Invasive Species and Maintaining Biodiversity in the Natural Areas – Rural and Urban – Subject to Strong Anthropogenic Pressure

Expansion of invasive species can be clearly seen all over Poland. Foreign tree and herbaceous plant species are effectively taking over more and more habitats competing with native vegetation. This phenomenon is strongly pronounced in the areas subject to strong anthropogenic pressure. The presence of invasive plants replacing the native vegetation is a threat for biodiversity and ecological balance. The research carried out by the authors between 2011 and 2017 on selected sites (comparatively: urban and open spaces, including a 600 ha park and a 10 km long forest strip along a river) confirms the pressure exerted by invasive species irrespective of the natural conditions of a particular site or its type – in each case it is most prominent in areas where vegetation is not properly maintained or where it is not maintained at all. The research was based on the dendrological inventories and phytosociological assessments. The inventories were used for a detailed assessment of both the condition and structure of treestands, including accounting for invasive species. Phytosociological assessment can, among others, form a basis for forecasting ecological stability of individual plant communities. Uncontrolled expansion of invasive species, especially in the areas of strong anthropogenic pressure, may cause unfavourable natural succession and in consequence – destabilisation of ecological system in a given area.


INTRODUCTION
Invasive plant species have been introduced into environment by man -either deliberately (by introducing utility plants) or inadvertently (accidentally introduced species).Currently, approximately 12 thousand foreign species have been identified, 10-15% of which are believed to have a negative influence of varying degree (Regulation (EU) No 1143/2014 of the European Parliament and of the Council of 22, October 2014).Such plants are mainly species cultivated in a controlled manner (agricultural cultivation, botanical gardens) which have found their way into the environment, such as Impatiens parviflora DC. -imported into botanical gardens in the 19th century from central Asia, now commonly seen in European forests and parks (Gwiazdowski, 2014).Some species, such as Prunus serotina (Ehrh.)Borkh., initially planted for practical reasons (acquiring precious wood, recreating tree-stands e.g. in industrial areas) 1 , then to improve biotic communities (enriching species pool of lower forest strata), have successfully acclimatized in new areas (Sudnik-Wójcikowska, 1987).Similarly, Quercus rubra L. -resistant to pollution, was commonly used, among other, as a fore-crop in poor habitats, including industrial idle lands (such as afforestation in sand pits) (Strzelecki & Sobczak, 1972).Both species turned out to be extremely expansive -black cherry, which in European forests has a bush form, grows into thick scrubs effectively blocking the development of other native tree species, thus hindering the forest renewal (Seneta & Dolatowski, 2012).A large number of black cherry patches have been found with a high number of specimens in each patch.This species keeps on taking over new plots and areas, easily invading the natural, semi-natural and anthropogenic vegetation communities.The areas where this species is a threat include forests, protected areas or habitats disturbed by anthropogenic pressure (Tokarska-Guzik et al., 2012).In a few decades after introduction of red oak into cultivated forests (in the beginning of 20th century [Sudnik-Wójcikowska, 1987]), its spontaneous spreading has been reported in numerous plots in various regions, as well as a large number of specimens in newly created patches (as a result of its fecundity -the species grows faster than other oaks and bears fruit already at a young age).Red oak continues expanding onto new plots and areas 2 .It may be harmful in forests and protected areas, since it easily penetrates into natural communities (Sudnik-Wójcikowska, 1987; Seneta & Dolatowski, 2012; Tokarska-Guzik et al., 2012).Another tree species -Acer negundo L. -was brought to Europe in the 17th century and in the 18-19th century to Poland (in Warsaw -approx.1880).Initially, it was considered valuable due to its quick growth, and as such used in gardens (in the beginning of the 20th century, often used in parks and planted in the country).Since 1940s, its strong expansion can be seen; in the 1980s it had already become one of the most common plants in Warsaw, often seen in anthropogenic habitats, acclimated in forest communities (such as riparian forests along Vistula River, border area of oak-hornbeam forests, pine and oak forests) (Sudnik-Wójcikowska, 1987).The presence of Acer negundo L. in riparian forests along the Vistula river should be considered a stage of secondary replacement succession (Matuszkiewicz & Roo-Zielińska, 2000).
Foreign herbaceous plants include Reynoutria japonica Houtt.-found in Europe since the middle of the 19th century; first reports of this taxon in Warsaw appeared in 1964.; in 1980s 2 Red oak currently is present in approx.3% of forest sub-compartments (nearly 5% of national forests' area).It is a dominant species in approx.0.5% of forest subcompartments, so 3 900 ha compared to approx.80 ha in mid 20th century -a 50-fold increase in area (Gazda & Augustynowicz, 2014).
it became common in anthropogenic ruderal and semi-natural habitats (along roads, at waste dumps, idle lands), but also at borders of willow riparian forests, alder forests, pine-birch-oak stands, in shrubs near water reservoirs (Sudnik-Wójcikowska, 1987).Another example includes Echinocystis lobata, cultivated after 1945 in Cracow; in 1980s it was commonly seen (e.g. in Warsaw) in anthropogenic habitats (along roads, waste dumps, near allotment gardens etc.) but also in semi-natural and natural communities (mainly in willow and willow-poplar riparian forests) (Sudnik-Wójcikowska, 1987).
Since the middle of the 20th century, a clear increased expansiveness of many foreign herbaceous plant species (e.g.Reynoutria japonica, Echinocystis lobata, Solidago gigantea, Impatiens parviflora) as well as tree species (e.g.Acer negundo, Prunus serotina, Robinia pseudoacacia) can be observed.Other species, such as Quercus rubra successfully reappear in anthropogenic habitats and forests (Sudnik-Wójcikowska, 1987).
Expansion of cities and industrial areas, exerting anthropogenic pressure over greater areas of land contributes to the transformation of habitats and their plant communities.This results in an increased number of sites with disturbed soil and water conditions, often polluted, which are suitable for highly tolerant species that continue to take over a particular ecosystem.Highly expansive foreign species may replace the local populations, leading to a reduction in the species count and finally to a change of ecosystem structure and destabilisation of the entire natural system in a given area (Gwiazdowski, 2014).As a result of introduction of foreign species into forests3 , entire forest sub-compartments became dominated by one "exot" or tree-stands composed of native species with an addition of foreign taxons with a varying degree of mix.Some foreign species may become invasive if they spontaneously spread and infiltrate natural biocenoses (Gazda & Augustynowicz, 2014).The presence of invasive vegetation replacing the native vegetation is a threat for biodiversity and ecological balance at species level (change of ecosystem species compositions) and super-species level (threat to habitat and ecosystem diversity).Additionally, some species, especially in the locations where humans are present, pose a health hazard (such as low ragweed pollen, which causes strong allergic reactions, or Sosnowsky's hogweed causing painful burns) (Gwiazdowski, 2014) 4 .

