The Introduction of Non-Native Tree Species to Present-Day Slovenia

Abstract

The introduction of non-native tree species to the territory of present-day Slovenia has not been well documented. The first documented introduction of a considerable number of species took place at the end of the 18th and the beginning of the 19th century. The estimated average time lag between the first known introduction of a particular species to Europe and its first record in Slovenia was 70–90 years. At first, species were planted in specialized plant collections, and their planting frequency was low. However, the planting frequency of some tree species, such as black locust and tree of heaven, was high since they were also widely used for afforestation of degraded soils. High planting frequency together with high seed dispersal rate resulted in their later invasiveness. Both species illustrate how human interference in ecosystems can generate long-term processes that cannot be stopped after a sudden change in social perspectives and values.

Full text

R. BRUS and D. GAJŠEK: The Introduction of Non-Native Tree Species ...
380
Robert Brus and Domen Gajšek
The Introduction of Non-Native Tree
Species to Present-Day Slovenia*
* We thank Nada Praprotnik for providing useful materials and valuable information as
well as Philip Nagel for proofreading the English.
BRUS, Robert, PhD,
Assoc. Prof., University
of Ljubljana, Biotechnical Faculty, Department
of Forestry and Renewable Forest Resources,
SI–1000 Ljubljana, Jamnikarjeva 101, robert.
brus@bf.uni-lj.si
GAJŠEK, Domen, researcher, University of
Ljubljana, Biotechnical Faculty, Department
of Forestry and Renewable Forest Resources,
SI–1000 Ljubljana Jamnikarjeva 101, domen.
gajsek@bf.uni-lj.si
The Introduction of Non-Native Tree Species
to Present-Day Slovenia
The introduction of non-native tree species to
the territory of present-day Slovenia has not
been well documented. The rst documented
introduction of a considerable number of species
took place at the end of the 18
th
and the beginning
of the 19
th
century. The estimated average time
lag between the rst known introduction of a
particular species to Europe and its rst record
in Slovenia was 70–90 years. At rst, species
were planted in specialized plant collections, and
their planting frequency was low. However, the
planting frequency of some tree species, such as
black locust and tree of heaven, was high since
they were also widely used for afforestation of
degraded soils. High planting frequency together
with high seed dispersal rate resulted in their
later invasiveness. Both species illustrate how
human interference in ecosystems can generate
long-term processes that cannot be stopped after a
sudden change in social perspectives and values.
Key words: non-native tree species, invasive
species, plant introduction, botanical history
Authors’ Abstract
BRUS, Robert, dr.,
izr. prof., Univerza v Ljublja-
ni, Biotehniška fakulteta, Oddelek za gozdarstvo
in obnovljive gozdne vire, SI–1000 Ljubljana,
Jamnikarjeva 101, robert.brus@bf.uni-lj.si
GAJŠEK, Domen, raziskovalec, Univerza v
Ljubljani, Biotehniška Fakulteta, Oddelek za
gozdarstvo in obnovljive gozdne vire, SI–1000
Ljubljana, Jamnikarjeva 101, domen.gajsek@
bf.uni-lj.si
Vnos tujerodnih drevesnih vrst na ozemlje
današnje Slovenije
Vnos tujerodnih drevesnih vrst na ozemlje
današnje Slovenije ni dobro dokumentiran. Prvi
zabeleženi vnos večjega števila vrst je potekal
ob koncu 18. in v začetku 19. stoletja. Ocenjeni
povprečni časovni zamik med prvim znanim
vnosom določene vrste v Evropo in njenim
vnosom v Slovenijo je bil 70–90 let. Vrste so
sprva sadili v specializiranih rastlinskih zbirkah
in pogostnost njihovega sajenja je bila nizka.
Vendar pa je bila pogostnost sajenja nekaterih
drevesnih vrst, na primer robinije in visokega
pajesena, visoka, saj so ju pogosto uporabljali za
pogozdovanje degradiranih tal. Visoka frekvenca
sajenja je skupaj z visoko stopnjo raznosa semen
povzročila njuno kasnejšo invazivnost. Obe
vrsti kažeta, kako lahko človekovo poseganje
v ekosisteme sproži dolgoročne procese, ki jih
po nenadni spremembi družbene perspektive in
vrednot ni mogoče ustaviti.
Ključne besede: tujerodne drevesne vrste, in-
vazivne vrste, vnos rastlin, zgodovina botanike
Avtorski izvleček

