FLORISTIC DIVERSITY AND THREATS FACTORS TO FLORISTIC DIVERSITY OF THE LEFT BANK OF THE DANUBE AND THE NETWORK OF CANALS OF SUBURBAN SETTLEMENTS OF BELGRADE AND

: The paper presents the floristic diversity of the left bank of the Danube and the canal network of Krnjača and Borča settlements, as well as the factors that endanger it. 216 autochthonous and allochthonous plant species were recorded in the investigated area. A large number of invasive plant species have been observed that spread rapidly and easily forming stable populations that displace native plants. Some of them are: Amorpha fruticosa L., Robinia pseudoacacia L., Ailanthus altissima (Mill.) Swingle, Asclepias syriaca L., Ambrosia artemisiifolia L. and others. The presence of 6 species that are rare and endangered in Serbia is especially significant, as well as 5 relict species. During the investigation, the factors which influence on natural habitats degradation and the reduction of autochthonous floristic diversity were identified. Accordingly, measures for protection and preservation of autochthonous floristic diversity are recommended.


INTRODUCTION
Floristic diversity and plant distribution are one of the indicators of ecological conditions of a certain ecosystem (Ko j i ć et al., 1994). Also, plant species are an important biological and economic resource of every country. Therefore, the preservation and conservation of the autochthonous flora of each area is very important. According to official estimates by the International Union for Conservation of Nature (IUCN), 70% of all known plants are endangered and threatened with extinction (www.iucn.org).
Factors that affect the reduction and loss of floristic diversity are numerous, and some of them are: urbanization, exploitation of autochthonous and introduction of allochthonous species, pollution of water, soil and air, etc. Urbanization completely destroys native habitats, creating semi-natural or artificial habitats that are unfavorable for the survival of autochthonous species. Even if the habitats do not change completely, but only if they become fragmented, significant disturbances occur within the ecosystem (S te va n o v i ć , Va s i ć , 1995). Another factor which influences on the loss of floristic diversity, especially in urban and suburban areas, is the introduction of invasive species. In order to preserve the biodiversity of any area, especially anthropogenic habitats, it is of special importance to prescribe and implement certain protection measures. Measures to protect biodiversity, and therefore floristic diversity, can be preventive and repressive.
An important prerequisite for plant species conservation is the conservation of their habitats. Habitats that are most endangered globally, such as wetlands have particular importance. Wetlands are fragile ecosystems that have been under decades of pressure, such as irrigation and drainage, but also the construction of embankments along river banks. These activities were imposed by the modern way of life, because the drainage of wet

FLORISTIC DIVERSITY AND THREATS FACTORS TO FLORISTIC DIVERSITY OF THE LEFT BANK OF THE DANUBE AND THE NETWORK OF CANALS OF SUBURBAN SETTLEMENTS OF BELGRADE AND
B. Sc, Suzana Petrović, JKP "Zelenilo -Belgrade" PhD Ivana Bjedov, Associate Professor, University of Belgrade -Faculty of Forestry (ivana.bjedov@sfb.bg.ac.rs) PhD. Dragica Obratov-Petković, full professor, University of Belgrade -Faculty of Forestry MSc Verica Stojanović, Institute for Nature Protection of Serbia Abstract: The paper presents the floristic diversity of the left bank of the Danube and the canal network of Krnjača and Borča settlements, as well as the factors that endanger it. 216 autochthonous and allochthonous plant species were recorded in the investigated area. A large number of invasive plant species have been observed that spread rapidly and easily forming stable populations that displace native plants. Some of them are: Amorpha fruticosa L., Robinia pseudoacacia L., Ailanthus altissima (Mill.) Swingle, Asclepias syriaca L., Ambrosia artemisiifolia L. and others. The presence of 6 species that are rare and endangered in Serbia is especially significant, as well as 5 relict species. During the investigation, the factors which influence on natural habitats degradation and the reduction of autochthonous floristic diversity were identified. Accordingly, measures for protection and preservation of autochthonous floristic diversity are recommended.
Key words: floristic diversity, forland, canal network, factors threatening flora, floristic diversity protection measures. habitats provided space for the expansion of urban settlements, agricultural areas, etc. However, this leads to drastic changes in habitat conditions and the extinction of a large number of species.
Floodplains along riverbeds represent extremely important areas from the aspect of biodiversity protection and natural values. The basic values of this area are related to the preservation of the specific phenomenon of periodic flooding in the river bank (Institute for Nature Conservation of Serbia, 2013).
The subject of the research is floristic diversity of the Forland on the left bank of the Danube near Belgrade and the network of canals of the suburban settlements Borča and Krnjača. Forland is an integral part of the ecological corridor of international importance. The corridor consists of the Danube River and green belts that stretch along it. Forland, as an extension of green areas along the Danube, is an important locality within the corridor that significantly supports the migration of plant and animal species. Forland on the left bank of the Danube near Belgrade, Gornji Podunavlje and Koviljsko -Petrovaradinski rit are the largest flood zones in Serbia. This vast floodplain performs the function of mitigating the effects of high waters on the population and the surrounding economy. In addition, natural and semi-natural habitats in Forland filter and purify water through biological processes. In the investigated terrain, the floodplains were reduced to narrow belts along the Danube River, which led to a significant biodiversity loss.
In accordance with the stated facts, the aim of the paper is to investigate the floristic diversity of the mentioned area and to determine the factors that endanger plant diversity in order to define measures for the protection of native floristic diversity.

