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1 Department of Botany, Faculty of Natural Sciences, Comenius University in Bratislava, Bratislava, Slovakia.
2 Department of Ecology, Slovak University of Agriculture, Nitra, Slovakia.
3 Department of Mathematics and Descriptive Geometry, Faculty of Civil Engineering, Slovak University of Technology in Bratislava, Bratislava, Slovakia. 
Comparison of the differences in the composition 
of ruderal flora between conventional tram tracks 
and managed green tram tracks in the urban 
ecosystem of the city of Bratislava
Abstract
Green infrastructure (GI) brings many benefits to urban ecosystems. Green tram 
tracks can be considered to be a part of GI. The presented study is focused on 
the comparison of the species’ frequency and composition between conventional 
tram tracks and green tram tracks in Bratislava, Slovakia, Central Europe. This 
comparison also provides an insight into the changes of the flora of tram tracks 
over time, as we compare the results of the older research with recent research on 
green tram tracks. The results revealed significant differences in the composition 
of flora between conventional tram tracks and green tram tracks. In particular, 
the total number of species has decreased over time, as green tram tracks host 
fewer spontaneously growing taxa than conventional ones. The frequency of 
occurrence of archaeophytes and neophytes has decreased on the strict rail yard 
while on the tracksides it has increased. Green tram tracks deliver positive features 
to ecosystems, but may also have negative aspects because they present a pool of 
alien, potentially invasive plants.
Izvleček
Zelena infrastuktura (ZI) prinaša urbanim ekosistemom številne koristi. Med 
ZI lahko uvrščamo tudi zelene tramvajske proge. V raziskavi smo se osredotočili 
na primerjavo frekvenc ruderalnih rastlinskih vrst in vrstne sestave med 
konvencionalnimi tramvajskimi progami z ozelenjenimi v Bratislavi (Slovaška, 
srednja Evropa). Primerjava omogoča tudi vpogled v spremembe flore tramvajskih 
prog v času, saj smo primerjali razultate s starejšimi raziskavami. Razlike med 
konvencionalnimi in ozelenjenimi progami so bile statistično značilne. Število 
vrst se je s časom zmanjšalo, saj je na ozelenjenih progah manj vrst kot na 
konvencionalnih. Frekvenca pojavljanja arheofitov in neofitov se je zmanjšala med 
tračnicami, na robovih prog pa se je povečala. Ozelenjene tramvajske proge imajo 
za ekosisteme določeno pozitivno vlogo, vendar imajo tudi slabe lastnosti, saj 
predstavljajo nabor tujerodnih vrst, ki so potencialno invazivne.
Key words: Central Europe, 
diversity, green infrastructure, 
invasive alien species, landscape 
planning, neophytes.
Ključne besede: Srednja Evropa, 
raznovrstnost, zelena infrastruktura, 
invazivne tujerodne vrste, krajinsko 
načrtovanje, neofiti.
Corresponding author: 
Alena Rendeková
E-mail: alenarendekova@gmail.com
Received: 13. 5. 2021
Accepted: 24. 8. 2021
Alena Rendeková1, Karol Mičieta1, Michal Hrabovský1, Eva Zahradníková1, 
Martina Michalová1 , Ján Miškovic1 , Mariana Eliašová2  & Dominika Ballová3
DOI: 10.2478/hacq-2021-002021/1 • 2022, 73–88
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Comparison of the differences in the composition of ruderal flora between 
conventional tram tracks and managed green tram tracks in the urban ecosystem
Introduction
With more and more people living in cities, a specific ur-
ban environment is emerging in more places across the 
world. The urbanisation in the past centuries has resulted 
in the reduction of natural habitats and biodiversity loss 
(Williams et al., 2009). Maintenance of remaining biodi-
versity in urban ecosystems and creating the most healthy 
environment possible for the citizens have became prior-
ity issues to be addressed by many environmentalists and 
other scientists, such as Williams et al. (2009), Burgin 
(2016), Horák et al. (2016), Kumar & Verma (2017), 
Castro et al. (2020). 
One of the valuable tools in this effort is green space 
planning and creation of green spaces and green infra-
structure (GI), which helps to maintain clean air and bet-
ter water quality, reduce pollutants, and mitigate noise, 
promote positive emotions in city’ residents, and has 
other beneficial functions in urban areas (Provendier & 
Damas, 2010; Steckler et al., 2012; Schreiter & Kappis, 
2013a, b; Wagner et al., 2013; Eglinton, 2014; Renter-
ghemi et al., 2015; Sikorski et al., 2018; Cameron et al., 
2020; Sikorska et al., 2020). Green infrastructure (GI) 
consists of the network of natural and human-made fea-
tures, such as land reserves, parks, green roofs, eco-ducts, 
cycle paths and other features covered by specific species 
of plants (Andreuci, 2013).
Plants have always represented an important compo-
nent of the urban environment. Spontaneously growing 
ruderal plants occur in many urban habitats, and orna-
mental plants have always been planted in public spaces, 
gardens, parks, and cemeteries (Wagner et al., 2013). The 
managed green spaces are perceived more positively by 
the city residents than unmaintained areas with ruderal 
weeds or sites without plants (Sikorski et al., 2018). The 
idea of green space planning came about precisely thanks 
to the effort to introduce more managed green spaces to 
the cities. While the first ideas in this area arose more 
than 100 years ago (Zube, 1995; Vasas et al., 2009), GI is 
still a relatively new area of interest in Europe (Andreuci, 
2013) and the number of studies focused on it should be 
increased. 
One of the important factors influencing the city envi-
ronment is public transport. With the increasing number 
of people living in cities, demands for more efficient ways 
of public transport emerged. In the context of the reduc-
tion of negative environmental impacts of travelling, tram 
transport appears to be one of the most suitable solutions 
compared to buses and other ways of transport (Sikorski 
et al., 2018). There is a readily apparent trend of the in-
creasing numbers of tramlines in many countries (e.g., 
Germany, Poland, Czech Republic, Switzerland, Neth-
erlands, Belgium, Bulgaria, France, Italy, Spain, USA) 
(Henze & Model, 2006; Banister, 2010; Eglinton, 2014; 
Pfautsch & Howe, 2018). Ecological aspects are being 
increasingly taken into account more and more in tracks 
and railway construction (Steckler et al., 2012). Tram and 
railway tracks can serve as additional green spaces in built-
up areas of cities if they are converted into 'green tracks' 
by replacing concrete between rails or ruderal weed veg-
etation on tracks by green carpets with planted vegeta-
tion (Sikorski et al., 2018). The plants which appear to 
be the most suitable for the greening of tracks are grasses 
and succulents, although in some cases herbaceous plants 
are also used (Steckler et al., 2012; Schreiter & Kappis, 
2013a, b; Eglinton, 2014; Sikorski et al., 2018).