MATHERIALS AND METHODS
In order to identify the invasive plants appearing in the environment, the authors have conducted research in selected sites between 2011 and 2017.These sites included managed urban sites and rural areas, recreational (5 parks, including one with an area of approx.600 ha) and technical (2 stripes of forests near water, inducing one 10 km long), as well as unmanaged sites (2 areas).The selected areas are subject to varying degrees of anthropogenic pressure.For the last 40 to 80 years, the vegetation succession (limited human interference) took place in whole or part of these areas.The research carried out in the sites has been preceded with the analysis of natural conditions (climate, soil, water conditions, habitat etc.) as well as functional and spatial analyses.The detailed research is based on the dendrological inventories and phytosociological assessments.The inventories were used for a detailed assessment of both condition and structure of tree-stands (spatial structure, species composition, health, age) including accounting for invasive and expansive species (e.g.Fortuna-Antoszkiewicz & Łukaszkiewicz 2017; Łukaszkiewicz & Fortuna-Antoszkiewicz 2017).These comprised: identification of taxons, their frequency and distribution, dendrometric measurements of trees/shrubs.Phytosociological assessment (phytosociological photographs using a 5-point Braun-Blanquet scale) was carried out to identify the vegetation communities and formed a basis for forecasting 4 In Poland, the management of foreign species is regulated by the Act of
In the existing tree-stands, invasive tree species can be found mainly around mother specimens, but they also invade open areas (unused park interiors, mini-interiors created after felling of dead large tree specimens, at fringes of the tree-stand).In the researched sites, invasive herbaceous plants can be found in open and well sunlit patches of land, mainly in synanthropic habitats (e.g.near communication routes).
Frequency and numbers of individual invasive taxons are varied: • they are more numerous in the intensively utilised areas with a stronger anthropogenic pressure, unmaintained or maintained only to a minimum degree (such as Chorzów -expansive part, on a hill; Warsaw: Ursynów parkreservation part at the foot of the escarpment, Żerań Canal -in the areas with high penetra-tion, Exhibition Canal; Radziejowice -eastern periphery near expressway; Sopot park); • in the areas with weaker anthropogenic pressure, the invasive plants are less numerous (such as Southern escarpment in Bydgoszcz, Kobyłka pond and scrub complex, Żerań Canal -in less used locations) and are less varied in terms of species (e.g.Wielgie park -only one taxon: Echinocystis lobata; grows seasonally in a depression, within the water way area on a patch with full sun exposition, covering 100% of available space); • in the heavily used sites with strong anthropogenic pressure in plots subject to maintenance activities (such as mowing, clearing/correction of small trees in lower forest strata), the invasive plants are basically absent (e.g.Ursynów park in escarpment crown) or are present only in controlled spots (e.g.Silesia Park -western intensive zone).