MATERIAL AND METHODS
Floristic and vegetation research includes the left bank of the Danube, between the riverbed of the Danube and the built embankment, observed downstream from Pupinovo to Pančevo' bridge, in the length of about 9 km, as well as the network of canals in the Borča and Krnjača settlements. The borders of the investigated area are determined by the natural borders that make up the Danube on the southwest side, the northwest border is a new road to Pupin's bridge, and the northeast is Zrenjanin road. In the northern part, there is a loop in which these two roads merge (Figure 1).

Floristic diversity of the invastigated area
216 autochthonous and allochthonous plant species have been recorded in the wider Forland area during field research and the study of the existing literatures. From the middle of the 19th century until today, 2.000 plant species have been described on the territory of Belgrade. By comparing these data, we can say that 10.8% of the entire flora of Belgrade is recorded within investigation area.
In the Forland zone, autochthonous woody hygrophilous species which are characteristic for wet habitats have been observed. The most numerous are: Alnus incana (L.) Moench, Salix alba L., S. triandra L., S. euxina IV Belyaeva, Populus alba L., P. nigra L., Fraxinus angustifolia Vahl and Quercus robur L. However, adventitious, invasive plants are dominated within this area. Some of them are: Fraxinus pennsylvanica Marshall, Acer negundo L., Amorpha fruticosa L., Robinia pseudoacacia L., Ailanthus altissima (Mill.) Swingle, Vitis riparia Michx., Ambrosia artemisiifolia L., Asclepias syriaca L., Solidago gigantea Aiton etc. Invasive species Amorpha fruticosa L. forms the most numerous populations in the forland zone ( Figure 2). It was noticed that all invasive plants spread very quickly in the researched area and displace the autochthonous flora.
Within Forland, the ponds represent ecosystems that especially contribute to the richness of native flora, but also biodiversity in general. Within the forests, or in some places near the embankment, ponds appear periodically or throughout the year. According to previous research (Institute for Nature Conservation of Serbia, 2013), two species of water lilies Nymphaea alba L. and Nuphar lutea (L.) Sm. have been recorded in the ponds. However, during the field research, they were not found in the forland ponds, but they were observed a little further north than the boundary of the study area. Lemna minor L. has been recorded on the surface of the ponds, and Typha spp. and Rorippa amphibia (L.) Besser have been recorded on the peripheral parts of the ponds. In the forland zone, a large number of native woody plant species have been recorded, such as: Cornus mas L., Corylus avellana L., Juglans regia L., Prunus cerasifera Ehrh., Rosa canina L ., Rubus caesius L., Sambucus nigra L.
The main function of the canals of the suburban settlements (Borča and Krnjača) is draining the surrounding terrain, but these canals also play an important role in protection of plant species. The floristic composition of the Krnjača and Borča canals is dominated by ruderal species, such as: Galium mollugo L., Plantago major L., P. lanceolata L., Urtica dioica L., Malva sylvestris L., Leontodon taraxacum L., Cichorium intybus L., Artemisia Figure 2. Population of invasive species Amorpha fruticosa in forland zone vulgaris L., Euphorbia cyparissias L., Calystegia sepium (L.) R. Br., Trifolium pratens L., Dactylis glomerata L. etc. It is important to mention that the species Iris pseudacorus, which is a rare, vulnerable and endangered species in Serbia, was also recorded in these habitats. Lemna minor L. and Ceratophyllum demersum L., Salvinia natans (L.) All., Trapa longicarpa Jankovic, Phragmites australis (Cav.) Steud and Typha spp. were found in the canal waters. Species Nuphar lutea (L.) Sm., Nymphaea alba L. used to grow in the canals, but they are now found in the canals north of the study area, but not within the study area. Some of the woody species recorded along the canals are: Salix alba L., S. euxina IV Belyaeva, Populus alba L., P. nigra L., Betula pendula Roth, Corylus avellana L., Junglans regia L., Sambucus nigra, Morus alba L., Prunus cerasifera Ehrh., Cornus mas L., Rubus spp., Robinia pseudoacacia L., Amorpha fruticosa L., Acer platanoides L., Ulmus laevis Pall., Syringa vulgaris L., Ailanthus altissima (Mill.) Swingle etc. Amorpha fruticosa L. is also one of the most dominant invasive species here. Robinia pseudoacacia L. and Ailanthus altissima (Mill.) Swingle also occur in large numbers (Fig. 30). In addition to the mentioned species, some herbaceous invasive species were also found: Ambrosia artemisiifolia L., Solidago gigantea Aiton, Symphyotrichum lanceolatum (Willd.) G. L. Nesom, Erigeron canadensis L., Asclepias syriaca L. and others.
Relict species give a special feature to the flora of each area, as a significant biological resource.
Five relict species were found in the study area: Juglans regia L., Carpinus betulus L., Hedera helix L., Helleborus odorus Willd., Humulus lupulus L. In addition to relict species, species protected by law as rare, endangered and vulnerable are of great importance for the flora of this area. These are: Iris pseudacorus L. (Figure 3a), Trapa longicarpa Jankovic (Figure 3b), Hypericum perforatum L., Althaea officinalis L., Crataegus monogyna Jacq. These species are on the list of the strictly protected and protected wild species of plants, animals and fungi ("Official Gazette of the Republic of Serbia", No. 5/2010, 47/2011, 32/2016and 98/2016.

