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Article

Natural Vegetation Recovery on Excavated Archaeological Sites: A Case Study of Ancient Burial Mounds in Bulgaria

by
Iva Apostolova
1,*,
Magdalena Valcheva
1,
Desislava Sopotlieva
1,
Nikolay Velev
1,
Anna Ganeva
1 and
Georgi Nekhrizov
2
1
Institute of Biodiversity and Ecosystem Research, Bulgarian Academy of Sciences, 2 Gagarin Str., 1113 Sofia, Bulgaria
2
National Archaeological Institute with Museum, Bulgarian Academy of Sciences, 2 Saborna Str., 1000 Sofia, Bulgaria
*
Author to whom correspondence should be addressed.
Sustainability 2022, 14(12), 7318; https://doi.org/10.3390/su14127318
Submission received: 9 May 2022 / Revised: 12 June 2022 / Accepted: 14 June 2022 / Published: 15 June 2022
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Abstract

:
As a distinctive component of the cultural landscape in Eurasia, burial mounds are well known for their historical value. Recently their role as biodiversity hotspots, especially in the homogenous agricultural landscape, has become particularly important. Archaeological excavations, although necessary, are destructive to the natural elements on the mounds. Restoration and vegetation recovery after such disturbances are needed for the preservation of biodiversity and for the cultural landscape integrity. In this study, we aimed to find out how effective is the natural vegetation recovery on the mounds after archaeological excavations. Successional stages between 2- and 30-years post-excavations have been studied. Vegetation sampling was performed on 15 mounds within 300 plots (1 × 1 m). Spontaneous succession was found to start immediately, and during the first decade, anthropophytes prevailed. In the subsequent years, their cover significantly decreased at the expense of species typical for the natural communities in the surroundings. Total species richness increased with the successional age and the vegetation composition became more similar to the semi-natural communities commonly established on mounds in Bulgaria. In the advanced successional stages, we registered a high rate of heterogeneity on the mounds, facilitated by the establishment of target plant species with different ecological requirements, including bryophytes. Provided the obtained results, we conclude that the natural vegetation recovery on the excavated and subsequently recovered mounds were very successful, and the current practice must continue in the future.

