollen diagram was constructed dependent on the state of preservation and abundance with the program TGView ver. 2.0.2 [32] (Figure 3). of the pollen grains. Spores of mosses, pteridophytes, The determination of the boundaries of the local pollen of aquatics and Cyperaceae were not included pollen assemblage zones (LPAZ) was prepared using in the pollen sum. Their presence was expressed as CONISS [33] in an attempt to correlate and synchronize percentages of the PS. The identification of spores with other previously studied lacustrine sequences and pollen was made using reference collections of from this area. Pollen accumulation rates (PAR; pollen A

B

Figure 3. Percentage pollen diagram from Lake Ribno. A. Pollen curves of selected trees and shrubs. B. Pollen curves of selected herbs and pteridophytes.

67 Postglacial vegetation history as recorded from the subalpine Lake Ribno (NW Rila Mts), Bulgaria

grains cm-2 yr-1) for the major tree/shrub and herb taxa, pollen assemblage zone boundaries (in cal. yrs. BP) complimentary to their percentage values, were also were indicated on the right side of the pollen diagram calculated (Figure 4) by the addition of 3 Lycopodium (Figure 3). tablets (13500/13911 spores each) per sample of 2 cm3 volume before preparation. PAR provide reliable evidence of the presence or absence of plant species 3. Results as they are directly dependent on the abundance of the given taxon in the surroundings of the studied lake Seven local pollen assemblage zones (LPAZ) [34,35]. Occasionally, a few fossil stomata were found were delimited on the pollen diagram (LR-1 to LR-7) on pollen slides from several sample depths (135, 140, (Figure 3). They correspond to successive changes in 255, 265 and 375 cm). All were identified as Pinus sp. vegetation development for the last ca. 18000 years. and their presence (*) was indicated on the pollen Short descriptions are as follows: diagram (Figure 3). The stomata can be used to infer LPAZ LR-1: 600-512 cm (Artemisia-Chenopodiaceae- local presence of conifers in the catchment area. Pinus diploxylon-type-Poaceae) (18000?–15100 cal. yrs. BP) 2.4 Radiocarbon dating and local chronology In this zone the amount of arboreal pollen (AP) was AMS 14C dating on 13 bulk sediment samples of gyttja/silty rather low, around 20%, attributed mainly by Pinus clay was performed at the Angstrom Laboratory, Division diploxylon-type (18-22%), some Pinus peuce, Betula, of Ion Physics, 14C-Lab, Uppsala University. Attempts Juniperus and Corylus. Pollen of Ephedra (distachya- to find and date macrofossils were not successful. and fragilis-type) was also determined. The components The dates have been calibrated to calendar years of the herb vegetation like Artemisia (40-60%), (±2σ range) with the computer program OxCal v3.10 Chenopodiaceae (up to 20%), Poaceae (up to 10%) [36] using the relevant atmospheric data [37] with dominated in the fossil assemblages, accompanied results shown in Table 2. An age/depth sedimentation by other taxa such as Achillea-type, Cichoriaceae, plot with a linear fit was also constructed (Figure 5). Centaurea jacea-type, Galium-type, Rumex, Apiaceae, Determined through extrapolation, the age of the Brassicaceae. lowermost part of the sequence could be accepted LPAZ LR-2: 512-463 cm (Pinus-Artemisia-Betula- provisionally ca. 18000 cal. yrs. BP. The ages of the local Poaceae) (15100–12900 cal. yrs. BP)

Figure 4. Pollen accumulation rates (PAR; pollen grains cm-2yr-1) for the main tree/shrub and herb taxa from Lake Ribno plotted against age (cal. yrs BP). LPAZ of Figure 3 are also indicated.

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Lab. code Sample depth 14C age 14C age Material dated (Ua) (cm) (BP) (cal BP, 2σ) mid-point

40623 50-51 2106±30 1995-2150 (2070) brown gyttja

40624 99-100 2569±30 2510-2756 (2630) brown gyttja

40625 186-187 4192±31 4621-4840 (4730) brown gyttja

40626 269-270 6359±37 6575-7741 (7160) brown gyttja

40627 354-355 8540±42 9476-9550 (9510) brown gyttja

40628 395-396 9662±47 10780-11210 (11000) dark brown gyttja

39221 427 10139±52 11400-12050 (11725) olive clay/gyttja

39222 437 10291±56 11800-12400 (12100) olive clay/gyttja

39223 452 10465±53 12100-12700 (12400) olive clay/gyttja

39224 472 11240±59 12990-13260 (13125) olive clay/gyttja

39225 491 12346±67 14000-14750 (14375) olive clay/gyttja

40629 498-500 12692±67 14650-15250 (14950) olive clay/gyttja

39227 532 13483±155 15450-16650 (16050) silty clay

Table 2. Results of radiocarbon measurements of Lake Ribno.