Factors threatening floristic diversity
Intensive anthropopression in this area have caused the degradation of natural habitats. Habitats have been degraded by urbanization, drainage and the raising of embankments, the introduction of allochthonous plants, many of which are invasive, and the planting of Euroamerican poplars plantations. All this has led to a decrease in the distribution and disappearance of some allochthonous plants.
Urbanization is a process that leads to changes in the floristic composition of all habitats in an area. The number of plant species is not declining, but the proportion of native species in the total number of plants decreases (Yo u h u a et al., 2012). Introduction of alien species, and many of them are invasive, presnts another factor that negatively affects the autochthonous floristic diversity. Spread of invasive alien species threatens other species and biodiversity generally (O b ratov-Pet kov i ć et al., 2009). Elodea canadensis, a very aggressive invasive aquatic species, was found in the forland ponds and in the canals of suburban settlements in the investigated area. It is a species of North American origin that rapidly colonized aquatic habitats in many European countries and caused serious environmental, sociological and economic problems (Ko l a d a , Ku t y ł a , 2016). The presence of the mentioned species could be one of the causes of Nymphaea alba L. and Nuphar lutea (L.) Sm. displacement from these habitats, where they were previously recorded (Institute for Nature Conservation of Serbia, 2013). The following invasive plants were recorded in the study area: Amorpha fruticosa L., Acer negundo L., Fraxinus pennsylvanica Marshall, Ailanthus altissima (Mill.) Swingle, Robinia pseudoacacia L., Asclepias syriaca L., Vitis riitis Michx., Echinocystis lobata (Michx.) Torr. & A. Gray, Ambrosia artemisiifolia L., Solidago gigantea Aiton, Symphyotrichum lanceolatum (Willd.) GL Nesom, Erigeron canadensis L. Field research has shown that restoration of native white willow (Salix alba) forests is hampered by the spread of invasive species, especially Fraxinus pennsylvanica and Amorpha fruticosa L. (Figures 9 and 10). Also, by comparing field re-search with previous research (Institute for Nature Conservation of Serbia, 2013), it was found that the species Populus alba L. and P. nigra L. had many wider distributions in these areas. One of the reasons is the cutting of these species due to the planting of Euroamerican poplars (Populus x euramericana) plantations, but also the pressure of invasive plants on the habitats of allochthonous Populus species. In this area, the habitat of Fraxinus angustifolia Vahl is threatened due to the spread of the invasive species Fraxinus pennsylvanica, which is planted as a fast-growing species. The species Fraxinus angustifolia Vahl has been displaced to such an extent that it is necessary to consider the protection of its habitats in investigated area (Puhalo et al., 2010). Quercus robur L. is most abundant in the central part of the forland. The spread of this species is limited by cutting and planting poplar plantations, draining floodplains and invasive woody plants introduction.
Within the studied area, large areas are under agricultural plantations. So, these areas have changed their purpose in the past, and natural habitats have been completely degraded and native species have been displaced. The planting of Euroamerican poplars plantations has caused the degradation of the habitats of a large number of autochthonous species in the forland area ( Figure  4). Planting poplar plantations leads to significant changes in the floristic composition of a particular habitat, most often in terms of declining floristic and vegetation diversity (J u ri š i ć, 2015).