1. Introduction

Cultural heritage is part of the physical environment affected by humans [1]. Together with the historical monuments, the landscape becomes a cultural landscape, reflecting the integrity between nature and humans [2]. The European Landscape Convention [3] defines the landscape as “an area perceived by people, whose character is the result of the action and interaction of natural and/or human factors”. Conservation of the cultural features supports the preservation of cultural landscapes and adds value by enhancing cultural ecosystem services [4].
Ancient burial mounds (also known as kurgans or tumuli) are distinctive elements of the landscape in Eurasia [5]. They are related to Yamnaya Culture from the Late Copper Age to the Medieval period [6]. The great historical value of the mounds is proven by a remarkable variety of burial tombs and artifacts found during archeological excavations that provide unique insights from the past [7,8]. Due to their numerosity, ancient burial mounds are a traditional landscape element in the lowlands of Bulgaria. They are emblematic features and play a considerable role in local people’s culture and traditions, as their spherical shape is well visible in flat lands and their names, which have been preserved in time, influence the name of a number of settlements and geographical locations in the country. Archaeological studies of burial mounds started in Bulgaria about 150 years ago [9]. Excavation procedures provided extremely important information about the burial practices, lifestyle, and religion of the local inhabitants from the Early Bronze Age till the end of Antiquity. Lavish graves and remarkable architectural monuments (tombs), built for Thracian aristocrats, were often found in the studied mounds [10,11,12,13].
On the other hand, being semi-natural landscape elements that have existed for a long period of time, their historical and aesthetic features could be considered part of the cultural ecosystem services. Vegetation cover and plant species diversity are considered generally important for the safeguarding of the historical landscape [14,15,16,17]. Along with their historical and cultural value, ancient mounds also serve as stepping stones for semi-natural vegetation in a human-modified landscape. Their role as biodiversity hotspots, along with other semi-natural fragments such as field margins and road verges, in an otherwise homogenous agricultural landscape has been recently studied [5,17,18,19,20,21,22,23,24].
Archaeological studies (including excavation) are necessary and important for the exploration of historical heritage, but unfortunately, they are also often destructive to the studied objects. The disturbance caused by archeological excavations harms the integrity of the cultural landscape and damages the natural elements on the mounds, such as flora and vegetation that have developed in time under the local environmental conditions [18].
In order to preserve the landscape integrity and authenticity, an appropriate restoration of every studied site after archaeological excavations is desirable at the European level [3]. The restoration procedures are supposed to allow the interaction of expert knowledge on both cultural heritage and natural processes [25]. The restoration, maintenance, and conservation of cultural heritage are addressed in several EU policies [3,26]. In the case of ancient burial mounds, the primary goals after excavation procedures are to restore the aesthetic value of the object and to prevent the down wash of the top soil horizon. In Bulgaria, the restoration of excavated mounds, by returning the soil substrate so that the mound regains its original shape, started approximately 30 years ago. This became a mandatory requirement for the National Archaeological Institute with Museum at the Bulgarian Academy of Sciences in 2006.
Vegetation restoration activities in disturbed sites have a long history throughout Europe [27,28,29,30,31,32,33]. There are several studies focused on vegetation recovery and management of historical monuments [14,34,35], and particularly of ancient burial mounds [36,37,38]. In Bulgaria, such observations are not available yet. Previous studies on vegetation restoration point out three main approaches: (1) spontaneous succession, (2) appliance of technical measures, and (3) a combination of both [39]. As for studies focused on vegetation restoration on ancient burial mounds, there are examples for both the appliance of technical measures (reintroduction of plants by seed sowing, transplanting green-house-grown plants, and transplanting individuals from threatened populations) [37] and observations of spontaneous regeneration [38].
