INVASIVE PLANTS AS A FACTOR OF FLORISTIC DIVERSITY DEGRADATION IN DELIBLATO SANDS SNR

: The floristic diversity of Deliblato sands SNR is significantly endangered by the spread of invasive plants. In addition to field research, which included the collection of plant material in the area of Deliblato sands SNR, primary and secondary data was collected in this paper. The primary data for the purposes of this study was obtained by applying a questionnaire technique to the management of the protected area - PE “Vojvodinašume” and an expert in terview technique to a representative of the Provincial Institute for Nature Conservation. The secondary data was collected to gain a better and broader understanding of the management of Deliblato sands SNR. as well as the manner of their propagation and reproduction was carried out, in order to make a recommendation on the preventive measures for the protection and suppression of invasive plant species and to improve the habitat. In the area of Deliblato sands SNR, 39 invasive plant species (4,33% of the total flora) were found with different invasiveness categories, among which herbaceous, annual, North Ameri can species from the Compositae family prevail. Protection measures include the mapping of habitats of invasive species, establishing cooperation with managers and scientific institutions, constant monitoring of endangered habitats and plant species, as well as the creation of a special sector responsible for enhancing biodiversity.


INVASIVE PLANTS AS A FACTOR OF FLORISTIC DIVERSITY DEGRADATION IN DELIBLATO SANDS SNR
In the area of Deliblato sands SNR, 39 invasive plant species (4,33% of the total flora) were found with different invasiveness categories, among which herbaceous, annual, North American species from the Compositae family prevail. Protection measures include the mapping of habitats of invasive species, establishing cooperation with managers and scientific institutions, constant monitoring of endangered habitats and plant species, as well as the creation of a special sector responsible for enhancing biodiversity.
Keywords: Deliblato sands Special Nature Reserve, floristic diversity, invasive alien plants, protection measures.

INTRODUCTION
The Republic of Serbia is one of the global centres of plant diversity. An 18% share of the total vascular flora of Europe, and 1,7% of the total world vascular flora is recorded in the territory of Serbia (2011/c).
The factors that threaten floristic diversity are related to the overall biodiversity. The threatening factors comprise the following activities: the change of land use purpose (habitat loss, fragmentation and habitat degradation), plantation cultivation of monocultural plant species, expansion of agricultural land, settlements and industrial areas, the over-exploitation of biological resources, the introduction of allochthonous species and genetically modified organisms, the pollution of air, water and soil, climate change and the changes of natural water levels (M i j ovi ć, 2012).
One of the measures of biodiversity protection for an area is to declare that area protected. This limits the use of natural resources as well as activities that threaten biodiversity.
In this paper an investigation of invasive plants of Deliblato sands Special Nature Reserve is presented. Deliblato sands Special Nature Reserve (SNR) is located in the Autonomous province of Vojvodina, in the southeastern part of the Pannonian Plain in south Banat. Its total area is 34.829,32 ha (Stjepanović -Veseličić, 1953;G a j i ć , 1983;. Deliblato sands is characterized by a relief, composed of sand dunes and depressions between dunes. These results in a huge difference in altitude, from 2 to 30m. Chernozem on carbonate aeolian sand (Ko š a n i n , 2002), as well as the disposition and type of vegetation, makes microclimate and environmental conditions unique in Europe (G aj i ć, 1983; 2016/b). For these reasons, in 1977, this area was designated as a Special Nature Reserve (Ku zman ovi ć, 1994; Medarević et al., 2004).
Inside the boundaries of Deliblato sands SNR there are numerous endemic, relict, endemo-relict species, as well as protected and endangered plant species. The importance of conservation of the Deliblato sands SNR has been recognized by many international organizations, and this area has been designated as one of the Important Plant Areas (IPAs) as well as an Important Bird area (IBA). In addition, inside the boundaries of Deliblato sands SNR, the Ramsar site "Labudovo okno" is situated. Deliblato sands SNR was also proposed for the UNESCO's Man and the biosphere programme (MAB) in 1997. (P u zovi ć, Pan j kovi ć, 2015).
The aim of this paper is to present the current state of floristic diversity of the Deliblato sands SNR, to investigate invasive plant species as a threatening factor of floristic diversity and to propose adequate protection measures.

