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Prevalence of alien versus native species of woody plants in Berlin
differs between habitats and at different scales
Zastoupení nepůvodních a původních dřevin v Berlíně závisí na typu stanoviště a prostorovém měřítku
Ingo K o w a r i k1, 2, Moritz v o n d e r L i p p e1, 2 & Arne C i e r j a c k s1, 2
1Department of Ecology, Technische Universität Berlin, Rothenburgstr. 12, D 12165 Berlin,
Germany, e-mail: kowarik@tu-berlin.de, moritz.vdlippe@tu-berlin.de, arne.cierjacks@
tu-berlin.de; 2Berlin-Brandenburg Institute of Advanced Biodiversity Research (BBIB),
14195 Berlin, Germany
Kowarik I., von der Lippe M. & Cierjacks A. (2013): Prevalence of alien versus native species of
woody plants in Berlin differs between habitats and at different scales. Preslia 85: 113–132.
Cities are hotspots for plant invasions and woody plants that have escaped from cultivation contrib-
ute significantly to this phenomenon. Yet whether the richness of alien species in the floras of
woody plants in urban areas also corresponds to a prevalence of alien species at the habitat and pop-
ulation levels is an open question. To explore the scale and context dependence of invasions by
woody plants of urban environments, we analysed the occurrence of alien and native species of
trees, shrubs and vines at the city, habitat and community scales in Berlin, Germany. The percentage
of alien species in the flora of spontaneously occurring woody plants increased from 16% at the end
of the 18th century to 67% two hundred years later. Of the 181 species of alien woody plants in
Berlin’s flora 32% have become naturalized. Species from other parts of Europe, the Mediterranean
and western Asia escaped and became naturalized more frequently than species from other areas.
Escape from cultivation did not increase the share of evergreens in the total flora of woody plants.
All habitats other than wetlands had more alien than native species, and the percentage of alien spe-
cies was significantly higher in green spaces, wastelands and residential areas than in forests and
wetlands. However, native species were more frequent at the habitat scale. Overall, the trees most
likely to be found in all habitats were native Acer platanoides,Betula pendula,Quercus robur and
alien Robinia pseudoacacia,Acer negundo and Prunus serotina, and the most frequent shrubs the
native Sambucus nigra and alien Mahonia aquifolium. At the communityscale, counts of the num-
bers of individual trees in two selected study areas revealed that native species prevailed in residen-
tial areas and alien species in urban wasteland. The results demonstrate that invasion success of
alien woody species in urban environments is strongly scale- and context-dependent. The clear
dominance of alien species in the total urban species pool was not similar at both the habitat and
community scales, particularly when the frequency of species is considered. In conclusion, assem-
blages of woody species in urban areas are not only characterized by high numbers of aliens but also
by an increase in the abundance of native species such as the formerly rare Acer platanoides and
A. pseudoplatanus, which now prevail due to enhanced propagule pressure and the eutrophication of
urban ecosystems.
Keywords:Ailanthus altissima, casual species, exotic species, functional traits, naturalization,
non-indigenous plant, ornamental species, urban heat island
Introduction
All over the world, urban regions are hotspots in terms of the wealth of woody species that
are cultivated there. Studies on parks (Zhao et al. 2010, Nagendra & Gopal 2011,
Abendroth et al. 2012), domestic gardens (Ringenberg 1994, Akinnifesi et al. 2010) and
tree plantings along streets (Jim & Chen 2008, Nagendra & Gopal 2010, Sjöman et al.
Preslia 85: 113–132, 2013 113
2012) reveal that among the cultivated plants there is a high percentage of alien species of
trees. In the plantings in green spaces in Hong Kong, China and Christchurch, New Zea-
land, for example, 73–84% of the trees are alien species (Jim 2000, Stewart et al. 2004).
Alien species also dominate the woody species cultivated in European landscape parks
(66% is the mean for three parks; Säumel et al. 2010). In Patras, Greece, introduced spe-
cies make up 59–75% of the planted woody species pool in various urban landscapes
(Tsiotsiou & Christodoulakis 2010).
Planting large numbers of non-native ornamentals usually results in a strong propagule
pressure in urban regions, which is an important driver of the subsequent naturalization of
these species (Mulvaney2001, Kowarik 2005, Lockwood et al. 2005, Křivánek et al. 2006,
Pyšek et al. 2009). Correspondingly, the frequency of species offered for sale by the horti-
cultural trade is associated with their later success in escaping from cultivation (Dehnen-
Schmutz et al. 2007). As a consequence, cities are usually rich in alien species (Kowarik
1995a, Pyšek 1998) of which escaped woody species are an important part. Moreover, the
steady supply from garden centres of semi-hardy plants from warmer regions along with
global warming is believed to foster the escape of these species from cultivation in many
parts of Europe (Niinemets & Peñuelas 2008).
The escape of ornamentals from cultivation can be enhanced by urban conditions, in
particular by urban heat island effects, which amplify the effects of global warming. As
a consequence, phenological changes such as earlier bud burst or flowering are recorded in
cities (Zacharias 1972, Roetzer et al. 2000, Shustack et al. 2009, Neil et al. 2010). The
interplay between urban heat islands, global warming and increased cultivation of alien
species may foster the establishment of plant species from functional groups that were pre-
viously under-represented in a region. A comparable example from natural habitats is the
increased spread of evergreen ornamentals in some regions associated with the tempera-
ture increase recorded during the last decades (Walther 2002, Walther et al. 2009). Also
species with other traits, such as animal dispersal, are over-represented in urban compared
to rural floras (Knapp et al. 2008, 2010). In New York, more alien than native species of
woody plants have fleshy fruits (Aronson et al. 2007), likely due to gardeners’ preferences
for species with shiny fruits (Knapp et al. 2009). There are other factors that enhance the
naturalization of introduced species in urban settings. Maintenance activities such as irri-
gation can help protect juvenile plants from detrimental environmental effects (Mack
2000), and tree plantings can facilitate population establishment by overcoming spatial
separation from adequate but otherwise inaccessible sites (Kowarik 2003).