We put several questions at the beginning of the study: (1) what is the rate of the natural vegetation recovery, (2) which species prevail during the decades of succession and when do target species establish, and (3) how effective is spontaneous succession for the revegetation on disturbed mounds after archaeological excavations. We assume spontaneous succession as an unintentional vegetation regeneration [40] that could be a beneficial and low-cost solution for the restoration of disturbed archaeological sites. Therefore, the objective of this study was to collect data on the spontaneous vegetation recovery on burial mounds excavated in the last 30 years in Bulgaria and to discuss the importance of these successional processes for the preservation of biodiversity.

2. Materials and Methods

2.1. Study Objects and Area

We selected 15 burial mounds from the Archaeological map of Bulgaria (http://www.naim-bas.com/akb/ accessed on 22 March 2021) that were excavated during 1992–2019 and subsequently restored by returning the relocated soil so that the mound regains its original shape. None of them included underground tombs. The selection of the mounds could not be based on preselected vegetation variables because it was limited to the accomplished archaeological research, which had completely different priorities. Historical imagery of Google Earth shows that prior to excavations, the studied mounds were covered by herbaceous vegetation with some scattered shrubs and small trees. Nevertheless, we were at least able to make a selection of mounds where flora and vegetation have developed under similar climatic conditions: all studied sites were situated in Southern Bulgaria, in the Upper Thrace lowland, within an area of approximately 5000 km2 (Figure 1). In this region, during the 5th century BC, emerged the first Thracian state—the Odrysian kingdom. Several significant urban centers have been formed there: Philippopolis (currently the town of Plovdiv), Kabile (close to the town of Yambol), and Seuthopolis (close to the town of Kazanlak), with numerous smaller settlements around them. Varying in shape and size, burial mounds have been built in the vicinity of those settlements. Systematic archaeological excavations of the burial mounds near Seuthopolis in the last 30 years revealed 15 tombs, each one with its unique architectural details and original decoration [41]. Two of the tombs discovered in these Thracian lands (Kazanlak and Sveshtari) are included in the UNESCO World Heritage List (https://whc.unesco.org/en/statesparties/bg, accessed on 15 February 2022), and nine other an application is under preparation.
The topographic characteristics of the studied mounds (Table 1) were calculated by the use of large-scale topographic maps.
The region where the studied sites are located is characterized by a continental macroclimate with a strong sub-Mediterranean influence. Mean annual temperature is 10.6 °C, and mean annual precipitation is 531 mm (data for the period 1971–2000, town of Kazanlak) [42]. The palaeoecological studies conducted in the same area unanimously assert domination of oak forests at the beginning of the Holocene [43]. These studies also reveal a strong human impact around 4000 BP and in the Late Bronze Age, using pollen diagrams that indicated deforested agricultural landscapes [43,44]. The human impact in the region was reflected by the presence of Cerelia-type pollen and pollen of Xanthium, Rumex, Scleranthus, and Galium, which indicates stockbreeding. Forests were fragmented, and scattered stands of Quercus, Tilia, and Ulmus remained until the 6th century AD [44].
The selected mounds, disturbed by archeological excavations, have all been studied according to the methodological approach for archeological research of burial mounds, which requires initial geodetic survey (tacheometry) of the mound and its surrounding area. For larger mounds, geophysical surveys are carried out in advance in order to find out whether there are archeological structures inside and where they are located. A georeferenced square network with a central reference point on the top of the mound is created. The mound is divided into four sectors by two axes: one directed north–south and another directed east–west. Each sector is then subdivided into 5 × 5 m plots, which allows for each finding to be precisely located and documented. During the excavation, “ribs” are left between the sectors for documentation of the profiles and the stages of accumulation of the mound (Figure 2). After completing the excavations in the sectors, the “ribs” are also removed. The original shape of the mound is restored under the supervision of an archaeologist by returning the soil substrate, removed during the excavation, using a bulldozer.