Although green infrastructure features consist mainly 
of non-linear green spaces (Andreuci, 2013), green tram 
and railway tracks can be considered as the part of the 
GI concept, as they provide multiple environmental, 
health, social and economic benefits and functions, e.g., 
noise reduction, improvement of the microclimate (via 
reduction of heat island effect etc.), water retention, dust 
absorption, and uptake of contaminants (Schreiter, 2010; 
Schreiter & Kappis, 2013a; Eglinton, 2014). 
Green tram and railway tracks have been recently in-
troduced in many European cities, and this has inspired 
research into this unique biotope. Consequently, recent 
years have seen the appearance of publications focusing 
on this area (Müller et al., 2001; Schreiter, 2010; Steck-
ler et al., 2012; Schreiter & Kappis, 2013a, b; Eglinton, 
2014; Pfautsch & Howe, 2018; Sikorski et al., 2018). 
In the majority of studies, green tram and railway tracks 
were researched separately from the spontaneously grow-
ing flora and vegetation of conventional tracks, or authors 
did not focus primarily on the comparison of these two 
types of habitats (Niemi, 1969; Suominen, 1969; Eliáš, 
1979, 1981; Brandes, 1983, 1984, 1993a, b; Hohla et 
al., 2000, 2002; Brandes, 2002a, b, 2003, 2004a, b, c, 
d, 2005a, b, 2008; Jehlík & Dostálek, 2008; Tret'yakova, 
2010; Galera et al., 2012, 2014; Sudnik-Wójcikowska et 
al., 2014; Wierzbicka et al., 2014; Májeková et al., 2014; 
Májeková & Limánek, 2016; Májeková et al., 2016; 
Woźnica et al., 2016; Wrzesień et al., 2016; Májeková et 
al., 2021). Sikorski et al. (2018) compared green tram 
tracks at different developmental stages from their estab-
lishment, but there is a lack of other similar studies, not 
to mention studies focused strictly on the comparison of 
conventional and green tracks. The findings of such study 
can be a valuable tool in landscape planning, and infor-
mation about floristic differences between these two types 
of tracks can facilitate the management of urban flora and 
vegetation as well as biodiversity conservation. For this 
reason our research is focused on this issue.
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Comparison of the differences in the composition of ruderal flora between 
conventional tram tracks and managed green tram tracks in the urban ecosystem
Until 2019, only spontaneously growing flora and veg-
etation occurred on tram tracks in Bratislava (Rendeková 
et al., 2020). In 2019, some parts of the tram tracks of 
Bratislava were repaired and at the end of October 2019, 
they were covered by planted succulents, mostly various 
taxa of the genus Sedum and tracksides were covered by 
various ornamental plants.
The aim of this study was to compare the number of 
species, frequency of their occurrence and species compo-
sition of the spontaneously growing ruderal flora between 
the past (i.e. conventional tram tracks) and present (i.e. 
green tram tracks). Specifically, we set up the following 
aims: (1) to record the species composition of the flora 
growing spontaneously on green tram tracks; (2) to analyse 
the differences in the total number of species, and a num-
ber of native species, archaeophytes, and neophytes; (3) to 
analyse the differences in the frequency of occurrence, i.e. 
the number of localities of all taxa, native taxa, archaeo-
phytes, and neophytes; (4) to analyse the differences in the 
species composition of the flora of tram tracks, comparing 
the past and present by means of cluster analysis.
Methods
Study area and data sampling
The study builds on our previous research published by 
Rendeková et al. (2020), which was carried out in the 
years 2014–2019. The recent research was performed 
during the vegetation season of the year 2020. Both, the 
older and the more recent data were collected in the area 
of tram infrastructure of Bratislava, the capital city of 
Slovakia. It is situated in south-western Slovakia, Central 
Europe. The climate is continental and has a moderate 
to warm character. Bratislava belongs to the warmest and 
driest parts of Slovakia. Its area is 367.9 km2 and it has 
approximately 425,500 inhabitants. 
Thanks to this relatively high number of city residents, 
efficient ways of public transportation are required. Tram 
transport solves part of this problem and represents the 
major component of public transport in Bratislava. The 
total length of the tram network of the city is 42 km 
(Feráková & Jarolímek, 2011; Hrnčiarová et al., 2006). 
Until 2019, only spontaneously growing ruderal flora 
and vegetation occurred on some parts of the tram lines 
(Figure 1a), while other parts were covered by concrete 
(Rendeková et al., 2020). At the end of the October 
2019, some parts of them were converted into green tram 
tracks (Figure 1b) by replacing spontaneously growing 
b
a
Figure 1: Photos showing the comparison between conventional tracks of Bratislava in the years 2014–2019 (a) and green tram tracks in 2020 (b)
Slika 1: Primerjava med običajnimi progami v Bratislavi med leti 2014 in 2019 (a) in zelenimi tramvajskimi progami leta 2020 (b).
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flora with planted succulents, mostly various taxa of the 
genus Sedum. Tracksides were also re-built, and were cov-
ered by new, mostly gravelly substrate with various orna-
mental taxa (Figure 1b).
The tram network of Bratislava with marked survey 
plots is shown in Figure 2a. Of the 30 localities previously 
studied (Rendeková et al., 2020), 15 of the strict rail yard 
plots (areas between rails up to 0.5 m from the outer rail 
of tracks) have been converted into green tram tracks, i.e. 
covered by planted succulents (Figure 2b), and the cur-
rent research was carried out on these localities. On 10 
of the selected 15 localities, the tracksides (plots running 
from 0.5 m to 3 m from the outer rail of the tracks) were 
also covered by planted ornamental plants (Figure 2c). 