MATERIALS AND METHODS
To analyse the floristic structure, field investigations were conducted during the vegetation season in 2016. The collected plant material was determined and herbarized. The analysis was performed by the standard floristic method, using relevant literature (Stjepanović -Veseličić, 1953; J o s i fov i ć , 1970-1986; J ávo r ka i C s a p od y, 1975; Gaj i ć, 1983; Sari ć, 1992).
In this paper, primary and secondary data was collected in addition to the field investigation. The primary data for the purpose of this study was obtained by applying the questionnaire technique, in cooperation with the management of PE "Vojvodinašume" and by conducting an expert interview with a representative from the Provincial Institute for Nature Conservation. The questionnaire was completed and it is composed of three groupes of questions. The first group was related to the protection of floristic diversity, the second group to the cooperation of management and local business of Deliblato sands SNR and the third set of questions was related to the threatening factors of Deliblato sands SNR.
The expert interview is an interview that focuses on the problem ("the problem-centred interview"). Using this technique, the researcher does not represent himself but provides expertise in the context of its institutional framework and organizational structure (M e u s e r and N a g e l , 1991). This expert interview aims to obtain deeper, concrete and qualitative information about a better understanding of protection of the floristic diversity and the invasive alien plants.
The purpose of secondary data is to have a better understanding and a broader view of the processed topic. This data includes the literature that presents the basis of research. For the purpose of a complete analysis of invasive plant species, their origin, the most represented life form, the way of reproduction, the degree of invasiveness and the other relevant information, the database "List of invasive species in AP Vojvodina" was used, as well as publications that deal with this topic (S ko čaj i ć et al. 2008;An a čkov et al., 2013;Ni ko l i ć et al., 2015).
The following documents and articles of the Law were analysed and researched to support the findings in this paper; the Law on Nature Conservation ("Official Gazette of the RS" No. 36/2009, 88/2010, 91/2010and 14/2016

The results of analysis of primary and secondary data
Based on the analysis of the data obtained by the questionnaire and the content of the expert interview, results indicate that natural factors threaten the flora of Deliblato sands SNR. One of them is frequent fires that occur due to the climatic conditions of the environment (high temperature during the summer) and the morphology of plants which inhabit this area (dried, base herbaceous vegetation). Then, the successive growth phase of shrub forms -Crataegus monogyna is also a threat to the biodiversity of Deliblato sands SNR. Namely, the shrub species are growing and expanding at the expense of the habitat of steppe herbaceous species, which is very important for floristic diversity of Deliblato sands. This problem incurred the prohibition of grazing in certain areas of Deliblato sands.
In addition to natural factors, the anthropogenic factors which threaten the biodiversity and floristic diversity of the Deliblato sands SNR are: collecting herbs, tourist activities, the introduction of allochthonous species and invasive species, illegal landfill, illegal construction (capture and disruption of natural habitats), sand opening (removal of the soil surface layer, the disturbance of natural habitats).
However, regardless of the many factors that threaten the floristic diversity of the area, the impact of invasive plant species can be clearly distinguished. The manager of Deliblato sands SNR recognized invasive alien plants as the leading threatening factor of floristic diversity of this area. The claims of experts were confirmed in the field research.
The presence of invasive alien plants is noticed by forest roads, as well as at steppe habitats, where these species are the most dominant.
Invasive alien plants are equally common on sand, forest and as well on wetland habitats which are on the border of the natural reserve. The localities with the most numerous populations of invasive plant species are situated in the areas of intensive human activity, then, in the contact zone of the natural reserve area and agricultural areas. The worrying fact is that locals from the settlements of Šušare and Deliblato grow some of the invasive species as ornamentals, and it contributes to their existence and spread outside the boundaries of the reserve. Invasive species make up 4.3% of the total plant species in the area of Deliblato sands SNR (Graph 1).