2.2. Data Sampling and Processing

The vegetation sampling of the mounds was performed from May to August 2021. We recorded all vascular plants and bryophytes, as well as their percentage cover, the total vegetation cover, and the average vegetation height, within five 1 × 1 m plots, randomly placed at each facing slope of the mound—north, south, east and west. Data from 20 plots (4 slopes × 5 plots) were collected from each of the 15 selected mounds, so in total, we had 300 plots (15 mounds × 20 plots).
The nomenclature and the native status of the registered vascular plants followed Euro + Med PlantBase [45]. The native status of the species not marked in Euro + Med PlantBase as distributed in Bulgaria (i.e., Allium rotundum, Brassica napus, Euphorbia cyparissias, Leontodon crispus, Leucanthemum vulgare, Vicia narbonensis) was defined according to national literature sources [46,47]. Bryophyte nomenclature followed Hill et al. [48]. The determination of phytogeographical (floristic) elements for bryophytes followed Ganeva and Düll [49]. Data concerning the biological type of the vascular plants were extracted from national literature sources [46]. The biological types were grouped as follows: (1) short-lived plants (annuals and biennials); (2) perennials and woody plants (dwarf shrubs, shrubs, and trees).
Following the suggestion of Prach [50], we classified the established species into three main groups according to their habitat preference: (1) anthropophytes (including ruderals, weeds, and alien plants)—synanthropic species found in heavily disturbed and man-made habitats, or on arable land, accompanying the vegetation of agricultural crops. Most species from this group are listed as diagnostic for anthropogenic vegetation classes, such as Artemisietea vulgaris, Chenopodietea, Epilobietea angustifolii, Papaveretea rhoeadis, Sisymbrietea; (2) grass- and shrubland species—plants typical for open (semi)-natural vegetation (herbaceous and shrub), predominantly listed as diagnostic for the classes Festuco-Brometea, Helianthemetea guttati, Koelerio-Corynephoretea canescentis, Molinio-Arrhenatheretea, Sedo-Scleranthetea, Stipo-Trachynietea distachyae; (3) species typical for woodlands—forest plants, most listed as diagnostic for the classes Alno glutinosae-Populetea albae, Carpino-Fagetea sylvaticae, and Quercetea pubescentis. We joined the last two groups for the purposes of the analysis, as we considered them both as target species for a successful vegetation recovery (Supplementary ESM S1). The association of vascular plant species to higher rank syntaxa was defined following Mucina et al. [51]. In cases when more than one phytosociological class was proposed for a certain diagnostic species, we selected the best representative for Bulgarian vegetation based on our expert knowledge.
For every mound, we calculated the post-excavation (and restoration) period—year of vegetation studies (2021) minus the year of excavation. Then, for the purposes of the analysis, we divided the studied mounds into 3 groups, according to their post-excavation period: (1) less than 10 years ago (5 mounds, 100 plots); (2) 10 to 20 years ago (6 mounds, 120 plots); (3) 20 to 30 years ago (4 mounds, 80 plots).
We calculated the basic features and variability of the data for the above-defined species groups both for all 15 mounds (300 plots) and separately for each group of mounds. Differences in vegetation parameters among the three mound groups were tested with one-way analysis of variance (ANOVA) and Kruskal–Wallis H test carried out in STATISTICA 13 [52]. Tukey’s HSD post hoc test at α = 0.05 was used to identify significant differences among groups.
We used NMDS (non-metric multidimensional scaling) to check the similarity among plots from different facing slopes for each mound, based on the Bray–Curtis similarity index by species composition. We also used similarity percentage analysis (SIMPER) [53] to determine the species that contributed the most to the resemblances between plots from mounds with different post-excavation period. Both the SIMPER analyses and the NMDS-visualizations were carried out in PRIMER 7 [54].
We calculated the area of two land cover categories within a 200 m circular buffer around the base of the mounds: (1) anthropogenic land (including urban areas, annual crops, and different types of perennial cultivation) and (2) (semi)-natural vegetation. The land cover types in the buffer area were taken from the Land Parcel Identification System (LPIS) database (with minimum mapping unit spatial resolution of 0.11 ha) and generalized into two mentioned categories. To test the correlation of defined species group’s richness and cover to (i) the mounds post-excavation period and (ii) the range of anthropogenic land and (semi)-natural vegetation in the buffer area, we used Pearson correlation coefficient in STATISTICA 13 [52].