Historical analyses of urban floras published over periods of decades and centuries
reveal a marked increase in the number of woody species (Chocholoušková & Pyšek 2003,
Knapp et al. 2010, Zhao et al. 2010, Gregor et al. 2012). This historical increase in alien
species richness goes along with an increase in the ranges of alien woody species in urban
landscapes over time as is recorded for the New York metropolitan region (Aronson et al.
2007).
At the habitat scale, most studies on woody species focus on forest remnants, either
addressing the establishment of native species (e.g. Lehvävirta & Rita 2002, Hauru et al.
2012), their role as sources of propagules for the reestablishment of native species in urban
areas (e.g. Doody et al. 2010) or invasion by alien species from urban areas (Bertin et al.
2005, Borgmann & Rodewald 2005, Duguay et al. 2007, Essl et al. 2011). There are few
studies on urban green spaces (Stewart et al. 2004), residential areas (Ringenberg 1994,
114 Preslia 85: 113–132, 2013
Stewart et al. 2004), wastelands (Kowarik 1990a, Trentanovi et al. 2013) or walls and
buildings (Jim 2008, Jim & Chen 2011). Comparative studies of the woody species that
have escaped cultivation occurring in several habitats, however, are rare (but see Kunick
1987, Stewart et al. 2009, Tsiotsiou & Christodoulakis 2010, Nowak 2012).
Escape from cultivation is an important conservation issue because urban plantings can
function as invasion foci for the spread of alien species into adjacent landscapes (Sullivan
et al. 2005, Vidra & Shear 2008), although studies of this risk have yielded ambiguous
results (Botham et al. 2009, Kowarik 2011). Moreover, plantings of native species could
provide a source of propagules for the (re)establishment of native woody vegetation in
urban habitats (Stewart et al. 2004, Doody et al. 2010, Woodall et al. 2010). Finally, novel
assemblages of urban species, which may be well adapted tourban conditions and provide
cities with a range of ecosystem services (Kowarik 2011), may be shaped by escapees. Yet,
as Lehvävirta (2007) states, there are significant gaps in our knowledge of the regeneration
of assemblages of urban woody species and the underlying mechanisms.
Taking the city of Berlin, Germany, as an example of a central-European metropolitan
region, we explored native and alien species in pools of woody plants at different scales. (i)
At the city scale, we analysed the richness of native and alien species of woody plants and
changes in the percentage of alien species since the 18th century, and (ii) tested whether
the origins of the woody plants that have escaped cultivation in Berlin reflect the origins of
all the introductions of woody species into central Europe. (iii) As alien species are known
to change the composition of urban floras in terms of functional traits (Aronson et al.
2007, Knapp et al. 2009), we analysed changes since the 18th century in the representation
of life forms and evergreen species within the flora of woody plants. (iv) To determine
whether the occurrence of species in different urban habitats is uneven (Lososová et al.
2011), we assessed the number and frequency of woody species in five major urban habi-
tats for which we calculated the percentage of alien species (v). To determine whether
these percentages are the same at the city, habitat and community scales, we analysed two
datasets from selected study areas in which there were trees in different vegetation layers.
Methods
Study area and data sources
The study area was Berlin, Germany, which has a population of about 3.5 million and cov-
ers an area of 892 km2. The prevailing natural vegetation is deciduous forest. Near-natural
forest remnants growing mostly on sandy acidic soils at the urban fringe are dominated by
Quercus robur, Q. petraea,Pinus sylvestris and by Alnus glutinosa on wet sites (Sukopp
1990). Currently, about half of Berlin’s surface consists of built-up areas, 20% is forested,
green spaces make up 10%, moving and standing water 6% and agricultural land 7%
(SenStadt 2008).
The long history of floristic research in Berlin offers excellentopportunities for assess-
ing changes in the flora of woody plants (Sukopp 1987, Krausch & Sukopp 2010).
Regional horticultural and silvicultural literature provides records of cultivation starting in
1594 (e.g. Franke 1594, Elssholtz 1663). The flora of Willdenow (1787) first differenti-
ated between cultivated and spontaneously occurring species and subsequent floras pro-
vide plenty of information on the latter (e.g. Ascherson 1864, Bolle 1887). Since the
Kowarik et al.: Alien and native woody plants in Berlin 115
1970s, there have been ecological studies on almost all near-natural and urban land use
types in Berlin, which include a substantial amount of data on native and alien species at
the habitat scale (see syntheses by Sukopp 1990, Kowarik 1992).
The origins of escaped species were determined by using data from Goeze (1916),
Fitschen (1987) and Meusel et al. (1965, 1978). The compilation by Goeze (1916) also
provides data on the regions of origin of 2645 species of woody plants introduced into cen-
tral Europe up to the beginning of the 20th century. This allowed us to test whether the ori-
gins of the species of woody plants that have escaped and possibly naturalized in Berlin
match that of all central-European introductions of species of woody plants.
To explore changes in the composition of life forms in the flora of woody plants, we
relied on the classification of Fitschen (1987) of tall, medium-sized and small trees, tall
and small shrubs, dwarf shrubs, half-shrubs and woody vines, and the same source was
also used to distinguish species with evergreen or deciduous leaves. Both traits are relevant
as leaf persistence and the capacity of a species to grow tall are often associated with inva-
sion success (Reichard & Hamilton 1997, Pyšek & Richardson 2007). We calculated their
representation in native and all escaped alien species, and separately for naturalized spe-
cies, i.e. the subcategory of alien species with records of at least two spontaneous genera-
tions over a period of at least 25 years (Kowarik 1992). Alien species consist of
archaeophytes (pre-1492 introductions) and neophytes (post-1492 introductions; Rich-
ardson et al. 2000). The nomenclature follows Fitschen (1987).