Lake RIBNO 0

100

200

300 depth (cm)

400

500

600 4 4 4 4 4 2000 4000 6000 8000 1 10 1,2 10 1,4 10 1,6 10 1,8 10

14 age C (cal BP)

Figure 5. Age/depth sedimentation plot for the core from Lake Ribno.

69 Postglacial vegetation history as recorded from the subalpine Lake Ribno (NW Rila Mts), Bulgaria

Important changes in the participation of AP were except for the determination of the first single grains of recorded for the interval 500-480 cm and P. diploxylon- Plantago lanceolata, Urtica, Cerealia-type. type and P. peuce reached up to 40% and 12%, LPAZ LR-6: 223–83 cm (Pinus–Abies–Fagus–Picea) respectively. Pollen of deciduous trees became also (5800–2440 cal. yrs. BP) more common - Betula (6%), Juniperus and Corylus Two subzones LR-6a (223–133 cm, Abies– (2-3% each), Quercus cerris – type, Q. robur – type, Alnus, Pinus-Fagus) and LR-6b (133–83 cm, Pinus–Fagus– Carpinus betulus, Humulus/Cannabis-type. Comparable Picea) were delimited. In subzone LR-6a (5800– to the previous zone pollen of Abies, Fagus, Picea, Tilia 3400 cal. yrs. BP) Abies was present at 25-30%, was also found. The components of the herb vegetation accompanied by P. diploxylon-type (40-45%), P. peuce such as Artemisia (up to 20%) and Chenopodiaceae (10-15%), and Fagus (up to 10%). A continuous pollen (7%) declined, while pollen of Poaceae reached 10-15%. curve of Picea (up to 5%) appeared. Slight increases LPAZ LR-3: 463-427 cm (Artemisia-Chenopodiaceae- for Betula and Carpinus betulus were recorded. Pollen Poaceae) (12900–11600 cal. yrs. BP) grains of Juglans were quite common. The main herb Notable for this pollen zone was the final decrease of pollen taxa were Poaceae (5%), Artemisia, Cichoriacae, the total pollen from trees and shrubs (P. diploxylon-type Rumex, Ranunculus-type, Apiaceae, Brassicaceae, and -15%, P. peuce 2-3%, Juniperus - 2%, Betula - below Plantago major/media-type. Pollen grains of Plantago 5%). The presence of pollen from other trees such as lanceolata, Cerealia-type, and Urtica were also Abies, Picea, Tilia, Fagus, Ulmus was insignificant. recorded. In subzone LR-6b (3400–2440 cal. yrs. BP) Similar to zone LR-1 the pollen frequencies for Artemisia the pollen curve of Picea rose to 8-10% together with (up to 40%) and Chenopodiaceae (15%) rised again Fagus, while Abies declined to 12%. High values were while those for Poaceae declined slightly. recorded for P. diploxylon-type (30–40%) and P. peuce LPAZ LR-4: 427–293 cm (Betula–Quercetum (10-15%). mixtum–Corylus–Pinus) (11600–7800 cal. yrs. BP) LPAZ LR-7: 83–0 cm (Picea–Fagus-Pinus–Abies) Two subzones LR-4a (427–363 cm, Betula– (2440 cal. yrs. BP till present) Quercetum mixtum–Pinus) and LR-4b (363–293 cm, In the