We have also recorded plant taxa on the tracksides here. 
Some parts of the tracks remained covered by concrete 
(2d); the survey was not performed here. Plot sizes were 
2 × 2.5 m2, as in the previous study of Rendeková et al. 
(2020). We chose this size because it is the same as in the 
previous study, to make the past and present data better 
comparable. The locations of the plots are presented in 
Table 1. Some of tram stops have been renamed; we pre-
sent the new names in Table 1.
We recorded the presence/absence of plants growing 
spontaneously on the tram tracks. In addition, Greenfond 
Polska Sp z o.o. and METRO Bratislava a.s., the groups 
which have carried out the 'greening' process of Bratis-
lava’s tram tracks, have provided us with lists of species 
planted on the tram tracks. While we also recorded the 
planted taxa from these lists, which were growing on the 
researched plots, these were not included into analyses, as 
the aim of the research was to study the changes in the 
spontaneously growing ruderal flora. Bryophytes were also 
recorded, but not included in the analyses because of their 
uncertain origin status. In order to provide a complete pic-
ture of tram track flora, we also present planted taxa and 
bryophytes in the lists of recorded taxa (Tables 2–3)
Taxa were recorded three times at each plot: 1) from 
April to the beginning of June, 2) from the end of June 
to the beginning of August, 3) at the end of August, and 
in September. This enabled us to record various stages of 
development of the flora. The same approach was applied 
in the previous study od Rendeková et al. (2020). In the 
previous study, we did not sample each locality every year 
between 2014–2019, each locality was sampled only in 
one year.
Figure 2: Tram network of Bratislava with 
marked survey plots (a). Explanation: 
numbers represent numbers of survey plots 
described in Table 1; numbers in brackets 
represent plots' numbering in the past – i.e. 
in the work of Rendeková et al. (2020); black 
line colour represents parts of tram tracks, 
where plants grew and where the research was 
done; black circles represent plots, where only 
strict rail yard was researched (b); red squares 
represent plots, where also tracksides were 
researched (c); grey line colour represents 
parts of tram tracks completely covered by 
concrete without flora and vegetation, which 
were excluded from the research (d).
Slika 2: Mreža tramvajskih prog Bratislave 
z označenimi popisnimi ploskvami (a). Leg-
enda: številke predstavljajo število popisnih 
ploskev opisanih v Tabeli 1; številke v oklepa-
jih predstavljajo oštevilčenje v preteklosti- t.j. 
v članku Rendeková et al. (2020); črne črte 
predstavljajo tramvajske proge, kjer uspevajo 
rastline in smo izvedli vzorčenje; črni krožci 
predstavljajo ploskve, kjer smo popisovali 
samo med tračnicami (b); rdeči kvadratki 
predstavljajo ploskve, kjer smo vzorčili 
tudi robove prog (c); sive črte predstavljajo 
dele prog popolnoma prekritih z betonom 
brez flore in vegetacije, ki smo jih izločili iz 
raziskav (d).
a
b c d
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Number 
of locality
Number of locality in the past 
[Rendeková et al. (2020)] Description of locality GPS coordinates
1 4 Bratislava, Dúbravka, tram track near Švantnerova street, 8 m from the tram stop Švantnerova
N 48° 10' 57.40",  
E 17° 02' 21.60"
2 7 Bratislava, Karlova Ves, tram track at the crossroad of the streets Karloveská × Svíbová
N 48° 09' 56.30",  
E 17° 02' 46.60"
3 8 Bratislava, Karlova Ves, tram track along the Karloveská street, 25 m from the tram stop Kútiky
N 48° 09' 53.30",  
E 17° 02' 48.30"
4 9 Bratislava, Karlova Ves, tram track at the crossroad of the streets Karloveská × Borská, 8 m from the tram stop Borská
N 48° 09' 44.60",  
E 17° 02' 54.10"
5 10 Bratislava, Karlova Ves, tram track at the crossroad of the streets Karloveská × Lackova
N 48° 09' 32.30",  
E 17° 03' 04.90"
6 11 Bratislava, Karlova Ves, tram track near Kempelenova street, 4 m from the tram stop Nad lúčkami
N 48° 09' 21.50",  
E 17° 03' 12.20"
7 12 Bratislava, Karlova Ves, tram track along the Segnerova street, 10 m from the tram stop Segnerova
N 48° 09' 13.60",  
E 17° 03' 26.90"
8 13 Bratislava, Karlova Ves, tram track near Jurigovo námestie street N 48° 09' 08.80",  E 17° 03' 39.70"
9 14 Bratislava, Karlova Ves, tram track along the Ladislava Sáru street, 25 m from the tram stop Riviéra
N 48° 08' 59.30",  
E 17° 03' 46.30"
10 15 Bratislava, Karlova Ves, tram track near Na Riviére street N 48° 08' 54.40",  E 17° 03' 50.20"
11 17 Bratislava, Karlova Ves, tram track along the street Botanická, 7 m from the tram stop Botanická záhrada
N 48° 08' 53.80",  
E 17° 04' 14.90"
12 18
Bratislava, Staré Mesto, tram track at the crossroad of the 
streets Nábrežie armádneho generála Ludvíka Svobodu × Nad 
lomom, 14 m from the tram stop Lanfranconi
N 48° 08' 43.40",  
E 17° 04' 40.50"
13 19 Bratislava, Staré Mesto, tram track along the Nábrežie armádneho generála Ludvíka Svobodu street
N 48° 08' 33.60",  
E 17° 05' 10.10"
14 20 Bratislava, Staré Mesto, tram track along the Dvořákovo nábrežie street
N 48° 08' 29.80",  
E 17° 05' 29.80"
15 21 Bratislava, Staré Mesto, tram track 20 m from Žižkova street N 48° 08' 31.60",  E 17° 05' 26.50"
Explanation: numbers in bold represent numbers of survey plots, where also tracksides were resea
Table 1: Locations of the survey plots. Tabela 1: Lokacije popisnih ploskev.