To assess numbers of species and frequency of native and alien species of woody plants
at the habitat scale, we used existing data on species occurrence in five groups of habitat
types (hereafter ‘habitats’): (i) remnants of natural forest (without wetlands; 405
phytosociological relevés from a larger data set; Kowarik 1990b), (ii) near-natural
wetlands (peat, swamp woods; 201 relevés from a larger data set; Kowarik 1990b), (iii)
urban green spaces (parks, cemeteries etc.; 221 relevés from eight studies), (iv) urban
wastelands (abandoned railway areas, brownfields etc.; 151 relevés from 11 studies), (v)
urban residential areas (courtyards, domestic gardens, paved habitats in densely built-up
areas, etc.; 405 relevés from six studies). The data, which consisted mostly of results from
unpublished research reports and theses (see Kowarik 1992 for references), were used to
assess species richness of native and alien trees, shrubs and woody vines in each habitat.
Then, frequency data from these studies were combined for each species for each habitat.
To allow for comparisons of the relative frequency of species among habitats, the value for
the most frequent native or alien species was set at 100% for each habitat and the frequen-
cies of all other species standardized to this maximum value.
Finally we illustrated the performance of native and alien species of tree at the commu-
nity scale by analysing two data sets from different areas, which include numbers of indi-
viduals (or ramets in species with clonal growth; for reasons of simplicity we henceforth
refer to both as ‘individuals’). The first data set is for a densely built-up part of the histori-
cal centre of Berlin (Spandauer Vorstadt, Berlin-Mitte; 46.2 ha; data from Mücke & Kliese
1991); the second is for woodland on urban wasteland, which is dominated by self-sown
alien Robinia pseudoacacia trees (28 plots 10 × 10 m; data from Kowarik 1990a). In both
surveys, the total number of spontaneously growing individuals of native and alien spe-
cies of trees was counted in the herbaceous (< 0.9 m), shrub (0.9–5.0 m) and tree layers
(> 5 m).
116 Preslia 85: 113–132, 2013
Statistical analyses
Numbers of species and individuals were summarized in two-way contingency tables,
which cross-classify the status as native/alien or as naturalized/non-naturalized (based on
the difference in the numbers of escaped and naturalized species) versus the region of ori-
gin (Table 1), life form (Table 2), leaf persistence, habitat (Table 3) and vegetation layer
(Table 6). In addition, we prepared contingency tables with the status introduced/escaped
and introduced/naturalized cross-classified to the species’ origin. These tables were used
to fit log-linear models with and without a possible interaction between the two categories.
Significant differences between the reduced model (model without interaction) and the
full model (with interaction) along with a lower Akaike Information Criterion (AIC) in the
full model indicated a significant interaction (Quinn & Keough 2003). In case of signifi-
cant interactions, we used Cohen-Friendly association plots to detect a deviation from
independence of a factor combination (Adler 2010). All statistics were calculated in R ver-
sion 2.15.0 (The R Foundation for Statistical Computing 2012).
Results
City scale
Seventeen species of escaped woody plants that are alien to Berlin were recorded in the
flora of Willdenow in 1787. Among these are fruit trees (e.g. Prunus cerasus,Malus
domestica) that have been cultivated since medieval times and some ornamentals first
cultivated more than 100 years before in the Berlin’s palace garden (e.g. Aesculus
hippocastanum,Syringa vulgaris; Elssholtz 1684). By the 1990s, the number of alien spe-
cies had increased nearly 11-fold to 181 species of woody plants that had escaped cultiva-
tion. This number is twice that of species of woody plants native to Berlin (89 species).
The percentage of alien species increased from 16.0% at the end of the 18th century to
66.8% at the end of the 20th century (Fig. 1A). Almost one-third (31.5%) of all escapees
have naturalized in Berlin (57 species).
Other regions of Europe and the Mediterranean are the most important donor areas for
escaped woody plants in Berlin (37.3%), followed by North America and other parts of
Asia (Table 1). The log-linear model showed a significant interaction between the origins
of alien species and their status as introduced/escaped (G2= 161.64, df = 5, P < 0.001, AIC
reduced model = 294.9, AIC full model = 173.0). Likewise, we found a significant interac-
tion between the origin of aliens and their status as introduced/naturalized (G2= 85.61, df
= 5, P < 0.001, AIC reduced model = 212.4, AIC full model = 166.3). In both models, the
number of escapees and naturalized species, respectively, was significantly higher than
expected for species from Europe and western Asia and lower for species from other
donor areas (Table 1).
In nearly all of the categories of life forms in the total woody flora of Berlin there were
higher numbers of alien than native species (except dwarf and half shrubs, Table 2).
Among alien species, shrubs were the prevailing life form (57%), but among naturalized
species, trees and shrubs contributed equally. The log-linear models showed no significant
interaction between life form and the alien/native or naturalized/non-naturalized status.
We thus found no evidence for shifts in life forms within the urban flora of woody plants
Kowarik et al.: Alien and native woody plants in Berlin 117
118 Preslia 85: 113–132, 2013
Fig. 1. – Percentages of alien species of woody plants in Berlin at different temporal and spatial scales; (A) city
scale: changes in the percentage since the 18th century; (B) habitat scale based on numbers of species: percentages
calculated for five habitats; (C) habitat scale based on species frequency; (D) community scale: percentages calcu-
lated for Robinia pseudoacacia woods in urban wastelands and an inner-city district (Spandauer Vorstadt), based on
the number of individuals or ramets in different demographic stages (i.e. herbaceous, shrub, tree layers). Lower case
letters refer to groups that deviate similarly in terms of their status as either alien or native species and habitat or veg-
etation layer from independence based on the Cohen-Friendly association plots. See text for further details.