uppermost part of this zone, P. diploxylon-type Corylus–Quercetum mixtum–Betula) were delimited. In reached a maximum (50–55%) after a short term sharp subzone LR-4a (11600–9800 cal. yrs. BP) tree pollen decline to 20% (level 45 cm) preceded by maximal reached 80% due to Betula (10%), Quercus robur-type proportions for Fagus (20%) and Picea (15%). The last and Q. cerris-type (each 15%), Tilia (8%), Ulmus (5%), finds of Ephedra fragilis-type pollen were recorded. P. diploxylon-type (15%), Alnus and Juniperus (below A slight increase for Corylus, Quercus, Alnus was 5%). Low proportions of Acer, Fraxinus excelsior, established. Important changes in the frequency and Carpinus betulus, Ephedra (distachya- and fragilis-type) diversity of the herb pollen taxa were not registered. were also found. The NAP is represented by Poaceae (15%), Artemisia (up to 15%), Chenopodiaceae (5%), Achillea-type, Rumex, Cichoriaceae, Ranunculus-type, 4. Discussion Brassicaceae, Galium-type, each below 3-5%. In subzone LR-4b (9800–7800 cal. yrs. BP) Corylus 4.1 Lateglacial dominated with 10–20%, together with Q. robur-type The palynological data from Lake Ribno and the (up to 20%), Q. cerris-type (10%). Tilia (10%), and Betula radiocarbon chronology contributed to the reconstruction (6-10%). The continuous pollen curve of Abies appeared, of the palaeoenvironment and vegetation development reaching 5%. Pollen grains of Fagus and Picea were in the Northwestern Rila Mountains since the end of the regularly found. Pollen of P. diploxylon-type was present Upper Pleniglacial. Recently, a detailed discussion of the at less than 20%. The pollen curve of Poaceae declined lateglacial vegetation and climate changes in the area of to 5-8%. the cirque of the Seven Rila Lakes has been provided LPAZ LR-5: 293–223 cm (Pinus–Abies) (7800– by evaluating the palaeoecological evidence from all the 5800 cal. yrs. BP) lakes studied (Figure 6), Lake Ribno included [19], so The pollen curve of Abies reached 25-30%, that only the most important characteristics are outlined. P. diploxylon-type (30-35%) and P. peuce (12%). Pollen The pollen stratigraphy at the time when sedimentation of deciduous trees (Betula, Q. robur-type, Q. cerris-type, in the lake had started, probably ca. 18000 years ago after Corylus, Tilia, Ulmus) declined. Carpinus betulus was the retreat of the mountain glaciers above 2300-2400 m present at 2–4%. Pollen of Fagus, Picea and Carpinus [11], indicated typical glacial conditions in the study orientalis/Ostrya was continously found. Important area. Open herb vegetation dominated by Artemisia- changes in the presence of NAP were not recorded Chenopodiaceae-Poaceae and other cold-resistant