Data analysis
Analyses were only done for the spontaneously growing 
recorded taxa of vascular plants. The taxa were divided 
into native and alien. The second group was divided into 
archaeophytes and neophytes. The origin status of taxa 
follows the list of alien vascular plants of the Slovak Re-
public (Medvecká et al., 2012). The spontaneously grow-
ing ornamental alien taxa (i.e. ornamental taxa, which 
were not planted on tram tracks by Greenfond Polska Sp 
z o.o. and METRO, but appeared on tram tracks after 
being planted in nearby gardens and parks), which are 
not included in the list of alien plants (Medvecká et al., 
2012), were evaluated as neophytes. 
Analyses of the records from the strict rail yard and 
the records from tracksides were done separately. The 
reason for doing the analyses this way was that from the 
previous 30 localities (Rendeková et al., 2020), 15 strict 
rail yard plots were converted into green tram tracks, but 
only 10 trackside plots were covered by planted orna-
mental plants.
The changes in the total number of native taxa, ar-
chaeophytes and neophytes between the older and more 
recent datasets were evaluated by the creation of graphs in 
the program R 3.5.1. (R Core Team, 2018). The number 
of taxa which were recorded only in the past, only in the 
present, and in both periods, and the number of taxa for 
which the frequency of occurrence declined, increased or 
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Comparison of the differences in the composition of ruderal flora between 
conventional tram tracks and managed green tram tracks in the urban ecosystem
remained the same compared to the past were also evalu-
ated in the same way.
In the statistical analyses, the normality of the data was 
tested by the Shapiro-Wilk test first. Since the data were 
not normally distributed, non-parametric analyses were 
used in the following steps. 
The differences in the frequency of occurrence (i.e. the 
number of localities) of native taxa, archaeophytes and neo-
phytes between the older and the more recent datasets were 
analysed by Sign test and by Wilcoxon two-sample test. 
The binary distance and average linkage method were 
used in the cluster analysis. These parameters proved to 
be the best for both, the data from the strict rail yard and 
data from tracksides, after testing. 
As explanatory feature, the changes in Raunkiær’s life 
forms of recorded spontaneously growing taxa between 
the old and the more recent period was calculated. We 
calculated the frequency of occurrence (i.e. the number of 
localities) of each Raunkiær’s life form in the old dataset 
and compared it with those of the more recent dataset by 
Wilcoxon test. The Raunkiær’s life forms were determined 
according to the ‘List of alien vascular plant species of the 
Slovak Republic’ (Medvecká et al., 2012) and according to 
the Pladias – Database of the Czech Flora and Vegetation 
(Chytrý et al., 2021). The statistical analyses were done in 
the R 3.5.1. (R Core Team, 2018).
Nomenclature
The nomenclature of the taxa follows The Plant List 
(2013) and the nomenclature of the syntaxa follows 
Jarolímek et al. (2008).
Results
Both datasets together contained 123 taxa spontaneously 
growing on the strict rail yard of Bratislava tram tracks 
(Table 2) and 96 taxa spontaneously growing on track-
sides (Table 3). 
From the number of taxa spontaneously growing on 
strict rail yard habitat, 60 taxa (49%) were only recorded 
in the older dataset (i.e. on conventional tram tracks), 23 
(19%) were only recorded in the more recent one (i.e. 
on green tram tracks), and 40 (33%) were present in 
both (Figure 3). From the total number of species of the 
trackside habitat, 47 taxa (49%) were only recorded in 
the past, 27 (28%) were only recorded in the present, and 
22 (23%) were recorded in both periods (Figure 4). In 
2020, planted taxa also grew on the tram tracks, and we 
recorded 17 of them on the strict rail yard (Table 2) and 
31 on the tracksides (Table 3).
From the total number of species of the strict rail yard, 
there were 72 taxa (59%) for which the frequency of 
occurrence (i.e. the number of localities) declined com-
pared to the past, 41 taxa (33%) for which the frequen-
cy of occurrence increased and 10 taxa (8%) for which 
the frequency of occurrence was the same as in the past 
 (Figure 5). On tracksides, the frequency of occurrence of 
57 taxa (59%) declined compared to the past, the fre-
quency of occurrence of 35 taxa (36%) increased and the 
frequency of occurrence of 4 taxa (4%) was the same as in 
the past (Figure 6).
Figure 3: Venn diagram for the number and percentages of spontane-
ously growing ruderal plant taxa recorded on the strict rail yard of 
conventional tram tracks in the past (2014–2019) and green tram 
tracks in the present (2020).
Slika 3: Vennov diagram za število in odstotke spontano rastočih 
ruderalnih rastlinskih vrst med tračnicami na konvencionalnih tram-
vajskih progah v preteklosti (2014–2019) in ozelenjenih tramvajskih 
progah v sedanjosti (2020).
Figure 4: Venn diagram for the number and percentages of spontane-
ously growing ruderal plant taxa recorded on the tracksides of conven-
tional tram tracks in the past (2014–2019) and green tram tracks in 
the present (2020) 
Slika 4: Vennov diagram za število in odstotke spontano rastočih 
rudralnih rastlinskih vrst na robovih konvencionalnih tramvajskih 
prog v preteklosti (2014–2019) in ozelenjenih tramvajskih progah v 
sedanojsti (2020).
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Comparison of the differences in the composition of ruderal flora between 
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From the total number of plant species spontaneously 
growing on the researched plots of the strict rail yard in 
the past, 31 were archaeophytes, 25 were neophytes, and 
44 were native (Figure 7). In 2020, the numbers in each 
of these groups were lower than in the past (18 archaeo-
phytes, 11 neophytes, 34 native taxa) (Figure 7). 
test). The analyses revealed a statistically significant differ-
ence in the frequency of occurrence (i.e. the number of 
localities) of archaeophytes (W = 890, p = 0.005; S = 21, 
p = 0.020). The number of localities of archaeophytes has 
decreased compared to the past (Figure 9a). The number 
of localities of neophytes also decreased with statistical 
significance (W = 525.5, p = 0.003; S = 19, p = 0.007) 
(Figure 9b). Analysis of the differences in the number of 
localities of native species did not reveal any significant 
differences (W = 2060, p = 0.300; S = 32, p = 0.300) 
(Figure 9c).
The Sign test and Wilcoxon two-sample test also re-
vealed some differences in the frequency of occurrence of 
plant species on the tracksides. The number of trackside 
localities of archaeophytes increased with statistical sig-
nificance compared to the past (W = 215.5, p = 0.008; 
S = 8, p = 0.015) (Figure 10a). The number of trackside 
Figure 5: Venn diagram for changes in the frequency of occurrence 
(i.e. the number and percentages of localities) of taxa between strict rail 
yard of conventional tracks (past) and green tracks (present).