Kowarik et al.: Alien and native woody plants in Berlin 119
Fig. 1. – Continued
due to escape from cultivation or naturalization. The same holds for evergreen species,
which made up almost the same percentage of native species (15.9%) as of escaped
(15.7%) and naturalized (14.8%) alien species.
Table 1. – Origins of the species of woody plants introduced into central Europe (calculated using data from
Goeze 1916) and invasion success of these species in Berlin, Germany (escaped, naturalized species; multiple
entries for species that originate from more than one area; including Rubus species). Species numbers (n) and per-
centages of the total (%) are given.
Origin Introduced into
central Europe
Escaped in Berlin Naturalized in Berlin
n% n% n%
Europe, nemoral zone 122 4.6 51 21.6 19 23.5
Europe, meridional zone 187 7.1 37 15.7 16 19.8
Western Asia (incl. Caucasus, Taurus) 128 4.8 32 13.6 14 17.3
Central and northern Asia 304 11.5 11 4.7 5 6.2
Eastern Asia (incl. Himalayas) 1047 39.6 37 15.7 9 11.1
North America 857 32.4 58 24.6 16 19.8
Unknown or horticultural origin n.a. 10 4.2 2 2.5
Total 2645 181 57
Table 2. – The different life forms recorded in the flora of woody plants of Berlin. Data are shown for native spe-
cies and escaped alien species, naturalized species are a subgroup of the latter (without Rubus). Species numbers
(n) and percentages of the total (%) are given. Because a species may exhibit more than one life form additions of
the values for the subcategories do not add up to the totals for each category.
Life form Native Escaped Naturalized
n% n% n%
Total tree species 29 42.0 68 39.5 26 48.1
Tall trees 19 32 10
Medium sized trees 9 23 9
Small trees 14 40 13
Total shrub species 38 55.1 98 57.0 26 48.1
Large shrubs 25 59 21
Small shrubs 15 51 11
Dwarf shrubs 6 5 0
Half-shrubs 4 3 1
Woody vines 2 2.9 6 3.5 2 3.7
Total woody species 69 100 172 100 54 100
Habitat scale
The flora of the more urbanized habitats (green space, wasteland, residential) was much
richer in spontaneously occurring species of woody plants than that of near-natural habi-
tats (Table 3). More than 170 species were recorded in both wastelands and green spaces;
even in residential areas more species (155) occurred than in near-natural forests (141) or
wetlands (40).
120 Preslia 85: 113–132, 2013
Table 3. – Richness of species of woody plants in urban habitats in Berlin (n) and percentageof native (%N) and
alien species of woody plants (%A). Data are shown for all species ofwoody plants (including vines) and for trees
and shrubs. Values calculated from presence/absence data (100% is the total number of species for a habitat) and
from frequency data (100% is the sum of all the frequencies of all native, or alien, species in each habitat) are
shown.
All species Trees Shrubs
Habitat type Measure n %N %A n %N %A n %N %A
Green spaces presence/absence 171 30.4 69.6 85 31.8 68.2 81 28.4 71.6
frequency 51.3 48.7 56.9 43.1 42.8 57.2
Wastelands presence/absence 173 33.5 66.5 76 38.2 61.8 92 29.3 70.7
frequency 51.5 48.5 56.0 44.0 49.9 50.1
Residential areas presence/absence 155 34.2 65.8 69 40.6 59.2 81 28.4 71.6
frequency 51.1 48.9 60.0 40.0 39.0 61.0
Forests presence/absence 141 43.3 56.7 67 43.3 56.7 70 42.9 57.1
frequency 71.1 28.9 70.7 29.3 74.4 25.6
Wetlands presence/absence 40 87.5 12.5 22 81.8 18.2 18 94.4 5.6
frequency 94.8 5.2 93.5 6.5 96.3 3.4
Across all habitats and life forms, except for wetlands, the number of alien species
exceeded that of native species of woody plants. About two-thirds of the species recorded
in green spaces, wastelands and residential areas were alien, with a higher percentage of
introduced species of shrubs compared to trees. Consistently, the log-linear model showed
a significant interaction between habitat and the status as native or alien (G2= 50.77, df =
4, P < 0.001, AIC reduced model = 148.1, AIC full model = 123.4) with a higher percent-
age of alien species than expected in the three more urbanized habitats compared to the
two near-natural habitats.
Considering the frequencies of species in the different habitats, alien species were less
prominent in urban environments (Table 3, Fig. 1C). Native species clearly prevailed in
wetlands and forests and were also more frequent than aliens in the more urbanized habi-
tats. While native species of trees were generally more frequent in all the habitats, alien
species of shrubs dominated in green spaces and residential areas.
Table 4 ranks native and alien species of trees according to their relative frequency in
the five habitats. In the top rank are native tree species (Acer platanoides,Betula pendula,
Quercus robur,A. pseudoplatanus), which were the species most likely to be found in all
the habitats except wasteland, where the North American Robinia pseudoacacia was most
frequent. Other highly frequent alien species of trees were Acer negundo (all habitats
except wetland), Prunus serotina (mostly in forests and degraded wetlands), Aesculus
hippocastanum (mostly in residential areas), Quercus rubra (mostly in green spaces) and
Ailanthus altissima (mostly in residential areas and wasteland).
Similarly to trees, native species of shrubs were more frequent in all habitats than alien
species of shrubs, with Sambucus nigra and three Rubus species at the top of the list (Table
5). Mahonia aquifolium, a cultivated hybrid complex of different North American species
(Ross & Auge 2008), was the most common alien species of shrub (mostly in residential
areas and green spaces), followed by Syringa vulgaris and Symphoricarpos albus.Ribes
uva-crispa was the most frequent alien species of shrub in forests. In wetlands, alien spe-
cies of shrubs only played a minor role, with Cornus stolonifera rarely established adja-
cent to the reed zones in riparian systems.