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Figure 6. Comparison of the palynostratigraphy and chronology for the lakes in the Northwestern Rila Mountains with sites in the Northern Pirin Mountains. species from Achillea, Centaurea, Rumex, Thalictrum, The rise of Pinus and Betula at the transition and Galium, was widely distributed (PAR for Artemisia LR-1/LR-2 is estimated at ca. 14800 cal. yrs. BP [19], are ca. 350 cm-2 yr-1). Isolated stands of Pinus (P. mugo/ and this result is apparently close to the oxygen-isotope sylvestris, P. peuce), inferred by their low PAR of ca. shift with a prominent rise in temperature ca. 14670 ice 150 cm-2 yr-1, along with some Betula and shrubland of core years before present recorded in the GISP2 core Juniperus-Ephedra were also present (LPAZ LR-1). The [38] and at 14700 ice core years in the GRIP ice core find of deciduous tree pollen of Betula, Alnus, Quercus, [39]. However, the decline of total arboreal pollen and Corylus proved that these trees had survived in (mainly for Pinus and Betula) after ca. 13100 cal. yrs. sheltered habitats at lower elevations. BP indicates a cooling trend at the end of the lateglacial After ca. 15100 cal. yrs. BP, with the improvement of interstadial. the climate, the stands of pines and birch had enlarged The pollen record after 13100 cal. yrs. BP points their area in the approaches to the lake as shown by a to a reversal towards stadial conditions in the Rila rapid increase in their pollen frequencies and influxes Mountains and the stratigraphic interval 463-427 cm (Figures 3, 4). The mountain herb vegetation was partly (LPAZ LR-3) can be assigned to the last cold phase of restricted in its distribution, the strongest for Artemisia the Lateglacial, the Younger Dryas stadial. Artemisia and Chenopodiaceae. At lower altitudes, deciduous and Chenopodiaceae pollen frequences and PAR trees (Quercus, Carpinus betulus, Corylus) were (1500 and 500 cm-2 yr-1 respectively) reached their regularly present and started to spread from their local maximal values, and the mountain herb vegetation refugia as the climate continued to improve. The time re-advanced, pushing the tree vegetation downslope interval 15100–12900 cal. yrs. BP (LPAZ LR-2) can be (Figures 3, 4). The transition from the Bölling/Alleröd correlated with the Bölling/Alleröd interstadial complex interstadial to the Younger Dryas stadial occurred from Western Europe. ca. 13000/12900 cal. yrs. BP, and this result appears