Graph 1:
The percentage of invasive plant species; Source: original According to taxonomic affiliation, the most invasive plants belong to the family Co (10 species). Then, according to the abundance the Amaranthaceae family (6)  Graph 3, presents the life forms of invasive plant species, showing that the most numerous species are actually herbaceous, annual plants (21), then herbaceous perennials (9), within which there are also biennial, herbaceous plants. When woody life forms are concerned, 6 invasive species are present, while the total of shrub woody life forms is 3.
According to origin, 17 species out of the total of 39 invasive plant species are from North America. 5 invasive species are native to North and South America, while only 2 are native to South America and 3 are native to North America, Australia and Oceania. Further, a considerable number of invasive plant species originate from Asia (4), Asia and North America (1), Asia and Europe (2). The other 5 species originate from different continents and different parts of Europe (Graph 4).
Graph 5 shows the way of introducing invasive alien plants to the investigation area. A total of 16 species were accidentally introduced, while 13 species were intentionally introduced in order to naturalize the species. For the remaining 9 invasive plant species, the way of introduction is unknown. Of all the invasive plants recorded in the study area, a total of 14 species spread "naturally"spreading by seeds, rhizomes, stolons and root shoots. Twelve species are expanded through seed trade by plants culture 12 species expanded through trade in seed crops ("transport spread"). Seven invasive species were introduced as cultivated to be cultivated for food, pharmaceutical and other purposes ("runaway species"). Two invasive plant species were cultivated in gardens, from which they spread to other habitats ("exempt species"). For 7 species the mode of propagation is unknown (Graph 6).
The degree of invasiveness of analysed plants is presented in Graph 7. Out of the total of 39 species, the degree of invasiveness is determined for only 20 species.
Highly invasive species recorded during field research are: Acer negundo, Ailanthus altissima and Amorpha fruticosa. These plants had wide use in landscape architecture. They are used in parks, alleys and private gardens. Species Erigeron annu- It has been recorded on steppe and sandstone habitats, and along forest paths. Ambrosia artemisiifolia and Asclepias syriaca are very frequent invasive plants in the territory of Deliblato sands SNR. The employees from PE "Vojvodinašume" describe Asclepias syriaca as a very aggressive plant. In the past, it was cultivated for beekeeping purposes (honey plant), and its mechanism of propagation (fruit cracking upon drying) has made it a highly invasive plant species. Robinia pseudoacacia belongs to the group of "highly invasive species". It has been used for sand bonding in the fight against aeolian erosion for almost two centuries. Strong offspring ability prevents the management from removing this species from the area of Deliblato sands SNR. The "Sporadically Invasive Species" Oenothera biennis and Solidago canadensis were introduced because of their decorative characteristics. Despite the fact that they are invasive plants, these two species are still cultivated in the gardens as decorative in the area of Deliblato Sands. One of the "Potentially Invasive Species" is Elaeagnus angustifolia which was introduced and cultivated as a decorative species and honey plant. According to the manager of the SNR, this species has a negative impact on herbaceous and shrubby autochthonous vegetation. An invasive species of Datura stramonium, which can spread rapidly and successfully, is cultivated as a decorative species in the wider area of Deliblato Sands. In addition, this species has important applications in the pharmaceutical industry.

Mechanisms of invasive plants spread
Invasive plants are species that adapt rapidly to new environmental conditions, forming stable populations, threatening native species and their habitats. Their aggressiveness is reflected in the spread mechanism, which depends on the adaptive morphological characteristics of the species. The analysis of the obtained data, the most common mechanism of the survival and spread of invasive plant species are a way of pollination, a large number of seeds production, vegetative propagation and intensive root spread.
Ailanthus altissima, Datura stramonium and Solidago canadensis have been developed by the self-pollination (autogamy) mechanism. The insect pollination or entomophily is the most common type of pollination of invasive plant species. A total of 28 invasive species are pollinated by insects. The second most common type of plant pollination is by wind (anemophily). 18 invasive alien plants Seed dispersal and fruit scattering are some of the mechanisms of spread. A total of 3 invasive species have developed a mechanism of self-dispersal by seed (autochory): Consolida orientalis, C. regalis and Cytisus scoparius. Out of the total of 39 invasive plant species, the seeds of 24 species are scattered by animals (zoochory), seeds of 21 are scattered by wind (anemochory), seeds of 8 species are scattered by water (hydrochory), while seed despersal of 12 invasive plant species depends on human activity (anthropochory). In addition, invasive plant species use 3 or 2 seed dispersal and fruit scattering mechanisms at the same time, which contributes to their aggressiveness.
The production of a large number of seeds per individual is one of the mechanisms that provide rapid species spread. For example, Ailanthus altissima produces about 325,000 seeds per individual, Amaranthus retroflexus produces about 500,000 seeds per individual and Conyza canadensis produces up to 200,000 seeds per individual (2016/c).
Five invasive plant species within the investigated area propagate vegetatively by root shoots. Four species reproduce by rhizomes.
Given that, as noted, invasive plants present a major threat to the autochtonous plant diversity, appropriate conservation measures need to be implemented. The implementation of laws and regulations, strategies and management plans for the protected area play a very important role in the protection of biodiversity. The problem of invasive species is primarily recognized by the national Law on Nature Protection ("Official bulletin RS" 36/2009RS" 36/ , 88/2010RS" 36/ , 91/2010RS" 36/ and 14/2016. Within this Law, according to Article 82, the introduction of allochthonous species into natural areas is prohibited. The Biodiversity Strategy of the Republic of Serbia for the period 2011 to 2018 (2011/c) prescribes biodiversity protection measures concerning invasive species. For the investigated area of Deliblato sands SNR, an important document in the fight against invasive plant species is the "Management Plan for Deliblato sands SNR for the period 2011 to 2020" (2011/a). Within this plan, PE "Vojvodinašume" as the manager envis-ages measures of fight and suppression of invasive plant species. All three documents can be actively applied.