SILESIA PARK IN CHORZÓW -CASE STUDY
Silesia Park is a special site, it has an area of 600 ha, and was established (in 1950-1968) on post-industrial and degraded lands within the Silesia agglomeration.It is an area subject to strong anthropogenic pressure (approx.3 million users annually).The current species structure of the park is a consequence of 60 years of often spontaneous growth, i.e. secondary succession.
In the initial stage of Park establishment, the tree and shrub species were introduced for the purpose of reclamation and phytoremediation.This decision was made because most of the Park area had poor soil (e.g.podzol), additionally degraded by mining and heavy industry.Forests were planted using pioneering species, hoping that as they grew, favourable conditions (habitat transformation) for more demanding and long living species would be created.The used native species included: birch, some poplars (aspen, black, white) as well as willows, hazel and elder.Two foreign species, already present in the area were also planted: Quercus rubra L. and Prunus serotina Ehrh.(Łukaszkiewicz & Fortuna-Antoszkiewicz 2017).In this period, these species were commonly used in western Poland for afforestation and as a forecrop for poor and degraded soils.
In 2013-2014, the authors conducted an evaluation of Silesia Park vegetation.The tree-stand structure analysis carried out within its current boundaries covered: spatial and species structure, age and health.Additionally, phytosociological evaluation was carried out in selected plots (Figure 2); the species composition and spatial structure of communities was analysed with special attention given to undergrowth strata (herbaceous plants).In general, the following phytocenoses are present in Silesia Park: a/ xerothermic turfs; b/ pasture-like and near pasture meadows -in the areas of expansive park lawns; c/ wet meadows, e.g. in terrain depressions or near water reservoirs; d/ water and near water communities

Potential vegetation:
Galio odorati-Fagetum in the lower part and on slopes of the escarpment; Stellario-Carpinetum -in the part of the upland.

Actual vegetation:
Overgrowing of glades with a degenerative form of substitute forest and shrub communities, mainly of features of oak-hornbeam forests.
No maintenance for about 40 years (bank area of park ponds); e/ communities similar to alder riparian, oak-hornbeam, oak forests and forest fringe communities -within expansive park tree-stands; f/ synanthropic and semisynanthropic communities -in the areas with an expansive management plan or where secondary succession was allowed.

BELT PLANTINGS AND UNDEVELOPED AREAS 6/ Area along the
Currently, among the pioneering tree species used for recultivation and as a fore-crop in Silesia Park, red oak and black cherry can be described as expansive.Numerous species grow in large parts of the area, especially in the extensive part of the Park (on the hill, where vegetation maintenance is limited) -in tree (A) as well as in other strata, and what is important -in the underbrush strata (B) -especially near older mother species (e.g.representative red oak specimens with breast height circumference of: 120/ 126/150/ 152/ cm and approx.25.0 m high and representative specimens of black cherry with trunk circumference of: 130 / 132 cm and approx.20.0 m high) or their clusters; they are also intensively renewing in the undergrowth strata (C) practically in the entire tree-stand area (Figure 3, 4).Highly concerning is that locally, the seedlings of both taxons cover 100% of area with no seedlings of other native tree species, which are also growing nearby.
The phytosociological evaluation (Figure 1) shows that there are 6 species of herbaceous plants, within the Park which are considered as highly  Generally, the phytocenoses in Silesia Park have reached a level of certain self-regulation and ecological stability, among other due to large area and compactness of the Park (approx.600 ha).Due to subsoil recultivation (renewal of physical and chemical properties) and formation of a particular phytoclimate within the Park, secondary succession takes place, which involves substitution of pioneering tree species (such as birch, robinia, poplars etc.), planted as a forecrop for more demanding trees.Generally, the tree species typical to oak-hornbeam, sometimes riparian and oak forest communities (mezo-and eutrophic deciduous forests -Querco-Fagetea class) are renewed.Simultaneous succession of expansive and invasive species is an undesired phenomenon; this pertains to both tree and herbaceous plants (such as red oak, black cherry, knotweeds, balsams, goldenrods).This issue is problematic and casts doubt on the optimistic forecast of tree-stand development via succession (towards natural, stable and undisturbed phytocenoses).