71 Postglacial vegetation history as recorded from the subalpine Lake Ribno (NW Rila Mts), Bulgaria

to conform with the ice core event GS-1 (Greenland macrofossils from Betula, Juniperus and occasional Stadial 1), which had started at 12900 cal. yrs BP [38]. finds of P. peuce needles at Lake Kremensko-5 Indications for climate improvement after 12100 cal. yrs. (2124 m) and Lake Bezbog (2250 m) between 10500 BP are shown by the rise of the total AP curve and the and 8000 cal. yrs. BP. These data suggest that the tree decline in Artemisia and Chenopodiaceae [19]. line in the early Holocene was placed higher compared The termination of the Lateglacial at ca. 11600 cal. to the Rila Mountains [16]. yrs. BP and the transition to the Holocene was manifested The fossil pollen record from Lake Ribno also by the steep rise of the arboreal pollen percentages, revealed the beginning of a wide spread of mixed mainly due to Pinus, Betula, Quercus, Corylus, and a Quercus forests with Tilia, Ulmus, Fraxinus excelsior, strong decrease in Artemisia and Chenopodiaceae. and Acer below the birch zone. These forests reached their maximal distribution ca. 10000-9800 cal. yrs. 4.2 Holocene ВР when Corylus started to expand in the study area The Holocene palynological record from Lake Ribno (subzone LR-4a). The regular presence of pollen grains confirms the basic stages in postglacial vegetation of Fagus, the slight increase of Humulus/Cannabis-type, development in the study area previously established and the maximum of Polypodiaceae spores, are quite from Lake Sedmo Rilsko [17] and Lake Trilistnika [11] probably indicative of a change toward a more humid (Figure 6). It contributes to a more precise definition climate. of the chronological boundaries and the driving Meanwhile Abies, being finally established in the mechanisms behind these stages, the time of migration, vegetation cover, began to gradually increase in the the arrival and expansion rates of the main deciduous Northwestern Rila Mountains prior to its quick expansion and coniferous trees and, last but not least, providing after ca. 7800 cal. yrs. BP (subzone LR-4b). The early data on human-environment interactions. Holocene presence of Abies was also traced from the The quick amelioration of the climate at the pollen records of Lake Sedmo Rilsko [17] and Lake beginning of the Holocene caused a decline of the Trilistnika [11], even as a short-term peak at the second lateglacial herb communities dominated by Artemisia site dated at 9900 cal. yrs. BP. The fossil record from and Chenopodiaceae and the initiation of afforestation. the Mozgovitsa peat-bog showed a rise of Abies pollen For the time interval 11600-9800 cal. yrs. BP (subzone after 7800 cal. yrs. BP, as low quantities had already LR-4a), pioneer forests of Betula with Pinus (P. mugo/ been established ca. 9500 cal. yrs. BP. However, sylvestris, P. peuce) and some Juniperus spread at macroremains from Abies were not recovered, which mid-higher altitudes on barren soil, replacing in many implied that fir never reached altitudes of 1800 m in this areas the remnants of the lateglacial herb vegetation. part of the Northern Pirin Mountains [42,43]. In the approaches to the lake, these forests were not The overall palaeoecological evidence from the dense. Birch with stands of Pinus shaped the tree line montane area of Southwestern Bulgaria indicated in the study area for nearly 4000 years after the onset that Abies had survived the lateglacial conditions in of the Holocene. The presence of Pinus was confirmed numerous local refugia in this part of the country [14,44]. for the first time by the discovery of fossil stomata. The Similar behaviour for an early Holocene establishment widespread presence of birch forests correlates well of Abies alba ca. 10700 cal. yrs. BP, reaching an altitude with the pollen data from Lake Trilistnika (till 7670 cal. of 2000 m at 9000 cal. yrs. BP, was recently reported yrs. BP) and from the lowermost Lake Sedmo Rilsko (ca. from the Retezat Mountains (South Carpathians) 7400 cal. yrs. ВР) (Figure 6). Another early Holocene based on pollen, macrofossils and stomata finds in the record from the Southwestern Rila Mountains (Lake sediments of two glacial lakes. This tree has been a Suho Ezero, 1900 m) showed high values of pollen and rare but important element of the local ecotone zone, fruits/bud-scales from Betula pendula, thus indicating and survived the Last Glacial Maximum on moist north- that the early Holocene tree line was running at the facing slopes [35]. above mentioned altitude [40,41]. Important changes in the forest composition and its Similarly, the vegetation reconstruction for the area altitudinal zonation had taken place after 7800 cal. yrs. of the Mozgovitsa valley in the Northern Pirin Mountains BP. In the course of ca. 2000 years the conifers (Abies, indicated an abundance of fruits and high pollen Pinus, P. peuce) expanded, shown by a substantial values (up to 20%) of Betula pendula, particularly for rise in their percentage and PAR values (Figures 3, 4), the time interval 9800-7300 cal. yrs. BP [42,43]. The replacing Betula forests at many areas and pushing palaeoecological information from the subalpine area of down slope the mixed mesophilous Quercus forests the Northeastern Pirin Mountains, where several glacial and Corylus (zone LR-5). Stomata of Pinus sp. found in lakes have been studied, pointed to the availability of the fossil record indicated local presence of pines, most