3. Results

In total, we identified 347 vascular plant taxa and 18 bryophytes in all plots (Supplementary ESM S1). Most of the vascular plants were identified to species level. Only 20 taxa (6%) were determined to genus level because they were either in a phenological stage unsuitable for correct species determination or too difficult to identify with certainty (e.g., Taraxacum spp.). These species were not included in the analyses based on the defined species group’s richness and cover. The majority of the registered vascular plants were native (including archaeophytes) to Bulgaria. Only four of the registered species were alien plants (Erigeron annuus, Erigeron canadensis, Sorghum halepense, and Cuscuta campestris) and two (Brassica napus and Triticum aestivum) were in large-scale cultivation in the country. A major share belonged to the short-lived plants (177 species or 52%), followed by the perennial and woody plants (156 species or 48%). Plants typical for woodlands were represented by 17 species only (5%), while grassland and shrubland species prevailed (177 species or 52%). Registered species related to anthropogenic vegetation were 133 (41%). Plants we considered for target species in the vegetation recovery process were 194 (59%).
The established species were diagnostic for 26 phytosociological classes (Supplementary ESM S1). Dry grassland species, characteristic of class Festuco-Brometea were most numerous (74), followed by species diagnostic to Molinio-Arrhenatheretea (26)—another class widely distributed at low and mid-altitudes in Bulgaria. Among the grasslands and shrublands diagnostic species, a significant number (33 species) belonged to pioneer open dry grassland vegetation (Koelerio-Corynephoretea canescentis and Sedo-Scleranthetea). The strong Mediterranean influence is reflected by the presence of 20 species diagnostic to (sub-) Mediterranean open and semi-open vegetation of five different classes (Supplementary ESM S1). Another substantial share of the established species was diagnostic for several anthropogenic vegetation classes—Papaveretea rhoeadis (42 species), Artemisietea vulgaris (32 species), Chenopodietea (27 species), Sisymbrietea (17 species), Epilobietea angustifolii (5 species), etc. Among species characteristic of woodlands, only five were diagnostic of oak and mixed deciduous forests.
Bryophytes were registered only on the mounds restored before more than 10 years. The listed species include nine pleurocarpic and nine acrocarpic plants. Rhynchostegium megapolitanum had the highest frequency followed by Syntrichia ruralis, Homalothecium lutescens, and Hypnum cupressiforme. The bryophytes registered in the plots are common for the temperate regions and characteristic of dry grasslands of Koelerio-Corynephoretea canescentis and Festuco-Brometea vegetation classes. Most of them are growing in considerably illuminated to full light sites, e.g., Bryum argenteum, Barbula unguiculata, Ceratodon purpreus, Syntrichia ruralis, Wesia controversa, Pleurochaete squsrrosa, so it was expected that these species comprise the moss flora of the mounds. Ceratodon purpureus and Bryum argenteum are pioneers on bare soil, while Rhynchostegium megapolitanum, Brachythecium rutabulum, Homalothecium lutescens, and Hypnum cupressiforme were found in places where other vascular plants and especially grass species have formed ground cover. All mounds where mosses were recorded were surrounded by semi-natural grasslands.
The total species richness per plot varied from 5 to 38 species. This parameter showed no significant correlation with the mound successional age expressed by the post-excavation period, but the mean species richness per plot increased from the early to the advanced successional stages (Table 2). All other correlation analyses between different species groups’ richness and cover and mounds post-excavation period were significant (p < 0.05). The highest positive correlation to the mound’s successional age was registered for the cover of short-lived plants (R = 0.6); the average short-lived plant cover was highest in the first-decade post-excavations and halved with the increase in the mound successional age. On the contrary, both richness and cover of perennial plants were low for the first period after the disturbance of the mounds and increased significantly in the latter successional stages. While species richness of anthropophytes was relatively constant in time, their cover differed greatly—it was highest for the first-decade post-excavations and decreased drastically in the last studied successional stage. As for target species, both their richness and cover increased at a relatively steady rate with the successional age of the mounds (Table 2).
Vegetation cover varied among the observed plots, respectively, among the studied mounds. We found significant differences in vegetation cover throughout mounds from different age groups (Kruskal–Wallis H test = 8.0321, p = 0.018; Figure 3a). Irrespective of the time period after excavation, the studied mound’s vegetation was predominantly closed (70% to 85–90% cover). The highest vegetation cover variation, as well as the highest maximum, median, and mean, was registered for the mounds excavated less than 10 years ago and decreased for the mounds from the other age groups. Results were similar for the average herb layer height (Kruskal–Wallis H test = 14.0393, p = 0.002; Figure 3b). The maximum height of the herb layer was recorded on the more recently recovered mounds, and in the late successional stages, the average height decreased.
The increase in the successional age promoted the differentiation of the species composition between N, S, E, and W expositions (Figure 4a–c). In the first mound group (<10 years of succession), only one of five mounds had a clear differentiation between N- and S-facing plots (Figure 4a). In the second mound group (10 to 20 years of succession), four out of six mounds had a clear differentiation between N- and S-facing plots (Figure 4b). The third mound group (>20 years of succession) represented the advanced vegetation development, and the difference in species composition between N- and S-facing plots was fully established—all four mounds were characterized with clearly differentiated N- and S-facing plots (Figure 4c).
Results from SIMPER analysis demonstrated that the species that contributed the most to the species composition similarity between plots from mounds excavated less than 10 years ago were Papaver rhoeas (14.29%), Xeranthemum annuum (8.33%), Erysimum diffusum (6.78%), Convolvulus arvensis (5.22%), Viola arvensis (5.21%) and Lolium perenne (5.1%). For plots from mounds excavated 10 to 20 years ago, they were Lepidium draba (10.33%), Arenaria leptoclados (6.14%), Dasypyrum villosum (5.97%), Convolvulus arvensis (5.54%) and Geranium dissectum (5.2%). For plots from mounds excavated 20 to 30 years ago Cynodon dactylon (7.26%), Euphorbia cyparissias (6.1%), Xeranthemum annuum (6.06%), and Arenaria leptoclados (5.77%) accounted the most for the species composition similarity.

4. Discussion

The natural vegetation recovery of the explored mounds could be interpreted as an ecological restoration in the sense of Clewell and McDonald [40] because it was facilitated by the intentional restoration of the mound’s shape by returning the soil removed during excavations. The vegetation recovery was accomplished without any manipulation of the biota.