DISCUSSION
In the territory of Vojvodina, most of the invasive plant species belong to the Compositae family (A n a č ko v et al., 2013; 2016/c), which is also the case in the area surveyed of Deliblato sands SNR. These species, known as cosmopolitan, are widespread in almost all climate zones, indicating their great adaptive capacity. This is the reason why species from Compositae family grow on all habitat types. In addition, these species produce fruits over the whole vegetative period, from May to October, which is another important strategy for their spread. In some species, such as ragweed, seed germination is maintained at low temperatures as well as after plant fading (2016/c). As mentioned above, many species of the Compositae family are cultivated as decorative, which also contributes to their spread.
Other invasive plant species can be found in all habitats of Deliblato sands SNR, but mostly in the areas of intensive anthropogenic influence. They are recorded along forest roads, at crossings between steppe and forest habitats, in the contact zone of the reserve and agricultural land, but also near watercourses. According to the manager and to the document "Management Plan of Deliblato sands SNR for the period from 2011 to 2020" (2011/a), the monitoring of the reserve is carried out once a week. Management plans, related to the investigated area are implemented in accordance with the law, but also in accordance with the natural, environmental, social and economic circumstances. The formatting of a sector for the suppression of invasive species is one of the possible solutions for the protection of floristic diversity. However, the problem is the lack of staff, because they control the area of 35,829 ha. In addition to that, issues and/or unforeseen circumstances may occur during the implementation of the management plans such as catastrophes or disasters such as fires or floods. In this case, adaptive management is applied according to the current situation.
Although the Law on Nature Conservation ("Official Gazette of the RS" No. 36/2009, 88/2010, 91/2010and 14/2016) clearly prohibits the introduction of allochthonous species into protected areas, in the earlier years they were used for afforestation purposes. One of the biggest problems with controlling the spread of invasive plants is that the locals cultivate some invasive plants as ornamentals, and the steering influence is impossible in those cases. This issue was considered in a study Skočajić et al. (2008), where a considerable number of plants reported to have been introduced for horticulture purposes subsequently showed their invasive characteristics. In addition to decorative species, such as: Amorpha fruticosa, Asclepias syriaca, Erigeron canadensis, Robinia pseudacacia and Solidago Canadensis are honey species (Ni ko l i ć et al., 2015) and they are highly represented in the reserve area and in settlements of the study area.
For these reasons, it is necessary to educate the local population, but also to control and monitor nearby nurseries that produce these species. To raise awareness and provide relevant information, it is necessary to map endangered biotopes and create an electronic database with easy access to the general public. After the mapping of endangered biotopes, it is also possible to isolate the endangered localities or certain smaller areas, in the form of experimental fields, where adequate suppression measures would be applied depending on the population of the invasive species, life forms and the degree of invasiveness.
The revitalization of certain habitats is one of the conservation measures implemented in the territory of the investigated area. This measure is implemented by PE "Vojvodinašume" and the Provincial Institute for Nature Conservation.
Steppe habitats, or more specifically the habitats of Paeonia officinalis subsp. banatica, are highly endangered due to the spread of invasive shrub species. Active conservation measures have proven to be a very successful way to combate invasive plants, but also a way to preserve and restore the native flora of the sands. However, such actions depend on the available financial resources, which is why they are not frequent.
In addition to that, the "Vojvodinašume"PE performs revitalization of pastures by establishing sustainable livestock farming that presents one of the active ways of habitat protection. In this case, the best solution is to control grazing in late spring or summer, after most of the endangered, rare, relict and endemic species complete their flowering and fruiting phases.

CONCLUSION
In accordance with the conducted research and the results obtained, the following conclusions can be drawn: • Deliblato sands SNR is characterized by a high degree of floristic diversity. • The factors that threaten the floristic diversity are a collection of medicinal plants, excursion activities, the introduction of invasive species, wild landfills, illegal construction and the opening of sand lakes. • According to the field observations, interviews with experts and managers, invasive plants present the dominant factor threatening to the floristic diversity of Deliblato sands SNR.
In the area of Deliblato sands SNR, a total of 39 invasive plant species were recorded in all habitat types (steppe, sandstone, forest and wetland), which makes 4.33% of the total flora of Deliblato sands. • The largest number of invasive species recorded in the territory of the "Deliblato sands" SNR belongs to the Compositae family. • The most common mechanisms of the survival and spread of invasive plant species in the area of the "Deliblato sands" SNR are: pollination, seed and fruit dispersal, vegetative propagation, and intensive root system spread. • In order to mitigate the impact of invasive plant species on the biodiversity of the Deliblato sands SNR, the implementation of the existing laws and regulations is necessary, the establishment of cooperation among the manager of the SNR, professional and scientific institutions, and the local population, as well as the creation of a separate sector whose task would be to improve biodiversity.
Anja Kalinić, Ivana Bjedov, Dragica Obratov-Petković, Jelena Tomićević-Dubljević Furthermore, it is important to educate and inform the public about all the negative effects that invasive plants can have. In addition, it is important to map endangered biotopes and create an electronic databases of the invasive plants that would be easily accessible to the general public. • When it comes to invasive plant species, direct control, the monitoring of endangered habitats, the protection of "cleared" habitats and potentially endangered habitats and the revitalization of indigenous species are crucial.