Slika 5: Vennov diagram frekvence pojavljanja (t.j. števila in odstotka 
lokacij) taksonov med tračnicami konvencionalnih prog (v preteklosti) 
in ozelenjenih prog (v sedanjosti).
Figure 6: Venn diagram for changes in the frequency of occurrence 
(i.e. the number and percentages of localities) of taxa between track-
sides of conventional tracks (past) and green tracks (present).
Slika 5: Vennov diagram sprememb frekvence pojavljanja (t.j. števila in 
odstotka lokalitet) taksonov med tračnicami konvencionalnih prog (v 
preteklosti) in ozelenjenih prog (v sedanjosti).
 In the past, 15 taxa of archaeophytes, 5 taxa of neo-
phytes and 49 native taxa grew on the tracksides (Fig-
ure 8). In 2020, the number of native species on track-
sides was lower than in the past, as there were 17 native 
taxa recorded. The number of archaeophytes and neo-
phytes on tracksides increased; we recorded 22 taxa of 
archaeophytes and 10 taxa of neophytes (Figure 8). 
The differences between the older and more recent data-
sets from the strict rail yard for alien taxa were confirmed 
by statistical analyses (Sign test and Wilcoxon two-sample 
Figure 7: Number of native taxa, archaeophytes and neophytes 
recorded on the strict rail yard of conventional tram tracks in the past 
(2014–2019) and green tram tracks in the present (2020)
Slika 7: Število domorodnih taksonov, arheofitov in neofitov med 
tračnicami konvencionalnih prog (v preteklosti) in ozelenjenih prog (v 
sedanjosti).
Figure 8: Number of native taxa, archaeophytes and neophytes 
recorded on the tracksides of conventional tram tracks in the past 
(2014–2019) and green tram tracks in present (2020)
Slika 8: Število domorodnih taksonov, arheofitov in neofitov na robo-
vih konvencionalnih prog (2014–2019) in ozelenjenih prog (2020).
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Figure 9: Comparison of the frequency of occurrence (i.e. the number 
of localities) of archaeophytes (a), neophytes (b), and native taxa (c) 
between strict rail yard of conventional tracks (past) and strict rail yard 
of green tracks (present). Explanation: – median, □ 25%–75%, I min-
max, ○ outliers.
Slika 9: Primerjava frekvence pojavljanja (t.j. števila lokacij) arheofitov 
(a), neofitov (b) in domorodnih vrst (c) med tračnicami konvencio-
nalnih prog (preteklost) in ozelenjenih prog (sedanjost). Legenda: – 
mediana, □ 25% 75%, I min-max, ○ osamelci.
Figure 10: Comparison of the frequency of occurrence (i.e. the 
number of localities) of archaeophytes (a), neophytes (b), and native 
taxa (c) between tracksides of conventional tracks (past) and tracksides 
of green tracks (present). Explanation: – median, □ 25%–75%, I min-
max, ○ outliers
Slika 10: Primerjava frekvence pojavljanja (t.j. števila lokacij) 
arheofitov (a), neofitov (b) in domorodnih vrst (c) med robovi konven-
cionalnih prog (preteklost) in ozelenjenih prog (sedanjost). Legenda: 
– mediana, □ 25%–75%, I min-max, ○ osamelci.
localities of neophytes also increased (Figure 10b), but 
this difference was not statistically significant (W = 39.5, 
p = 0.054; S = 4, p = 0.388). The number of trackside 
localities of native taxa decreased significantly compared 
to the past (W = 2566, p < 0.001; S = 45, p < 0.001) 
(Figure 10c).
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Comparison of the differences in the composition of ruderal flora between 
conventional tram tracks and managed green tram tracks in the urban ecosystem
We recorded the changes in the spectrum of the Raun-
kiær’s life forms. The statistical analyses on the level of 
significance 0.1 revealed significant increase in the fre-
quency of occurrence (i.e. the number of localities) of 
therophytes (W = 2641.5, p = 0.071) and on the level 
of significance 0.05 significant decrease of hemicrypto-
phytes (W = 2497.5, p = 0.003) on the strict rail yard 
(Figure 11). Changes in other life forms on the strict rail 
yard were not statistically significant (geophytes: W = 46, 
p = 0.118; chamaephytes: W = 4, p = 0.999; phanero-
phytes: W = 38, p = 0.110). 
Figure 11: Comparison of the frequency of occurrence (i.e. the 
number of localities) of Raunkiær’s life forms between strict rail yard 
of conventional tracks (old data) and strict rail yard of green tracks 
(recent data). Explanation: Th – therophytes, Ge – geophytes, He – 
hemicryptophytes, Ch – chamaephytes, Ph – phanerophytes, o – old 
data, r – recent data.
Slika 11: Primerjava frekvence pojavljanja (t.j. števila lokacij) Raunki-
erjevih življenskih oblik med tračnicami konvencionalnih prog (stari 
podatki) in ozelenjenih prog (novi podatki). Legenda: Th – terofiti, Ge 
– geofiti, He – hemikriptofiti, Ch – hamefiti, Ph – fanerofiti, o – stari 
podatki, r – novi podatki.
The statistical analysis of the changes of Raunkiær’s 
life forms on tracksides revealed significant increase in 
the frequency of occurrence (i.e. the number of localities) 
of therophytes (W = 607.5, p = 0.011) and significant 
decrease of hemicryptophytes (W = 2076.5, p < 0.001) 
on the level of significance 0.05 (Figure 12). Changes in 
other life forms on stracksides were not statistically sig-
nificant (geophytes: W = 37.5, p = 0.118; chamaephytes: 
W = 8.5, p = 0.999; phanerophytes: W = 27.5, p = 0.733). 