4.1. Vegetation Changes

Our results show that restored mounds host a high number of vascular plants, supporting the opportunity for the preservation of both natural and historical components [14]. Following the current restoration methodology, the mounds were restored by the same soil substrate removed during the excavations. This means that the local soil seed bank of the restored mounds was more or less preserved. Therefore, we expected that after the restoration of the mound’s original shape, the revegetation process was primarily initiated by the soil seed bank [55] and subsequently by the immigration of species from neighboring (semi)-natural territories [56], as observed in similar studies [57].
Weeds and ruderals are well known for their fast regeneration strategy, as well as for their ability to maintain a long-term persistent diaspore bank [56]. Hence, the prevalence of anthropophytes (both in species richness and in cover, Table 1) at the beginning of the revegetation process was an expected result. During the first decade of recovery, the total projective cover of the vegetation was high due to the presence and high abundance of anthropophytes such as Papaver rhoeas, Xeranthemum annuum, Convolvulus arvensis, Carduus nutans, Anisantha sterilis, etc. The average vegetation height during the first decade was also the greatest as a result of the strong competition for light in the dense vegetation cover registered for this period of succession. Total vegetation cover decreased in the following decades. While the species richness of anthropophytes did not differ between different successional stages, their cover significantly decreased in the second- and third-decade post-excavations at the expense of tufted grasses and some forbs. Almost all species involved in the natural vegetation recovery process were native to the country. The presence of alien plants was negligible, and these were species such as Erigeron annuus, Erigeron canadensis, and Cuscuta campestris, widespread in the semi-natural habitats in Bulgaria. The low presence of alien plants was also mentioned for other historical sites [16], but this threat should be considered [15]. The natural recovery process during the second decade was distinguished by modest disturbances, the so-called “intermediate disturbance”, which is known to increase the local number of taxa. During this time, the number and cover of perennial and woody plants increased. The presence of species diagnostic for different vegetation types demonstrated the potential of the mounds to serve as stepping stones for a variety of plant species with a wide range of ecological specifications. The late successional stages of the natural vegetation recovery on the studied mounds were characterized by a closed vegetation cover, usually assessed above 80%, and by the presence of species diagnostic for dry grasslands, representing the most common characteristics of the vegetation in Bulgaria [58]. Apart from grasses, the expansion of forbs and mosses was also observed after the second decade of succession. Research on the recovery success of cryptogams is scarce [59,60]. The results from this study showed that ten years of vegetation recovery were sufficient for the establishment of bryophytes. Our observations confirm the mentioning of Jeschke [61] for possible complete cryptogam restoration in less than 20 years and also support the previous finding of weak or no link between the cover of vascular plants and the cover of bryophytes [59]. The presence of both pleurocarpous and acrocarpous mosses signals a successful recovery [61]. We can only speculate where the bryophyte propagules came from. Some studies show that pleurocarpous mosses could be transported on the fleece of sheep or goats [61]. Supposedly, bryophyte colonization was facilitated by the surrounding patches of semi-natural grasslands, often managed as pastures. The establishment of woody species was low, which could be explained by the strong competition in the rapidly developing herb layer [50]. The dense cover of trees not only reduces the overall biodiversity of the mounds [17], but it could be harmful if underground structures are present [15,34].