Figure 4. Euroamerican poplars (Populus x euroamericana) plantation in forland zone
Also, the pollution of the canals of suburban settlements, pesticides that are used to treat agricultural crops, but also waste that is thrown into the canals is one of the reasons for the displacement and disappearance of native plants.
Low public awareness of the importance of conservation certain species and their habitats also presents one of negative factors that affect the reduction of floristic diversity of the study area.

Measures to protect floristic diversity
According to the General Plan of Belgrade until 2021, it is envisaged that this area will remain a green area (Official Gazette of Belgrade, no. 27/2003, 25/2005, 34/2007and 63/2009). The green belt mitigates the negative effects of the increased water level of the rivers, represents a habitat for many endangered and protected plant species, forms a balance against the urban tissue and is part of the green corridor. However, as already written, this area is dominated by altered and disturbed natural habitats. Given the continuous and constant anthropogenic pressure, it is important to take appropriate measures that would lead to biodiversity conservation. This primarily means the revitalization of natural habitats or the conservation of existing native habitats. In order to the measures have adequate results, different socio-economic measures that require organizational and material engagement by local self-government, competent services and economic entities should be undertaken.
Some of the measures that can be proposed for biodiversity conservation are: • Increase of areas under autochthonous plant communities, • Restoration and conservation of pond ecosystems, • Removal of invasive plants from habitats, primarily those that are mostly colonized, which will enable natural regeneration of populations of native plant species, • Restoration of devastated forest habitats.
Partially preserved natural floodplain forests and meadows should be placed under such a level of protection that would prevent their further degradation, • Significantly reduce the area under clonal Euroamerican poplar plantations. In that sense, cooperation with PE "Srbijasume" is necessary; • Prevention of canal pollution, as well as their regular cleaning, • Prevention of unplanned construction and resolving the status of illegally constructed facilities, • Education and informing the local population and competent services about the importance of protection and conservation of natural habitats and autochthonous species, • Strengthening the awareness and will of citizens to preserve the entire environment, including floristic diversity, • declaring invasigated area as a protected area.

CONCLUSIONS
216 plant species have been recorded in the study area, of which 5 are protected by law and 5 species are relicts. The investigated area is characterized by significant degradation of native habitats, which caused the displacement and disappearance of certain autochthonous plants. Urbanization, introduction and spread of invasive plants, planting Euroamerican poplar plantations, draining wetlands and forming agricultural land are factors that threaten native floristic diversity.
In order to prevent further displace or loss of autochthonous species in the study area, appropriate measures should be undertaken. These measures include activities such as restoration of native habitats, reduction of areas under Euroamerican poplars plantations, prevention of introduction and removal of invasive plants from all habitats, cleaning of the canal network, as well as strengthening public awareness of the importance of conservation native habitats and autochthonous plants.

SUMMARY
The paper presents a study of the flora of Forland on the left bank of the Danube in Belgrade and the canal network in the suburbs Krnjača and Borča. The aim of this study was to examine the floristic diversity and its endangerment in the study area in order to determine adequate measures for the protection and preservation of autochthonous flora.
The natural habitats of the investigated area are mostly degraded. This influenced the extinction and displacement of certain plant species from sudied area. The distribution of many autochthonous plants is significantly limited. With regard to studied decades of anthropopression, habitat degradation factors have been identified in the study area: urbanization, drainage of wetlands, large areas under agricultural plantations, planting of clones of Euroamerican poplars, introduction of invasive plants and canal network pollution. Habitats have been degraded to such an extent that the biodiversity of the entire area is endangered. In addition to the negative impact on biodiversity, all these factors have disrupted the aesthetics of the landscape, as well as the quality of life. In accordance with the current situation, adequate measures have been proposed that are realistic to implement in order to protect and preserve the autochthonous flora, and biodiversity in general. Some of the measures are: restoration of natural habitats, prevention of introduction and spread of invasive plants, reduction of areas under plantations of clonal Euroamerican poplars, cleaning of canals and education and informing the public about the importance of protection of autochthonous species.