Vascular and Cryptogam Flora on Bulgaria’s Ancient Mounds: History
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Subjects: Environmental Sciences
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Iva Apostolova

The mounds are important for the preservation of the native vascular and cryptogam flora. A total of 1059 vascular plants, 58 bryophytes and 61 lichen taxa are recorded. Despite their small area, the mounds were shown to preserve nearly a quarter of the Bulgarian flora. The vegetation cover on the mounds included 61% perennials indicating a long-term persistence and stability. The majority (98%) of the established vascular plants were native species. 

  • bryophytes
  • generalist plants
  • grassland specialists
  • historical monuments
  • invasive alien plants
  • kurgans
  • lichens
  • native plants

1. Introduction

Ancient mounds (also called tumuli or more commonly kurgans) were constructed in temperate Eurasia between 4th millennium BC and 4th century AD and used primarily for burial purposes. A remarkable number of these mounds have been preserved due to their spiritual and cultural importance [1][2][3][4]. Bulgaria is exceptionally rich in ancient mounds with a known number of approximately 50,000 [5]; 11,000 of these mounds have been registered in the Archaeological Map of Bulgaria (http://www.naim-bas.com/akb/ accessed on 3 November 2021). Some of Bulgaria’s ancient mounds are remarkable historical monuments, including massive underground stone buildings often decorated with wall paintings (e.g., Kazanlak and Aleksandrovo tombs). The most attractive of these structures are important tourist destinations open to the public. At present, most of Bulgaria’s ancient mounds are surrounded by vast agricultural lands [4]. Similar to ancient mounds in other European countries, these structures are often standing as sole “islands” of semi-natural vegetation in an otherwise human-modified landscape [3][6][7]. Along with field margins, road verges and buffer strips adjacent to arable land, the ancient mounds preserve small semi-natural fragments and provide an opportunity for the long-term survival of indigenous flora. Moreover, ancient mounds consist of different microhabitats, which enrich the suitability for the development of an ecologically diverse flora [8][9]. The long-term persistence of the mounds within agricultural lands, primarily due to sacred and religious respect, naturally makes them a part of the Green and Blue Infrastructure defined at the European level as a “strategically planned network of natural and semi-natural areas with other environmental features designed and managed to deliver a wide range of ecosystem services” [10].
Recently, there has been an increased interest in burial mounds as biodiversity hotspots situated in an otherwise homogenous agricultural landscape [2][3][7][11][12][13][14][15]. Plants, and especially flowering plants (Angiosperms), are one of four groups of living organisms (along with Heteroptera, Symphyta and aculeate Hymenoptera) that have been shown to be best served for the biodiversity evaluation of cultivated areas [16]. Recent research has shown that ancient mounds preserve a remarkable plant diversity [3][11][13][15][17][18]. To date, no studies on the natural value of Bulgaria’s ancient mounds have been conducted and no records of their floristic diversity are known to exist. At the beginning of the current research, researchers assumed that the cultural significance and principal sacrosanct nature of Bulgaria’s ancient mounds, akin to other countries, provided long-term repository conditions for natural communities and that they served as refugia for indigenous flora in anthropogenically transformed areas. 