72 S. Tonkov et al.

probably P. mugo at this altitude. Hornbeam (Carpinus areas occupied by single dominant or mixed communities betulus) also increased slightly in the transitional zone of Fagus (PAR of ca. 500 cm-2 yr-1; Figure 4) in the between the deciduous and coniferous forests, together Northwestern Rila Mountains, a continuing increase with the continuous though still restricted presence of for Pinus diploxylon and P. peuce, and the maximal stands of Fagus. All these changes in the vegetation distribution of Abies at 4730 cal. yrs. BP (the highest cover confirm that the climate shifted to a more humid PAR of ca. 1500 cm-2 yr-1; Figure 4) which coincided with one, with cooler summers and warmer winters in the the regular – though still restricted occurrence of Picea Northern Mediterranean region during the second half of abies in the study area. the Atlantic period [45,46]. Pollen and plant macrofossils The first indications of human interference are (needles, cones) from conifers (P. sylvestris/mugo, visible in the pollen diagram ca. 3700 cal. yrs. BP, when P. peuce, Juniperus sibirica) preserved in the pollen grains of Cerealia-type and Urtica, together with sediments of other sites in the Rila Mountains (Lake the continuous presence of Plantago lanceolata, were Suho Ezero [40,41], Lakes Ostrezki [15]) suggest their established, accompanied by a decline in the pollen existence in the altitudinal zone 1900-2200 m around curve of Abies. Identical evidence was gathered from 6500-6000 cal. yrs. BP. In the coniferous belt, or above it Lake Sedmo Rilsko [17] and Lake Trilistnika [11], where on rocky places, stands of Betula were also preserved as the appearance of anthropogenic indicators such as demonstrated by the finding of fruits ofBetula pendula in P. lanceolata and Rumex agrees with the starting point Lake Sedmo Rilsko, radiocarbon dated at 7360 cal. yrs. of increase for Fagus and P. abies. The archaeological BP [17]. The tree line in the Rila Mountains was running information points to a rise in the number of the Late higher compared to the present day situation [15]. Bronze Age settlements (3400–3200 cal. yrs. BP) in The formation of a coniferous belt with similar the foothills of the lower (Maleshevska, Vlahina) and composition, synchronous in time with the Rila Mountains, higher (Pirin, Rila) mountains surrounding the Struma was also inferred by pollen and plant macrofossil valley, and an intensification of human activity including evidence for the Northern Pirin Mountains after ca. deforestation [49]. At least in the lower mountain 7900/7800 cal. yrs. BP. For instance, in the Mozgovitsa ranges of Southwestern Bulgaria, the spread of Fagus peat-bog, the regular finding of needles of P. sylvestris/ in place of the coniferous Pinus-Abies communities mugo and P. peuce, seeds of Pinus sp. and P. peuce, was favoured by anthropogenic activities [14,50]. This and wood of Pinus sp., dates back to ca. 7300 cal. yrs. evidence is in accordance with the estimations of BP [43]. In the sediments of Lake Ribno Banderishko Giesecke et al. [51] for the lowlands of Central Europe (2190 m), needles of P. peuce, Pinus sp. and budscales and for the southern Prealps [52], where disturbance of Pinus sp. were detected and radiocarbon dated for the and anthropogenic clearance accelerated the growth interval 8300-6500 cal. yrs. BP [10]. The results of the of Fagus sylvatica populations during the Holocene. plant macrofossil analysis from Lake Popovo Ezero-6 In many areas of Romania, the increase of Fagus was (2185 m) on the Northern Pirin Mountains showed the most likely climatically driven ca. 3600 cal. yrs. BP, and appearance of needles of P. peuce as well as bud-scales such changes may have also influenced the human and microsporophylls of Pinus sp. in the surroundings of activities [53]. the lake after 7500 cal. yrs. BP [47]. Abundant needles After 3400 cal. yrs. ВР (subzone LR-6b), P. abies and seeds of P. peuce, needles of Abies alba, and bud- started to invade areas in the coniferous belt, reaching its scales and microsporophylls of Pinus sp. were recorded first expansion maximum ca. 2630 cal. yrs. BP together at lower altitude in the sediments of the Praso peat-bog with P. peuce, P. diploxylon, Fagus, and partly Abies, (1900 m) [48]. All these findings undoubtedly indicate clearly indicated by a rapid increase in the PAR values of that during the Holocene climatic optimum, the tree line the corresponding taxa (Figure 4). These changes in the on the Northern Pirin Mountains reached its maximum vegetation cover were mainly climatically driven, as a shift to height above 2000 m compared to the present day lower average temperatures and an increase in precipation situation. The absence of macrofossils of Abies in the occurred in the northern hemisphere [54]. In the Eastern Northwestern Rila Mountains thus far could be explained Mediterranean the period of ca. 2800 cal. yrs. BP onward by the localization of Abies communities predominantly was characterized by a rise in humidity and temperatures in the lower zone of the coniferous belt, far away from [55]. At the same time, in the fossil record of Lake Sedmo the lakes studied. Rilsko [17] the rise of P. abies and the tendency for a Dynamic changes in the vegetation cover appeared decline of Abies coincided with the appearance of pollen after 5800 cal. yrs. BP (zone LR-6) and lasted for 3400 of Secale, Triticum-type, Cerealia-type, and an increase years. These were comprised of the final establishment in the participation of other anthropogenic indicators such and the first enlargement (ca. 4300 cal. yrs. BP) of the as P. lanceolata and Rumex. The evidence for human