Kowarik et al.: Alien and native woody plants in Berlin 121
Table 4. – Relative frequency of spontaneously occurring native and alien species of trees in the five habitats in
Berlin. Species that are alien to Berlin are in bold. The values are frequency numbers for the different studies, which
were combined for each of the five habitats. In order to compare the relative frequencies in the different habitats the
maximum frequency of a species in a habitat was set at 100. Shown are species with a relative frequency ≥10 in at
least one habitat (Kowarik 1992). Species are ranked according to their mean frequency value (sum of habitat fre-
quencies divided by 5) in all habitats. * Mostly did not include the planted individuals in the tree layer.
Green spaces Wastelands Residential
areas
Forests Wetlands Mean
frequency
Acer platanoides 100 88 100 100 2 78
Betula pendula 81 91 78 29 100 76
Quercus robur 69 65 47 67 68 63
Acer pseudoplatanus 81 86 90 56 2 63
Robinia pseudoacacia 67 100 56 37 1 52
Acer negundo 70 70 56 38 3 47
Sorbus aucuparia 55 40 45 86 8 47
Prunus serotina 37 50 22 92 27 46
Pinus sylvestris 17 21 12 34* 91 35
Aesculus hippocastanum 34 32 63 37 0 33
Crataegus monogyna 26 74 39 24 1 33
Salix caprea 15 49 69 9 12 31
Quercus rubra 57 31 19 37 1 29
Ulmus glabra 39 53 38 14 0 29
Acer campestre 51 48 25 18 0 28
Tilia cordata 27 35 40 37 0 28
Ailanthus altissima 38 44 52 2 0 27
Populus tremula 45632261226
Fraxinus excelsior 36 35 15 25 7 24
Carpinus betulus 29 26 21 35 0 22
Prunus padus 28 33 22 19 7 22
Populus alba et canescens 10 61 20 5 0 19
Prunus avium 20 28 31 15 0 19
Quercus petraea 17 13 <1 50 9 18
Malus domestica 13 34 32 9 0 18
Taxus baccata 38 11 22 5 0 15
Alnus glutinosa 68294915
Populus ×canadensis 74413 6 014
Fagus sylvatica 91654014
Betula pubescens 013 0 74914
Salix ×rubens 621 6 42412
Prunus domestica 13 19 20 5 0 11
Pyrus communis 10 23 17 5 0 11
Juglans regia 19 16 15 5 0 11
Tilia platyphyllos 21 17 6 11 0 11
Prunus mahaleb 836 7 3 011
Laburnum anagyroides 18 14 20 2 0 11
Salix alba 22911 3 710
Ulmus laevis 711 913 3 9
Ulmus minor agg. 8 21 8 4 0 8
Prunus cerasus 10 8 17 <1 0 7
Sorbus intermedia 6220106
Celtis occidentalis 2193005
Populus nigra 'Italica' 7104004
Ulmus pumila 1334004
Hippophae rhamnoides 1161104
Elaeagnus angustifolia 2121103
Larix decidua 3001003
Quercus cerris 1100002
122 Preslia 85: 113–132, 2013
Table 5. – Relative frequency of spontaneously occurring native and alien species of shrubs in five habitats in
Berlin. Species that are alien to Berlin are in bold. The values are frequency numbersin each of the studies, which
were combined for each of the five habitats. In order to compare the relative frequencies of the species in each of
the habitats the maximum frequency of a species in a habitat was set at 100. Shown are those species with a rela-
tive frequency ≥10 in at least one habitat (Kowarik 1992). Species are ranked according to theirmean frequency
value (sum of habitat frequencies divided by 5) for all habitats.
Green spaces Wastelands Residential
areas
Forests Wetlands Mean
frequency
Sambucus nigra 100 100 100 100 11 82
Rubus idaeus 20 49 15 97 27 42
Rubus fruticosus agg. 38 69 14 64 4 38
Rubus caesius 23 90 6 51 4 35
Mahonia aquifolium 75 37 49 9 0 34
Rosa canina 28 83 24 34 0 34
Cornus sanguinea 28 57 21 26 0 26
Syringa vulgaris 27 52 40 11 0 26
Symphoricarpos albus 29 46 25 26 0 25
Euonymus europaea 31 48 9 33 0 24
Corylus avellana 33 23 38 24 0 24
Ligustrum vulgare 30 50 17 20 0 23
Frangula alnus 4 6 1 46 53 22
Vaccinium oxycoccos 0 0 0 <1 100 20
Philadelphus coronarius 16 34 33 7 0 18
Ribes rubrum 19 26 10 20 0 15
Lycium barbarum 21 38 10 5 0 15
Cornus stolonifera 13 24 16 7 12 14
Prunus persica 32040 3 013
Ribes alpinum 17 20 16 11 0 13
Salix aurita 20035612
Lonicera tatarica 13 34 8 4 0 12
Ribes aureum 841 5 3 011
Lonicera xylosteum 21 17 5 11 0 11
Ribes uva-crispa 33 30 15 35 0 11
Colutea arborescens 2354108
Vinca minor 2750508
Salix cinerea 113 2 417 7
Cornus alba 13 20 1 2 0 7
Rosa rugosa 5216307
Viburnum lantana 10 11 9 3 0 7
Viburnum opulus 5416917
Buddleja davidii 4623007
Cytisus scoparius 1193506
Sambucus racemosa 2212306
Vaccinium myrtillus 2002505
Berberis vulgaris 8102505
Prunus spinosa 0123805
Salix purpurea 1131265
Rosa corymbifera 021<1104
Salix triandra 0003194
Sorbaria sorbifolia 3115104
Rubus laciniatus 514 0<1 0 4
Rosa rubiginosa 212 4<1 0 4
Calluna vulgaris 3001043
Caragana arborescens 12814203
Kowarik et al.: Alien and native woody plants in Berlin 123
Community scale
The sampling of species of trees in Robinia woods undergoing succession in a residential
area revealed similar numbers of native and alien species of trees when the total species
richness and numbers in each of the three vegetation layers were compared. In terms of
abundance, native species of trees strongly prevailed in the residential area and alien spe-
cies were more abundant in the Robinia woods (Table 6).