The cluster analysis also showed the differences be-
tween the past (conventional tracks) and present (green 
tram tracks) data. Figure 13 shows the results of the clus-
ter analysis for the strict rail yard. Most of the records 
from the strict rail yard were separated based on the years 
in which they had been recorded. Most of the recent re-
Figure 12: Comparison of the frequency of occurrence (i.e. the 
number of localities) of Raunkiær’s life forms between tracksides of 
conventional tracks (old data) and tracksides of green tracks (recent 
data). Explanation: Th – therophytes, Ge – geophytes, He – hemic-
ryptophytes, Ch – chamaephytes, Ph – phanerophytes, o – old data, 
r – recent data
Slika 12: Primerjava frekvence pojavljanja (t.j. števila lokalitet) Raun-
kierjevih življenskih oblik med robovi konvencionalnih prog (stari 
podatki) in ozelenjenih prog (novi podatki). Legenda: Th – terofiti, Ge 
– geofiti, He – hemikriptofiti, Ch – hamefiti, Ph – fanerofiti, o – stari 
podatki, r – novi podatki.
cords were merged in the clusters A, C, D and E, whereas 
most of the records from the past were merged in the 
cluster B by hierarchical clustering (Figure 13).
Figure 13: Dendrogram of cluster analysis of the flora of different 
strict rail yard localities (binary distance and average linkage method 
were used). Explanation: A, B, C, D, E – clusters A, B, C, D, E; X – 
plots from conventional tracks recorded in the past (2014–2019), 
Y – plots from green tracks recorded in the present (2020), numbers 
represent numbers of localities – for explanation see Table 1.
Slika 13: Dendrogram klastrske analize flore med tramvajskimi 
tračnicami na različnih lokacijah (uporabili smo binarno razdaljo in 
povprečno metodo povezovanja). Legenda: A, B, C, D, E – klastri A, 
B, C, D, E; X – ploskve na konvencionalnih progah med leti 2014 in 
2019, Y – ploskve na ozelenjenih progah leta 2020, številka predstavlja 
število lokacij – za razlago glej Tabelo 1.
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The results of the cluster analysis of records from the 
tracksides are shown in Figure 14. The records from the 
tracksides were separated into five clusters, most of which 
were formed based on the period in which the records 
were recorded. Clusters A and D predominantly merge 
the old records, cluster C the recent ones. Only cluster 
B contains both the old and the more recent trackside 
records (Figure 14).
of dry habitats with open vegetation, they both offer suit-
able conditions for the growth of archaeophytes (Chytrý 
et al., 2008). It is interesting that in Bratislava archaeo-
phytes prevailed over neophytes not only on the strict rail 
yard, but also on tracksides (Figure 8), where vegetation is 
denser than on the strict rail yard. Nevertheless, tracksides 
still present dry habitat with relatively open vegetation 
compared to other city habitats, which could account for 
the high proportion of archaeophytes on tracksides. 
The majority of studies from the urban environment 
which evaluate the changes in the representation of al-
ien species find an increase of the number of alien spe-
cies, mainly neophytes (Chocholoušková & Pyšek, 2003; 
Lososová & Simonová, 2008; Medvecká et al., 2009; No-
bis et al., 2009; Gregor et al., 2012). Our study shows 
the same results for the trackside habitat (Figure 10a, b), 
but on the strict rail yard, the opposite pattern holds. 
Both the Sign test as well as the Wilcoxon two-sample 
test showed a significant decrease in the occurrence of ar-
chaeophytes and neophytes compared to the past (Figure 
9a, b). In the presentation of our results, we have to take 
into account the fact that our study was done on specific 
tram track habitats and does not evaluate other urban 
habitats, while studies of other authors have researched 
a wider spectrum of habitats. Moreover, in our case, the 
habitat conditions changed significantly and in a univer-
sal direction, as conventional tram tracks were replaced by 
green ones. This difference could explain why the studies 
which are focused mainly on changes generate different 
results compared to ours.
From the total of the strict rail yard species, 72 taxa (the 
majority) saw a decline in frequency in comparison with 
past (Figure 5). The decrease in the frequency of the ma-
jority of total species number makes the decrease in fre-
quency of the alien taxa group more probable. Moreover, 
not only the frequency of occurrence, but also the species 
numbers are lower than they were in the past (Figures 3, 
7), a fact which could also have an impact on the results.
The most probable explanation for the decrease of the 
species numbers and frequencies on the strict rail yard 
seems to be a conversion of conventional tram tracks 
into green ones. Compared to the past (i.e. conventional 
tram tracks), green tram tracks host mainly planted suc-
culents, which compete with spontaneously growing rud-
eral plants. Succulents planted on green tram tracks now 
occupy the major part of the habitat, and therefore only 
a few spontaneously growing ruderal plants can succeed. 
From this we can conclude that on the strict rail yard area 
of tram tracks, succulents represent strong competitors, 
as the numbers and frequency of occurrence of spontane-
ously growing ruderal plants on this habitat is lower than 
it was on the tracks without planted succulents (i.e. in the 
Figure 14: Dendrogram of cluster analysis of the flora of different 
trackside localities (binary distance and average linkage method were 
used). Explanation: A, B, C, D – clusters A, B, C, D; X – plots from 
conventional tracks recorded in the past (2014–2019), Y – plots from 
green tracks recorded in the present (2020), numbers represent num-
bers of localities – for explanation see Table 1
Slika 14: Dendrogram klastrske analize flore med robovi tramva-
jskih prog na različnih lokacijah (uporabili smo binarno razdaljo in 
povprečno metodo povezovanja). Legenda: A, B, C, D, – klastri A, B, 
C, D,; X – ploskve na konvencionalnih progah med leti 2014 in 2019, 
Y – ploskve na ozelenjenih progah leta 2020, številka predstavlja število 
lokacij – za razlago glej Tabelo 1.
Discussion
The ruderal flora of tram and railway tracks consist of 
both, native and alien species. Non-native plant taxa are 
an important components of these habitats (Hansen & 
Clevenerg, 2005; Kelcey & Müller, 2011; Wiłkomirski et 
al., 2012; Zaliberová & Májeková, 2014; Májeková et al., 
2016; Denisow et al., 2017; Jehlík et al., 2017; Wrzesien 
& Denisow, 2017). Our results from Bratislava con-
firmed this phenomenon. Both older, conventional tram 
tracks and newer green tram tracks host a relatively high 
proportion of alien species (Figures 7–8).
In both datasets, the older and the more recent, ar-
chaeophytes prevailed over neophytes and this pattern was 
found in both strict rail yard and tracksides (Figures 7–8). 