4.2. Rate of the Recovery Success

Obtained results show that spontaneous succession starts immediately, and in the second year of natural recovery, we observed an almost completely revegetated mound surface where the evaluated vegetation cover was the highest. Fast recovery of the vegetation cover on the mounds prevents the down wash of the top soil horizon and limits erosion. The total species richness increases during the natural recovery process, and with the progression of the successional stages, the cover of target species increases. A previous study of the flora of Bulgarian mounds established that they maintain a considerable share of the national flora with the prevalence of Festuco-Brometea representatives [23]. Therefore, we expected dry grassland specialists as target species. Considering this, the registered significant number of species diagnostic for Festuco-Brometea and Koelerio-Corynephoretea in the plots prove the establishment of vegetation common for most of the mounds in the country. This also shows that the restored mounds tend to regain their original appearance. The registered fast recovery of target species and the increasing abundance of clonal grassland species during the recovery process is consistent with results from other studies [33,57], where after the third year of succession, the species richness and cover of specialists become apparent.
The site climatic conditions in the surroundings of the studied mounds [42] and the position of most of the mounds close to arable lands indicated a fertile environment. In accordance with the conclusions in previous studies [39,62], which reported that the speed of recovery in highly productive sites is expected to be fast, we observed a relatively fast revegetation process. In about 20 years, the species diversity doubled, and both richness and cover of target species tripled till the late successional stages of the recovery process. In the advanced vegetation of the studied mounds, we observed several competitors, such as Bothriochloa, Stipa, and Festuca, which are also among the target species, which appear as evidence of successful natural recovery with native plants, well adapted to the environment. The results showed that the relatively small mound size favored the fast development of vegetation similar in species composition to semi-natural communities commonly established on ancient mounds in Bulgaria [23]. The advanced stages of the natural vegetation recovery included some rare and important conservation species such as Achillea clypeolata, Linum thracicum, Salvia aethiopis, and Silene frivaldszkyana, which further emphasizes the successful outcome of the recovery.
Studies on spontaneous vegetation recovery in Central Europe show convergent succession inside a locality (except for stone quarries), which was explained by the increasing habitat diversification over time [50]. Habitat heterogeneity was proved to be one of the reasons for greater biodiversity on the mounds and often supports higher plant diversity than in the neighboring plain areas of similar size [63]. We found out that the age of recovery played a substantial role in the establishment of a high rate of species composition heterogeneity on the mounds. Habitats and species assemblages related to N, S, W, and E exposition were better distinguished in mounds excavated before more than 20 years. The difference in species composition between N- and S-faced plots clearly increased along with the time of succession. Habitat heterogeneity was facilitated by the establishment of target species with different ecological requirements. Since humidity is one of their most important requirements, bryophytes were usually confined to the northern and eastern slopes of the mounds.

5. Conclusions

Our study confirms the high efficiency of natural vegetation recovery restored after archaeological excavations in burial mounds. The collected data show that in such circumstances, the so-called “passive restoration” is an effective strategy, even if initial disturbances have been severe. The overall success of recovery we assess as positive, as the restoration of vegetation cover, species diversity, reestablishment of target species, and habitat diversification become evident in a relatively short time. The presence of rare plant species and the low number of alien plants are evidence of a recovery process that is favorable for the local biodiversity. Vegetation recovery prevents the mound’s surface from erosion and thus allows to maintain the landscape integrity. Obtained data could be useful for the selection of species suitable for other revegetation projects. Our results clearly state that in the case of excavated mounds, ecological restoration is a promising perspective. In conclusion, we approve the recommendation for the restoration of ancient mounds after archaeological research, as it will contribute to the conservation of the biodiversity in the area.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/su14127318/s1, Electronic Supplementary Material S1: Original vegetation data from 15 studied mounds in Bulgaria.

Author Contributions

Conceptualization, I.A. and G.N.; methodology, I.A., M.V. and D.S.; formal analysis, M.V. and D.S.; data curation, I.A., M.V., N.V. and A.G.; writing—original draft preparation, I.A., M.V. and D.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Bulgarian National Science Fund (contract KΠ-06-H21/2, 2018).