DISCUSSION
High number of the researched taxons: Robinia pseudoacacia L., Acer negundo L., Quercus rubra L., Prunus serotina Ehrh., Impatiens parviflora DC. found within researched sites are consistent with the results of research carried out in forest areas in entire Poland.For example: in the eastern part of Opoczyńskie Hills, the same species have been found to have highest frequency and expansiveness potential (Quercus rubra L.
The research on the spreading of invasive species e.g. in Ladzka primeval forest (bordering Białowieża primeval forest on south-west) also confirm that the most numerous species include Impatiens parviflora DC. and Prunus serotina Ehrh.(the species showing preference for non-fresh and poor habitats and pine tree-stands aged from 20 to 60 years growing in the area).In context of anthropogenic factors, the identified species showed preference for the areas with a dense road network (>50 m/ha) and areas less than 0.5 km away from buildings and forest border (Fyałkowska et al., 2014).The research on the expansion of Prunus serotina Ehrh. in Kampinos National Park (covering the entire Park with an area of 385 km²) shows that the species is frequently6 found in the ecosystems which underwent anthropogenic transformation of soil conditions and vegetation.The disturbances caused by human interference may be a decisive factor for the vulnerability of a given ecosystem to invasive species (Otręba & Kondras, 2014).The analysis of invasive species in 9 research sites (Table 1), in the context of natural conditions and anthropogenic pressure shows the necessity of taking preventive actions (vegetation maintenance, tree-stand management) -following many years of negligence or no maintenance whatsoever -to protect native phytocenoses and maintain local biodiversity.
Other examples of areas which require adapting a rational vegetation maintenance and treestand management plan are the areas protected by law (such as thermophilous oak forest Potentillo albae-Quercetum in King John Sobieski reservation in Warsaw -under recession [Ciurzycki et al., 2014]), or locations were secondary succession develops towards forest communities -due to the abandoning of land (e.g.secondary forest succession in the meadows in Małe Pieniny [Frączek & Dziepak, 2014]).In such cases, a permanent destruction of cultural landscape and local decrease of biodiversity may result, similarly as in the case of uncontrolled invasive plant introduction.

CONCLUSIONS
• Generally, the presence of invasive plant species is symptomatic of an unfavourable vegetation succession in a given area.To a large degree, it is connected with increasing anthropogenic pressure on the environment.• Excessive expansion of invasive species may disturb the ecological balance of an ecosystem in a given area, replacing less expansive and less competitive species.This may lead to impoverishing of the species structure and decreasing biodiversity.The current research proves (Tokarska-Guzik et al., 2012) that the environmental functions are best achieved by afforestation composed of native species, especially if they are of a free and non-schematic structure.
• In the selected sites which are subject to anthropogenic pressure, the presence of invasive species was detected to a varying degree and was most prominent in intensively utilised areas.This resulted in alarger area taken over by individual species and greater species variety.
In the case of tree species, all tree-stand strata have been taken over, with intensive renewal in the undergrowth strata.• The authors' own research carried out in the selected sites confirms invasive species' pressure is present irrespective of location, natural conditions of terrain and site type -in each case, it is most prominent in the plots where vegetation is not properly maintained or is not maintained at all.• In order to maintain optimum vegetation systems (stable native phytocenoses) in the areas subject to strong anthropogenic pressure (urban, recreational and tourist areas) it is necessary to introduce systemic supporting activities: constant monitoring of succession and a rational, planned maintenance of vegetation to reduce negative impact of environmental changes, and in the case of invasive species -elimination in the early stages of expansion.

Figure 1 .
Figure 1.Location of research polygons in Poland (prepared by: P. Wiśniewski)

Table 1 cont.
7/ South Escarpment in Bydgoszcz / Bydgoszcz, South Escarpment, Toruńska str./ surroundings: multi-family buildings / undeveloped land / spontaneous plants communities -habitat of slope oak-hornbeam / 2017 / ca.0.6 ha (slope with a length of 200 m and a width of 30 m)Potential vegetation:subcontinental oak-hornbeam forests from the belt of the great valleys of the Wielkopolska-Kujawska Region, from the Kujawski District.Actual vegetation:Tilio-Carpinetum typicum sub-continental forest of the slope variety with a small amount of invasive plants in the undergrowth and undergrowth.8/Thearea along the Żerański Canal / fragment -west bank / Warsaw, Żerań / municipality Nieporęt / open areas; development of single and multi-family housing / / water-side, protective woodlots / spontaneous vegetation with the remaining composed plantings from the 1960s / 2015 -2016 / ca. 25 ha (strip of land length ca. 10 km / average wide 20,0 [30,0] m)Potential vegetation:communities of deciduous forests with robinia similar to poor oakhornbeam forests and mixed coniferous forests with elements of classes: Querco-Fagetea, Vaccinio-Piceetea and Robinietea (Cl.); communities of thermophilic deciduous forests with robinia, similar to bright oakwood forests (with elements of Quercetalia pubescetis order (O.) and Robinietea class (Cl.).water and by-water communities (peripheral zone of ponds); nitrophilous shrub-thicket communities (Sambuco-Salicilion), being a further stage of succession in the forest regeneration process; communities with the characteristics of riparian forests and low oakhornbeam forests.