In both samples, about 90% of all the individuals occurred as seedlings or young sap-
lings in the herbaceous layer. In the residential area, native species of trees clearly pre-
vailed in the herbaceous layer, but this dominance was not pronounced in the shrub and
tree layers. In contrast, alien species of trees dominated in all layers in the Robinia woods
(Table 6). For both surveys, log-linear models showed a significant interaction between
vegetation layer and whether the individual trees were alien/native (G2= 35.34, df = 4, P <
0.001, AIC reduced model = 100.8, AIC full model = 79.9 for residential area; G2=
213.81, df = 2, P < 0.001, AIC reduced model = 294.1, AIC full model = 97.9 for urban
wastelands). The shrub and tree layers consistently had a higher number of individuals of
alien species, whereas in the herbaceous layer the number was lower than expected.
Table 6. – Number of species and populationsizes of alien and native species of trees in (A) a residential area in
the historical centre of Berlin (Spandauer Vorstadt, 46.2 ha; adapted from Mücke & Kliese 1991) and (B) woods
in urban wastelands dominated by Robinia pseudoacacia (0.28 ha; adapted from Kowarik 1990a). Shown are
numbers of individuals (or ramets for species with clonal growth) per hectare in the herbaceous (> 0.9 m), shrub
(0.9–5.0 m) and tree layers (> 5.0 m). In addition, data for the three most abundant native and alien species of trees
in each study area are shown, with total counts in bold. The share of alien species refers to the abundanceof alien
and native species of trees.
A. Residential area B. Robinia woods
Layer Herb Shrub Tree Total Herb Shrub Tree Total
Alien trees
Species number 17 19 9 20 10 7 5 11
Individuals (n) 291 51 13 355 12,668 657 971 14,296
Individuals (%) 81.9 14.5 3.6 100.0 88.6 4.6 6.8 100.0
Most abundant:
Ailanthus altissima 146 30 7 184 0000
Acer negundo 67 11 3 81 6050 114 32 6196
Aesculus hippocastanum 37 0.4 0.3 38 0000
Robinia pseudoacacia 14 4 1.4 20 3000 464 918 4382
Prunus serotina 0.1 0.4 0 0.5 1886 29 0 1914
Native trees
Species number 15 16 9 17 10 8 4 10
Individuals (n) 968 60 10 1039 9246 379 43 9668
Individuals (%) 93.2 5.8 1.0 100.0 95.6 3.9 0.4 100.0
Most abundant :
Acer platanoides 602 28 5 635 4136 36 3.6 4175
A. pseudoplatanus 301 16 3 320 929 75 3.6 1007
Fraxinus excelsior 21 1.3 0.1 23 25 0 0 25
Quercus robur 1.2 0.1 0.2 1.5 1750 11 0 1761
Crataegus monogyna 0 0.1 0 0.1 1075 114 11 1200
Total tree species
Species number 32 35 18 37 20 15 9 21
Individuals (n) 1259 112 23 1349 21,914 1036 1014 23,964
Individuals (%) 90.3 8.0 1.7 100 91.4 4.3 4.2 100.0
Proportion of alien species (%) 23.1 46.0 55.4 25.5 57.8 63.4 95.8 59.7
124 Preslia 85: 113–132, 2013
In the residential area, the most abundant species of native tree was Acer platanoides,
of which there were twice as many individuals as of its congener A. pseudoplatanus. The
most abundant alien species of tree was Ailanthus altissima, followed by Acer negundo
and Aesculus hippocastanum. The total counts were generally higher for native than for
alien species of trees. However, the alien Ailanthus altissima was the most abundant spe-
cies in the tree layer (Table 6).
In the Robinia woods, Acer platanoides, again, was the most abundant native species,
followed by Quercus robur and Crataegus monogyna. Yet alien species prevailed in all
vegetation layers. Robinia pseudoacacia dominated the tree and shrub layers, but due to
high seedling numbers in the herbaceous layer Acer negundo was even more abundant.
The third among the top aliens was Prunus serotina.
Discussion
City scale
Our study highlights the important role of horticulture in plant invasions by repeatedly
introducing and encouraging the planting of ornamentals (Dehnen-Schmutz et al. 2007,
Kowarik & von der Lippe 2007). As in other cities (e.g. Chocholoušková & Pyšek 2003,
Zerbe et al. 2004, Knapp et al. 2010), alien species of trees and shrubs that escape cultivation
make up a major part of the alien flora of urban areas. The pronounced increase in species
that escaped cultivation during the second halves of the 19th and 20th centuries (Figure
1A) coincided with important periods in Berlin’s history. Berlin was named the German
capital in 1871 and the subsequent urban growth was certainly associated with an increase
in propagule pressure produced by the numerous newly established green spaces. In the
second period new sites became available for plants to grow following bombing in World
War II. Early studies report abundant populations of Acer negundo,Ailanthus altissima,
Clematis vitalba and Robinia pseudoacacia in demolished areas (Scholz 1960, Kohler &
Sukopp 1964). As these species were either previously absent or rare in the spontaneous
flora, their post-war occurrence is a good example of the unpredictable episodic temporal
variation in environmental conditions as a driver of plant invasions (Crawley 1989,
Hastings et al. 2005).