Acknowledgments

The authors would like to thank Nadya Tsvetkova for her help with the GIS procedures. We thank the two anonymous reviewers for their comments.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. A map of Bulgaria with marked locations of the sampled mounds (red dots).
Figure 1. A map of Bulgaria with marked locations of the sampled mounds (red dots).
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Figure 2. A view of the excavation process of a mound from 2021 with the “ribs” left between the sectors.
Figure 2. A view of the excavation process of a mound from 2021 with the “ribs” left between the sectors.
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Figure 3. Differences in (a) cover of herb layer and (b) average height of herb layer among three mound groups (mounds excavated and restored less than 10 years ago; 10 to 20 years ago and 20 to 30 years ago). Means (square), medians (line), 25th to 75th percentiles (box), non-outlier range (whiskers), outliers (circle) and extremes (asterisk) are shown.
Figure 3. Differences in (a) cover of herb layer and (b) average height of herb layer among three mound groups (mounds excavated and restored less than 10 years ago; 10 to 20 years ago and 20 to 30 years ago). Means (square), medians (line), 25th to 75th percentiles (box), non-outlier range (whiskers), outliers (circle) and extremes (asterisk) are shown.
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Figure 4. NMDS-visualizations of the distances between plots with different exposure (N—north; S—south; E—east; W—west) from three mound groups (mounds excavated and restored (a) less than 10 years ago; (b) 10 to 20 years ago; (c) 20 to 30 years ago), based on the Bray–Curtis similarity index by species composition.
Figure 4. NMDS-visualizations of the distances between plots with different exposure (N—north; S—south; E—east; W—west) from three mound groups (mounds excavated and restored (a) less than 10 years ago; (b) 10 to 20 years ago; (c) 20 to 30 years ago), based on the Bray–Curtis similarity index by species composition.
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Table
Table 1. Basic topographic characteristics of the studied mounds.
Table 1. Basic topographic characteristics of the studied mounds.
Height (m)2D Area (m2)
Age GroupMinMaxAverage ± SDMinMaxAverage ± SD
Mound excavated less than 10 years, n = 52.03.42.3 ± 0.678.5736.4337.5 ± 250.7
Mound excavated between 10–20 years, n = 62.07.94.3 ± 2.378.51481.2588.5 ± 5.9.1
Mound excavated between 20–30 years, n = 42.23.53.2 ± 0.678.5570.9338.6 ± 188.9
Table
Table 2. Characteristics of the studied mounds vegetation. Means and standard deviations are provided. Different letters indicate significant differences between communities at α = 0.05 from Turkey’s HSD test; p-values derived from ANOVAs (**—p < 0.01; ***—p < 0.001).
Table 2. Characteristics of the studied mounds vegetation. Means and standard deviations are provided. Different letters indicate significant differences between communities at α = 0.05 from Turkey’s HSD test; p-values derived from ANOVAs (**—p < 0.01; ***—p < 0.001).
<10 Years10 to 20 Years20 to 30 Yearsp
Number of cases (plots)10012080
Parameter
Total species richness14.3 ±6.3 a18.9 ± 7.5 b23.8 ± 4.4 c**
Species richness of short-lived plants9.9 ± 4.5 a10.45 ± 5.3 a13.7 ± 4.2 b***
Species richness of perennial plants4.3 ± 2.6 a8.2 ± 4.1 b9.8 ± 3.5 c**
Species richness of anthropophytes8.3 ± 2.68.5 ± 3.79.2 ± 2.60.125
Species richness of target species5.9 ± 4.7 a10.1 ± 6.3 b14.3 ± 3.6 c**
Cover of short-lived species 73.4 ± 20.4 a40.8 ± 34.9 b34.6 ± 18.5 b**
Cover of perennial species 16.5 ± 15.0 a50.9 ± 27.4 b50.7 ± 28.4 b**
Cover of anthropophytes 52.6 ±25.6 a43.4 ± 35.9 b22.1 ± 14.9 c***
Cover of target species 37.3 ± 25.3 a48.3 ± 31.0 b63.2 ± 20.8 c***
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Apostolova, I.; Valcheva, M.; Sopotlieva, D.; Velev, N.; Ganeva, A.; Nekhrizov, G. Natural Vegetation Recovery on Excavated Archaeological Sites: A Case Study of Ancient Burial Mounds in Bulgaria. Sustainability 2022, 14, 7318. https://doi.org/10.3390/su14127318

AMA Style

Apostolova I, Valcheva M, Sopotlieva D, Velev N, Ganeva A, Nekhrizov G. Natural Vegetation Recovery on Excavated Archaeological Sites: A Case Study of Ancient Burial Mounds in Bulgaria. Sustainability. 2022; 14(12):7318. https://doi.org/10.3390/su14127318

Chicago/Turabian Style

Apostolova, Iva, Magdalena Valcheva, Desislava Sopotlieva, Nikolay Velev, Anna Ganeva, and Georgi Nekhrizov. 2022. "Natural Vegetation Recovery on Excavated Archaeological Sites: A Case Study of Ancient Burial Mounds in Bulgaria" Sustainability 14, no. 12: 7318. https://doi.org/10.3390/su14127318

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