Findings of many authors, who have researched railways 
(Galera et al., 2014; Májeková and Limánek, 2016; Máje-
ková et al., 2016; Jehlík et al., 2017) bring the same results 
as our study; archaeophytes were more abundant than 
neophytes. As both tram and railway tracks are examples 
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past), and that this also applies for spontaneously growing 
alien plants. 
Another factor which could lead to the lower frequency 
of occurrence of ruderal species in present could be the 
fact that the green tram tracks were built in Bratislava only 
recently. As a  result, this research could be done only in 
2020. Therefore, the habitats studied could lack species 
from the later succession stages. In addtion, the substrate 
from before 2020 was removed when railway tracks were 
rebuilt. Underground organs of perennial species were re-
moved by this process, so annual plants could dominate 
the first year. Also, the seed bank could be impoverished if 
the previous substrate was removed. The results of the ex-
planatory analysis of the life forms agree with these ideas, as 
they revealed the increase of the therophytes and decrease 
of hemicryptophytes between old data (i.e. conventional 
tracks) and recent data (green tracks) (Figures 11–12).
Species such as Achillea millefolium and Cichorium inty-
bus occur on the strict rail yard less frequently in the year 
2020 than in the past and species such as Ballota nigra, 
Clematis vitalba, Elymus repens, Lolium perenne, Lotus cor-
niculatus, Pastinaca sativa, Potentilla argentea, which were 
recorded in the past, did not occur on the strict rail yard 
of tram tracks in 2020 at all (Table 2). Most of these spe-
cies are hemicryptophytes, which grow typically in the 
vegetation of the phytosociological classes Artemisietea 
vulgaris and Molinio-Arrhenatheretea, which emerge in 
the later succession stages. It is possible, than, that some 
spontaneously growing ruderal plants will emerge in the 
coming years, but the number of these species should not 
be high, as they will always have to compete with planted 
succulents. 
The differences between conventional tracks (old data-
set) and green tram tracks (recent dataset) were confirmed 
by the cluster analysis. The majority of clusters (Figures 
13–14) were separated based on the period in which they 
had been recorded. A minority of clusters merge the old 
and more recent records, e.g. cluster B in the dendrogram 
of the records from tracksides, which could be caused by 
the fact that the clusters merge the old and more recent 
records from the same localities (Figure 14). The cluster 
analysis showed that the species composition in these par-
ticular plots remained similar, but that it did change on 
the majority of localities, which form other clusters. 
Compared to  conventional tracks, green tram tracks 
hosted fewer spontaneously growing ruderal 'weed' 
plant taxa (Figures 3–4), which are perceived as unde-
sirable by city residents, unlike planted taxa on green 
tracks (Özgüner & Kendle, 2006). However, the process 
of building green tram tracks can also carry risks. The 
number of ruderal species in 2020 is lower than it was 
in the past, but we have recorded some particular taxa 
such as Amaranthus blitoides, Carduus acanthoides, Ce-
rastium fontanum subsp. vulgare, Geranium pyrenaicum, 
Oxalis dilenii, Rumex acetosella, Urtica dioica, Urtica urens 
growing directly on green tram tracks in the present and 
none of these species occurred in the ruderal flora of con-
ventional tram tracks in the past (Table 2). The major-
ity of the mentioned species are alien species in Slovakia. 
Propagules of some ruderal species, which were recorded 
only in the year 2020, could have been brought in with 
the construction material in the process of reconstruc-
tion of tram tracks. Construction sites are good environ-
ments for the spreading of alien species, as they present 
constantly disturbed habitats with many empty niches, 
and they experience strong disturbances (Chytrý et al., 
2005; Simonová & Lososová, 2008). Since the construc-
tion material is imported from foreign countries (Poland 
in our case), propagules of some alien plant taxa, as well 
as some potentially invasive, can be introduced into the 
new territories in this way. 
Moreover, some of the species planted on green tram 
tracks (e.g. Sedum hispanicum, Sedum spurium) and their 
tracksides (e.g. Calendula officinalis, Cerastium tomento-
sum, Fagopyrum esculentum, Ligustrum ovalifolium, Malva 
moschata, Spiraea japonica) in Bratislava are listed as neo-
phytes in the List of alien species of Slovakia (Medvecká 
et al. 2012). 
Many authors (Reichard & White, 2001; Dehnen-
Schmutz et al., 2007; Hulme, 2011; Pyšek et al., 2012; 
Lehan et al., 2013; Pyšek et al., 2015; Perg et al., 2016; 
Mayer et al., 2017) have proven the trend of alien plants’ 
escapes from cultivation (horticulture, botanical gardens 
etc.) and of becoming invasive in the future. Therefore, 
it follows that plants cultivated on tram tracks may also 
constitute a risk of being an source of alien, potentially 
invasive taxa. 
From the aspect of the potential risk of plant invasion, 
the strict rail yard of green tram tracks covered by suc-
culents turned out to be a biotope with lower risk than 
conventional tracks, as they hosted a lower number of 
spontaneously growing alien taxa, and their frequency 
of occurrence also was lower than on conventional tracks 
(Figures 7, 9a, b). However, they may still present a risk 
of being a pool of planted alien potentially invasive taxa. 
But the strict rail yard of conventional tracks presents 
even higher risk, as this habitat hosted a higher number 
of alien taxa, which occurred more frequently (Figures 7, 
9a, b) than on the strict rail yard of green tracks. 
Covering the tracksides of green tracks with ornamen-
tal non-succulent plants did not prove to be a very good 
choice, as they hosted more alien plant taxa, and these 
also occurred more frequently than on conventional 
tracksides (Figures 8, 10a, b). 