Alien species of woody plants in Berlin were more likely to have come from European
and western-Asian than American or eastern-Asian sources (Table 1). This result confirms
those of previous studies on assemblages of alien species in urban habitats (e.g. Lososová et
al. 2012). Consequently, the number of naturalized plants showed a similar geographic bias.
This seems to be related to introduction history. Residence time is known to be an important
predictor of plant invasions (Pyšek et al. 2009), and species from the former sources gener-
ally have been present for a longer time in central Europe and thus more likely to have
escaped and naturalized than species from the latter sources (Kowarik 1995b).
The log-linear models did not reveal significant interactions among life forms and the
species’ status as native, escaped or naturalized (Table 2). The fact that no life form predomi-
nates in these categories may be explained by the high diversity of urban habitats, which pro-
vides suitable conditions for all the woody life forms. In contrast to other studies (Walther
2002, Walther et al. 2009), our data do not show a general increase in evergreen species over
the last 200 years. The spread of some species, however, is likely to have been mediated by
Kowarik et al.: Alien and native woody plants in Berlin 125
favourable urban climates. Ailanthus altissima is currently one of the most abundant species
of tree in Berlin’s densely built-up areas (Tables 4, 6), i.e. where urban heat island effects are
strongest. Saplings of this species grow well at high temperatures in climate chambers
(Kowarik & Säumel 2007, Säumel 2007). Another striking example is Juglans regia.
Records of the cultivation of this species in Berlin date back to medieval times, but it started
to spread only about 500 years later, in the 1960s (Kowarik 1995b), asit did in other regions
(Loacker et al. 2007, Hetzel 2012). Determining the years of high germination revealed that
milder winters enhance seedling establishment (Loacker et al. 2007).
The milder urban climate is also expected to favour native species. An exposure experi-
ment revealed a higher percentage survival of saplings of Acer platanoides at inner-city
sites than in the adjacent countryside following a sudden severe frost in late autumn (von
der Lippe et al. 2005). Moreover, studies on urban populations of Taxus baccata that were
descendants of park plantings suggest that November temperatures below –7.5 °C limit
seedling establishment in this species (Iszkulo & Boratynski 2005). Nevertheless, the
warmer urban climates obviously do not compensate totally for harsh winters. This is pos-
sibly why Buddleja davidii occurs at a low frequency in Berlin (Table 5) despite the strong
propagule pressure generated by the numerous plantings of this species. In regions with
a mild oceanic climate, this species is very abundant (Ebeling et al. 2008, Tallent-Halsell
& Watt 2009, Wittig 2012).
Habitat and community scales
The log-linear models revealed a significantly higher number of alien species in urban
areas than in near-natural ecosystems. This confirms the well-known general invasion pat-
tern for species of woody plants. There are high numbers of alien species in many man-
made habitats because they are subject to frequent disturbances and there is a high avail-
ability of nutrients there (Kowarik 1990b, Chytrý et al. 2008).
With the exception of Aesculus hippocastanum, the most common alien species of trees
in Berlin (Robinia pseudoacacia,Acer negundo,Prunus serotina, Quercus rubra, Ailan-
thus altissima) are reported as invasive in other parts of Europe (e.g. Pyšek et al. 2012a)
and beyond (Richardson & Rejmànek 2011). Most populations of alien species, however,
do not conflict with conservation objectives in the densely built-up parts of Berlin as they
are components of novel urban ecosystems that are well adapted to urban sites and contrib-
ute an array of ecosystem services (Kowarik 2011). As a consequence, Robinia woods in
a former railway area have been included in a conservation area to allow the development
of novel urban forests (Kowarik & Langer 2005).
When considering species frequency, the percentage of individual trees and shrubs that
are alien does not reflect that most of the species are alien. Despite the high total species
richness of alien species of woody plants, native species were found more frequently in
urban areas, especially in forests. Even in the more urban habitats native species were
slightly more frequent than alien species (Table 4, Fig. 1C). Studies at greater spatial
scales have shown that alien species are often less frequent than native species (Hulme
2008, Knapp et al. 2009, Pyšek et al. 2012b). Our study confirms this pattern for assem-
blages of urban woody species at the habitat scale.
Tree counts in residential areas revealed a clear dominance of native species, but in the
tree layer, the alien Ailanthus altissima was most abundant (Table 6). Correspondingly, the
126 Preslia 85: 113–132, 2013
log-linear models revealed a significantly higher percentage of individuals of alien species
of trees in the tree and shrub layers compared to the herbaceous layer. Prolific populations of
species of native trees in the herbaceous layer might suggest a future shift to a greater domi-
nance of native species in higher vegetation layers. As this survey assessed scattered popula-
tions of trees in residential areas that are highly fragmented and usually subject to frequent
disturbances, the prevalence of alien species in the tree layer there possibly indicates that
these species have a greater chance of maturing there. Other studies have found Ailanthus
altissima to be among the most abundant species of alien tree on urban land (e.g. Pan &
Bassuk 1986, Lenzin et al. 2001, Tsiotsiou & Christodoulakis 2010). Its frequent presence
in the tree layer can be explained by its higher growth rate, both in shoots and roots, com-
pared to Acer platanoides (Pan & Bassuk 1986, Säumel 2007). Moreover, Ailanthus
altissima responds quickly to disturbance by regenerating vegetatively (Kowarik & Säumel
2007), which is likely to be highly advantageous in habitats subject to repeated disturbances.