73 Postglacial vegetation history as recorded from the subalpine Lake Ribno (NW Rila Mts), Bulgaria

presence gathered from the subalpine zone of the Central An enlargement of P. abies communities after 500 Rila Mountains (Lakes Ostrezki) was comprised of the cal. yrs BP in the lower part of the coniferous belt, appearance of the pollen curves of P. lanceolata, Rumex, concurrent with the time span of the Little Ice Age (1550- Scleranthus, Urtica, along with cereals at 3770 cal. yrs. 1850 AD), was established [18]. It should be pointed out BP, while wood charcoal – indicating forest fires – was that in the pollen diagrams from the subalpine lakes determined ca. 2800 cal. yrs. BP [15]. Trilistnika, Ribno and Sedmo Rilsko indications for such In the Northern Pirin Mountains, P. abies invaded the a rise of P. abies were not detected. coniferous belt ca. 4400 cal. yrs. BP and later on, after Further south, the palaeoecological evidence from 3300 cal. yrs BP, it started to occupy new areas [10]. By the area of Lake Ribno Banderishko on the Northern that time macrofossils of P. abies (needles and charred Pirin Mountains confirmed that after 2200 cal. yrs. BP wood) were already dominant constituents in the fossil the upper tree-line was shaped by P. peuce and P. abies. record of the peat bog Mozgovitsa at an elevation of The abundance of pollen from P. dipoxylon-type and 1800 m [43]. By contrast, the plant macrofossil record the presence of macrofossils (needles) from Pinus from Lake Popovo Ezero-6 (2185 m) lacked any sylvestris/mugo indicated that dwarf-pine had enlarged macroremains of Picea [47], suggesting that never its areas in the subalpine belt [10]. The results from other reached altitudes above 2000-2100 m on the Northern investigated sites located on the Northern Pirin Mountains Pirin Mountains even though its pollen possessed good demonstrated close similarities in the vegetation dispersal capacity in the subalpine zone. development for the last two millennia [13,16,47]. The vegetation history of the study area after 2440 cal. yrs. ВР until present (zone LR-7) was characterized by an intensification of the anthropogenic 5. Conclusions impact in all vegetation belts. In many places, the upper tree line was artificially lowered in order to The new results from the glacial Lake Ribno, supported extend mountain pasture land. The PAR of the major by a reliable chronology and compared with the tree taxa (P. diploxylon, P. peuce, Abies, Fagus, Picea) evidence from the nearby lakes, provided a detailed steeply declined ca. 2000 cal. yrs. BP. Even at high information on the postglacial vegetation development, altitude the appearance of pollen from cereals (Triticum- tree establishment and expansion, and anthropogenic type, Cerealia-type) and from Juglans pointed to well impact on the natural forest cover. The new information developed agriculture and cultivation of walnut in the was successfully correlated with the available pollen foothills of the mountain, whereas pollen of Rumex, and plant macrofossil data from other studies in the Rila P. lanceolata and Urtica was indicative of livestock and Northern Pirin mountains. From this, the following grazing in the mountain meadows. main conclusions can be presented: The final enlargement in the distribution of P. abies 1. The palaeoenvinronmental record dates back and Fagus was reached in Medieval times after to ca. 18000 cal. yrs. BP when sedimentation in Lake 1600 cal. yrs. BP. Subsequently, both trees partly Ribno started after the retreat of the cirque glaciers. declined, whilst larger areas were occupied by Pinus 2. The lateglacial landscape at high-mid altitudes (mostly P. mugo in the subalpine belt and P. sylvestris). was dominated by open herb vegetation composed of A more detailed picture of the vegetation history and Artemisia, Chenopodiaceae and Poaceae, with scattered human impact at lower altitudes (1300-1400 m) for the stands of Pinus and Betula, and shrubland of Juniperus- last 2000 years in the Northwestern Rila Mountains was Ephedra. This vegetation expanded during the stadials obtained from the investigation of sediments of Lake and partly retreated during the Bølling/Allerød interstadial. Panichishte (1345 m) [18,56]. These results complement 3. The afforestation in the early Holocene (11600- the palaeoenvironmental reconstruction for the study 7800 cal. yrs. BP) began with pioneer Betula forests along area. The plant macrofossil and pollen records revealed with groups of Pinus and Junipers at high-mid altidudes that after 1220 cal. yrs. BP, A. alba and F. sylvatica were and mixed deciduous Quercus forests with abundant replaced at many places by P. sylvestris and P. abies, Tilia, Ulmus, Acer, and later on Corylus below this zone. while stands of Betula, Corylus and Juniperus occupied 4. A coniferous belt composed of P. sylvestris, open areas. This successional change suggests a likely P. peuce and Abies was shaped in the conditions of a transition from a natural forest community to one that more humid climate (7800-5800 cal. yrs. BP) as indicated was maintained by regular disturbance and clearing. by the pollen and plant macrofossil findings. The upper For example, the evidence from Northwestern Romania tree line was running higher compared to today. showed that the distribution of A. alba was seriously 5. The last tree immigrants that started to enlarge their restricted on a regular scale [57]. presence in the study area were Fagus after 4300 cal. yrs.

74 S. Tonkov et al.

BP and P. abies after 3400 cal. yrs. BP. Both trees featured Acknowledgements several expansion maxima after ca. 2600 cal. yrs. BP. 6. Reliable evidence for destructive changes in the The coring of the lake was performed under the natural vegetation cover and indications of agricultural guidance of Prof. H. E. Wright, USA, and Prof. B. and stock-breeding activities were continuously recorded Ammann, Switzerland. Assoc. Prof. E. Marinova since the Late Bronze Age (3400-3200 cal. yrs. BP). helped with the identification of the fossil stomata. 7. The basic stages of the postglacial vegetation Dr. A. Tosheva was responsible for the drawings of development in the Northwestern Rila Mountains Figures 1, 2 and 6. The two reviewers and the editors exhibited close similarities with those of the Northern provided useful suggestions and comments to improve Pirin Mountains. the manuscript.

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