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Tram and railway habitats can serve as the refuge areas 
for endangered and rare species (Májeková et al., 2014; 
Májeková & Limánek, 2016). Results of our study re-
vealed, that rare taxa such as Filago arvensis, Misopates 
orontium, and Petrorhagia saxifraga, which grew on con-
ventional tram tracks of Bratislava in the past, were not 
recorded on green tracks in 2020. We also recorded a 
lower total number of spontaneously growing vascu-
lar taxa as well as bryophyte taxa on green tracks than 
on conventional tracks (Figures 3–4, Tables 2–3). The 
maintenance of the highest biodiversity possible should 
be one of the priorities in the efforts of environmental-
ists. Bryophytes have many beneficial functions for the 
urban ecosystem (e.g. they contribute to biodiversity, im-
prove microclimatic conditions, accumulate pollutants 
and heavy metals, increase humidity, serve as a shelter 
for invertebrates etc.) (Haines & Renwick, 2009; Bahu-
guna et al., 2013; Jiang et al., 2018). Therefore, from the 
perspective of biodiversity, the green tracks of Bratislava 
do not appear to be a very good solution, but we must 
remember, that only the number of spontaneously grow-
ing taxa was assessed. If we also include the planted taxa 
into the analyses, the species number would be higher 
and biotope would not present itself as species-poor. 
Moreover, it is possible that some species (including 
bryophytes) might recolonize these habitats. Neverthe-
less, several other authors have confirmed that unman-
aged or lightly managed urban areas have higher biodi-
versity than intensively managed green areas (Säumel et 
al., 2016; Unterweger et al., 2017). This is not related 
only to the plant biodiversity, but also to the diversity of 
animals. For example, Walker et al. (2017) suggest that 
demolition activities, which are connected with the in-
troduction of green spaces in cities do not increase avian 
species richness.
On the other hand, Özgüner and Kendle (2006) proved 
that from an aesthetic point of view, managed green spac-
es are more valued by the majority of city residents than 
places covered by more diverse spontaneously growing 
ruderal vegetation. 
The question of whether it is better to leave tram tracks 
and other urban habitats to self-development or to main-
tain them remains a matter of worldwide discussion. Our 
conclusion is that, at least in Bratislava, there are positive 
and negative aspects of both green tracks and the con-
ventional ones. It must be remembered that we can not 
consider the conventional tracks as habitat which has 
been left entirely to self-development, as some controlled 
human intervention (e.g. trackside mowing) takes place 
here from time to time. On the other hand, controlled 
interventions on green tracks were more comprehensive, 
as these areas have been covered by a completely new 
spectrum of planted species. The upside of green tracks 
in Bratislava is that the tracks were covered with carpets 
of many evergreen Sedum species, which have only low 
ongoing maintenance requirements and thus require less 
human interventions. The resulting environmental and 
economic advantages of Sedum green tracks have been 
noted e.g. by Schreiter (2010). However, there is a nega-
tive side with respect to Sedum tracks; namely, that these 
do little to contribute to biodiversity, a fact which is also 
confirmed in a technical report from Toronto (Eglinton, 
2014). Sikorski et al. (2018) have stated that it was not 
planted species, but mostly spontaneously growing plants 
which have contributed substantially to the greenness 
coverage of green tracks in Warsaw. The authors of this 
study suggested that low-maintenance green tram tracks 
undergoing spontaneous succession may be a good en-
vironmental solution, but they admit, that such strat-
egy requires further studies. Another advantage of low-
maintenance green tracks is the fact that they present an 
economically better solution than intensively maintained 
green tracks (Sikorski et al., 2018). 
Based on our results we suggest attempting to imple-
ment more native species into green carpets on tram 
tracks in Bratislava to increase their biodiversity and 
eliminate the risk of alien species invasions. Many native 
species have high aesthetic value and could prosper on 
tram tracks’ biotope. This idea needs to be considered in 
advance and the spectrum of these species should be com-
piled by specialists (technicians, engineers, botanists, en-
vironmentalists etc.). A similar approach has been taken 
on some sections of Vienna’s tram tracks (Steckler et al., 
2012), where particular attention has been paid to the 
ecological aspects of green tracks. In this effort, self-suffi-
cient salt and drought resistant flowering plants of native 
origin were added into commercial mixtures of planted 
species. 
All species planted in tram tracks are presumably treat-
ed by breeding measures. With the taxa having near rela-
tives within the local flora genetic interactions could oc-
cur (e.g. hybridizing, backcrossing) which may be able to 
change the local gene pool. Greening process which use at 
least partly nonresident species could be counterproduc-
tive. In Germany the concept of „local seed and plants“ 
was developed, which propose to carry out greening of 
traffic infrastructure only with seeds locally yielded and 
aggrandized (Prasse et al., 2010). We also recommend to 
rather avoid to use alien taxa and try to use more autoch-
thonous ornamental species in the greening process of 
tram tracks. These should be such native species which 
are no able to change lokal genetic pool dramatically and 
which can also exist in the environmental conditions of 
tram tracks, e.g. Sedum acre, S. album.
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Green spaces and green tram tracks should serve as 
areas, which help to maintain the biodiversity of cities 
(Sikorski et al., 2018), but invasive species, which can 
potentially arise due to the construction of green areas, 
could contribute to the reduction of biodiversity at the 
end. Our study revealed the decrease of the total num-
ber of species and the decrease of the frequency of oc-
currence of archaeophytes and neophytes on the strict 
rail yard and the increase on the tracksides. Although the 
recent research was only conducted in 2020, it presents 
an important first view of the situation of plant dynamics 
on tram tracks and an interesting comparison between 
conventional and green tram track habitat. Fundamental 
changes during the next years are likely to expect. The 
study of tram track habitats of Bratislava shall continue 
in next years. 
Acknowledgements
We thank Dr. George Scott Burgess for English grammar 
proofreading of the manuscript. We are grateful to Doc. 
Katarína Mišíková, PhD. for identification of bryophytes. 
This study was supported by the Operation Program of 
Research and Innovation for the project: Advancing Uni-
versity Capacity and Competence in Research, Develop-
ment and Innovation,  ITMS2014+: 313021X329, co-
financed by the European Regional Development Fund. 
The research was also supported by the APVV-18-0052, 
APVV-17-0066, and the Grant Agency VEGA, Grant 
No. 1/0767/17. We are grateful to Greenfond Polska Sp z 
o.o., which has provided us with the list of Sedum species 
of green tram tracks of Bratislava. 
Alena Rendeková  https://orcid.org/0000-0003-2083-4929
Karol Mičieta  https://orcid.org/0000-0003-0676-3933
Michal Hrabovský  https://orcid.org/0000-0002-7834-4167
Eva Zahradníková  https://orcid.org/0000-0002-0632-6146
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