The survey of the Robinia woods, which were about 40 years old when sampled,revealed
a clear prevalence of R. pseudoacacia in both tree and shrub layers (Kowarik 1990a). Again,
the log-linear models revealed a significantly higher percentage of individuals of alien spe-
cies of trees in the tree and shrub layers compared to the herbaceous layer. The dominance of
R. pseudoacacia, which continues today (I. Kowarik, personal observation), strongly con-
trasts with the development of stands of this species in its native North American range,
where this pioneer tree is quickly replaced after 20–30 years by shade-tolerant native trees,
mostly due to it being strongly attacked by insects (Boring &Swank 1984). The shade-toler-
ant native A. platanoides was also able to establish prolific populations in the herbaceous
layer of Robinia woods in Berlin but as yet not in the upper vegetation layers. In contrast to
early predictions (Kohler & Sukopp 1964), R. pseudoacacia appears to remain dominant in
woods undergoing succession in its European range for longer than in its native range, possi-
bly because it is not attacked here as much by insects as in North America.
The high frequency of native species in urban habitats (Table 4) may suggest an impor-
tant potential for the recovery of native woody vegetation in urban areas (Stewart et al.
2004). Forest remnants can function as sources of propagules of native trees that colonize
adjacent gardens (Doody et al. 2010) but as few parts of Berlin’s built-up areas are close to
forests, it is much more likely that urban trees are the major source of propagules for the
recolonization by native species. This can induce biological invasions at the gene level
since the horticultural sector usually provides plants grown from other than local seed
sources (Petit 2004). Growth or leaf anomalies of some spontaneously occurring species
of native woody plants indicates that the seeds come from particular cultivars (Ringenberg
1994, Seidling 1999) and are mainly dispersed by birds (Moller et al. 2012). Plantings in
urban areas can also lead to the establishment of forest populations of native species,
which mainly consist of the descendants of garden plants. This is most evident for Acer
platanoides (Sachse 1990) but is also true for Taxus baccata (Seidling 1999), a native for-
est tree, which was extirpated in the Berlin-Brandenburg region during the 19th century
(Benkert 1978). Due to the fact that all the current populations grew from seed from other
provenances, T. baccata is currently listed as an alien species in Berlin.
Plantings of trees in urban areas havealso obviously changed the abundance patterns of
native species. The high abundance of Acer platanoides and A. pseudoplatanus at the hab-
itat and community scales (Table 4, 6) is a surprising result from a historical perspective
because both species were rare in Berlin up to the end of the 19th century (Ascherson
Kowarik et al.: Alien and native woody plants in Berlin 127
1864, Sachse et al. 1990). This pronounced increase and the invasion success of
A. platanoides in North America can be related to frequent planting that multiplied the
propagule pressure (Sachse et al. 1990, Hunter & Mattice 2002). In addition, both these
species of maple respond positively to increased availability of nitrogen (Sachse et al.
1990, Pröll et al. 2011), a typical feature of many urban environments (McDonnell et al.
1997, Alberti 2005). The fact that A. pseudoplatanus is less frequent in urban areas in
Berlin than its congener could be due to its lower tolerance of drought (Hemery et al.
2010). Overall, these results highlight that both alien and native species respond to the
urban environment with converging trends at different scales.
Acknowledgements
IK was funded by BMBF (FKZ 0319304A) and all authors were supported by institutional resources of
Technische Universität Berlin. We thank Petr Pyšek and three anonymous reviewers for helpful comments and
Kelaine Ravdin and Tony Dixon for improving our English.
Souhrn
Invazní druhy jsou ve městech důležitou složkou květeny; k tomuto jevu významně přispívají zplaňující pěstova-
né dřeviny. Otázkou zůstává, zdase celková druhová bohatost dřevin projevujetaké na úrovni stanovišť a popula-
cí. Cílem práce je porozumět tomu, jak invaze dřevin ve městech závisejí na měřítku studia; za tímto účelem jsme
analyzovali výskyt původních a nepůvodních druhů stromů keřů a popínavých dřevin spontánně se vyskytujících
na území Berlína. Zastoupení nepůvodních dřevin ve flóře Berlína vzrostlo z 16 % na konci 18. století během 200
let na 67 %. Z celkového počtu 181 zaznamenaných druhů je 32 % považováno za naturalizované. Druhy z ostat-
ních částí Evropy, z Mediteránu a západní Asie zplaněly a zdomácněly častěji než druhy z jiných oblastí.Zplaňo-
vání nezvýšilo podíl stálezelených druhůve flóře. Všechny typy stanovišť vyjma mokřadů hostí více nepůvodních
než původních druhů, procentální zastoupení nepůvodních je vysoké zejména na plochách s městskou zelení, ne-
udržovaných místech a v obytných čtvrtích. Na úrovni stanovišť jsou však původní druhy zastoupeny relativně
více. Nejčastějšími stromy bez ohledu na stanoviště jsou původní druhy Acer platanoides,Betula pendula,Quer-
cus robur a nepůvodní Robinia pseudoacacia,Acer negundo aPrunus serotina, mezi keři pak původní Sambucus
nigra a nepůvodní Mahonia aquifolium. Analýza počtu jedinců ve dvou vybranýchoblastech ukázala, že původní
druhy jsou početnější v obytných čtvrtích, zatímco nepůvodní v neudržovaných částech města. Výsledky potvr-
zují, že invazní úspěch dřevin v městském prostředí je nutno posuzovat v kontextu. Jasná převaha nepůvodních
druhů v celkové flóře se nepřenáší na úroveň stanovišť a rostlinných společenstev, měřeno frekvencí výskytu. Pro
městské oblasti je typické nejen vysoké zastoupení nepůvodních druhů, ale take nárůst početnosti nekterýchpů-
vodních dřevin, jako jsou Acer platanoides and A. pseudoplatanus, které v současnosti převládají díky zvýšené-
mu přísunu diaspor a eutrofizaci městských ekosystémů.
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Received 24 October 2012
Revision received 3 March 2013
Accepted 13 March 2013
132 Preslia 85: 113–132, 2013