2. Diversity and Species Characteristics

2.1. Vascular Plants

The list of registered vascular plants includes 1059 taxa (Supplementary ESM S1, could be found in https://www.mdpi.com/2223-7747/11/5/705#supplementary). The average number of species per mound was 69.9 ± 22.6 SD (min 27, max 152). Researchers identified 971 plants to species level, accepted 3 taxa as aggregates or species groups, and identified 85 taxa to genus level. The floristic diversity was confined to 82 vascular plant families. Flowering plants (Angiosperms) made up the majority of the observed species diversity and only seven species belonged to other groups: one horsetail—Equisetum hyemale, two ferns—Polystichum aculeatum, Pteridium aquilinum, and four Gymnosperms—Juniperus communis, J. oxycedrus, Pinus nigra, P. sylvestris. There were 23 families represented by more than 10 taxa, the most species rich of these were Asteraceae—126 taxa (11.9% of the established taxa), Fabaceae—111 taxa (10.5%), Poaceae—106 taxa (10%), Lamiaceae—61 taxa (5.8%), Brassicaceae—58 taxa (5.5%), Caryophyllaceae—57 taxa (5.4%), Rosaceae—48 taxa (4.5%), Apiaceae—46 taxa (4.3%), Boraginaceae—31 taxa (2.9%) and Plantaginaceae—31 taxa (2.9%). Twenty-three other families (28.1%) were represented by a single species. The flora of the studied mounds was composed primarily of perennial herbaceous plants (61%), followed by short-lived plants, and a low number of shrubs and trees. Generalists and grassland specialists dominated the species composition of the mounds. A major part (98%) of the established vascular plants consisted of native species (Table 1). Only 21 plants belonged to other categories (alien (status unknown)—13, naturalized alien—5, in large-scale cultivation—2 and doubtfully native—1). Invasive alien plants included Acer negundo, Ailanthus altissima, Amaranthus albus, Conyza canadensis, Cuscuta campestris, Datura stramonium, Erigeron annuus, Phytolacca americana, Robinia pseudoacacia, Sorghum halepense, Xanthium orientale subsp. italicum and X. strumarium. These taxa represent 20% of all plants included in the list of invasive or potentially invasive alien plants in Bulgaria and three of them (Acer negundo, Ailanthus altissima and Robinia pseudoacacia) are among the “top 10” invasive alien plants in Bulgaria. The largest number of invasive plant species researchers registered on a single mound was four. A substantial number of the studied mounds (40 or 35.4%) had at least one invasive alien plant.
Table 1. Biological characteristics of the registered vascular plants. Total number and descriptive statistics are given.
  On a Mound
Parameter Total Min Max Mean SD
Biological type          
Short-lived 378 4 64 24.77 10.90
Perennial 495 10 88 36.13 13.57
Dwarf-shrub 17 0 7 0.80 1.09
Shrub 49 0 12 4.02 2.80
Tree 35 0 7 2.55 1.87
N/A 85 0 10 2.20 2.26
Plants with conservation importance          
Critically Endangered (CR) 2 0 1 0.04 0.19
Endangered (EN) 9 0 2 0.22 0.43
Vulnerable (VU) 7 0 2 0.15 0.39
Near Threatened (NT) 6 0 1 0.07 0.26
Least concern (LC) 2 0 2 0.21 0.45
Bulgarian Biodiversity Act 10        
Habitat Directive 1        
CITES 11        
N/A 85 0 10 2.20 2.26
No conservation status 947 27 137 67.50 20.87
Native status          
Native (including archaeophytes) 948 27 141 66.76 21.30
Naturalized alien 5 0 2 0.15 0.39
Doubtfully native 1        
In large-scale cultivation 2        
Alien (status unknown) 13 0 5 0.83 0.97
N/A 85 0 10 2.20 2.26
No data 5 0 2 0.45 0.63
Invasive alien plants (for Bulgaria)          
Invasive alien plants (IAP) 12 0 4 0.50 0.80
Not classified as IAP 962 27 142 67.76 21.38
N/A 85 0 10 2.20 2.26
Critically endangered species present on the mounds included Anchusa stylosa and Limonium asterotrichum. Endangered plants present on the mounds included Astragalus haarbachii, A. wilmottianus, Chamaecytisus frivaldszkyanus, C. kovacevii, Dianthus pallidiflorus, Erysimum cheiranthoides, Festuca thracica, Goniolimon besseranum and Jurinea ledebourii. Fourteen of the registered taxa were Balkan endemics: Achillea clypeolata, A. pseudopectinata, Armeria rumelica, Astragalus wilmottianus, Asyneuma anthericoides, Cytisus eriocarpus, Dianthus moesiacus, Festuca thracica, Heptaptera triquetra, Koeleria simonkaii, Dichoropetalum vittijugum, Polygala supina subsp. rhodopea (syn. P. rhodopaea (Velen.) Janch.), Scabiosa triniifolia and Thymus longidentatus. Eleven plants were included in the CITES Convention. 

2.2. Bryophytes

Pottiaceae included the highest number of species, all of which were confined to dry, skeletal and sandy substrates. The second most diverse family was Bryaceae, a group which includes species with diverse ecological preferences. On the mounds, this family was represented by species typical of dry eroded terrains. No bryophytes were registered on 40 of the studied mounds (36.04%). In cases where bryophytes were found, their number ranged from 1 to 10 species (average 3.1 ± 2.1SD). The only typical epiphytic moss recorded during this entry was Orthotrichum pumilum, which was found on trees of the genus Quercus. The rest of the listed species usually occupied the soil substrate and were occasionally also found on woody stems. Abietinella abietina, Barbula unguiculata, Pterygoneurum ovatum, Rhynchostegium megapolitanum and Thuidium assimile were confined to herbaceous habitats. Bryum pallescens has broad ecological affiliation, while Atrichum undulatum, Fissidens taxifolius and Plagiomnium affine were observed only within the forested northern slopes. Nearly all of the bryophytes registered during the present entry are taxa commonly found in Bulgaria where they are known to occur in a variety of different plant communities, usually in lowland areas. Phytogeographically, most of the bryophytes (35 species) belong to the temperate region. A significant share of the registered bryophytes were cosmopolites (Polytrichum juniperinum, Polytrichum piliferum, Funaria hygrometrica, Schistidium apocarpum, Ceratodon purpureus, Weissia controversa, Syntrichia ruralis, Bryum argenteum, Pohlia nutans and Hypnum cupessiforme). The Ceratodon conicus species, previously known from a single locality in Bulgaria (Vitosha Mt. [19]), deserves special attention. This taxon was registered on a mound in the Thracian lowland near the town of Plovdiv (ID249), a substantial distance from the previously reported location.

2.3. Lichens

A total of 61 lichen taxa, 56 identified to species level, were registered on 52% of the studied mounds (Supplementary ESM S1, could be found in https://www.mdpi.com/2223-7747/11/5/705#supplementary). The highest number of recorded lichens were epiphytes (35 taxa), followed by epigean taxa (16 taxa), and by species with wide ecological breadth occurring on all substrate types (2 taxa). Researchers recorded eight epilithic lichen taxa. The number of species registered on a mound ranged between 1 and 18 taxa (average 4.0 ± 2.1SD). Lichens were not registered on 57 of the studied mounds. The most common family found on mounds was Parmeliaceae, followed by Cladoniaceae and Physciaceae. Frequently registered lichen species were Xanthoria parietina, Parmelia sulcata and Physcia adscendens. Arthopyrenia salicis is recorded for the first time for the country. Until present, Caloplaca cerinella and Catillaria nigroclavata were known from single Bulgarian localities and are reported here for a second time. Phytogeographically, many of the lichens (31 taxa) are typical for the temperate region. In general, there is a clear preponderance of warm-temperate lichens. Arctic–alpine lichens, common for the Bulgarian alpine and subalpine zones, were not encountered during the present entry. It is interesting to note that the new country record and the two other poorly recorded taxa were found together on a single mound (ID 562) situated in the eastern part of the country, near the Black Sea.

This entry is adapted from the peer-reviewed paper 10.3390/plants11050705

References

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