Man, Nature and Environment Between the Northern Adriatic and the Eastern Alps in Premodern Times
381
Introduction
Man can inuence his environment and nature in various ways, such as by
the excessive use of natural resources and by the transformation of natural habitats
through settlement building and other activities. The introduction of plant species
from other countries or continents has also been a long-standing and important
means of inuencing or transforming the environment. Traditionally, such species
have been called non-native plants or alien, allochthonous or introduced species.
They can be brought to a certain area for several reasons, a very important one
being the introduction of a plant as a food source. Many recent European food
crops, such as wheat, barley, pea, maize, potato, tomato, bean, sunower, and peach,
have been introduced from other environments or continents.1 Another reason for
introduction is the healing property of a particular species; several tree species
were rst introduced to Europe as medicinal plants, an example being arborvitae
(Thuja occidentalis). Many plant species were introduced for the establishment of
various kinds of functional plantations. Lombardy poplar, for instance, is a well-
known tree species for wind control barriers. Although in Europe, as well as in
Slovenia, most of the original surface was covered by forests rich in domestic tree
species, the desire to introduce exotic species with presumably better and faster
growing timber (e.g. Douglas r from the Western USA) has long been present.
The aesthetic beauty of trees is one of the most important reasons to plant trees
in urban environments. In the case of the rst introductions, this reason was often
associated with curiosity, the desire for new knowledge or simply the need to draw
attention with novelty.
Introduction can be either intentional or unintentional. Intentional introduc-
tions are much more common and are normally associated with the reasons alre-
ady mentioned, while the unintentional introduction of tree species is in fact rare.
Unintentional introductions are more common among various diseases (Dutch elm
disease), pests (chestnut gall wasp) or animals (grey squirrel). Today, approximately
six new plant species capable of naturalization occur in Europe each year, with
most (68.2%) being intentional introductions.2
With respect to residence time, i.e. the time since the arrival of a non-native
species in the territory of Europe, we distinguish archaeophytes (primarily species
1 Hancock, Plant Evolution, p. 106–111.
2 Lambdon et al., Alien Flora of Europe, p. 127, 130.

R. BRUS and D. GAJŠEK: The Introduction of Non-Native Tree Species ...
382
introduced before the discovery of America in approx. 1500 A.D.) and neophytes
(introduced after that date).3 Good examples of archaeophytes are introductions
to the Mediterranean and Adriatic region where species cultivated in the eastern
Mediterranean were spread westward by various ancient civilizations. It is worth
mentioning that archaeophytes have only rarely become invasive. Neophytes on
the other hand are plant species that were introduced to Europe after 1500 A.D.,
signifying the discovery of the New World and the initiation of relatively rapid and
substantial changes in human movement, demography, agriculture, commerce and
industry.4 This caused several dramatic changes in the plant cover of the Old World.
The consequences of the introduction of non-native species can vary widely.
One scenario is that there is no consequence due to a failure, a typical example
being the unsuccessful planting of eucalyptus on the eastern Adriatic coast.5
Cultivation can also remain conned to cultivated environments. This is typical
of crops as well as of cultivated fruit and ornamental trees. As a result of their
intensive breeding, they have lost the capacity to settle or spread into the natural
environment. Several tree species, such as peach, apricot, sour cherry, and loquat,
are included in this group. Some species, such as g and Mediterranean cypress,
however, were historically capable of naturalization but have so far not started to
spread invasively. Hybridization with wild relatives is not very common, but is a
possible consequence. Due to the lack of sufciently powerful tools, it has rarely
been detected in the past. But there have been several data on such events lately, an
example being the gene ow from hybrid poplars to native black poplar.6 The last
possible effect of introduction is invasiveness, which denes the most controversial
and most studied group of introduced species. It is very difcult to predict whether
introduced species will become invasive. Recent extended research across Europe
has shown that the naturalization success of trees and shrubs originating from North
America is signicantly increased by planting frequency (even more pronounced
in shrubs than in tree species), the time that has passed since the rst introduction,
range size (either original or later established), maximal plant size and seed spread
rate.7 It is therefore important to have good knowledge of these factors in order to
better understand the dynamics and drivers of particular invasive species. While
other factors are well known and for the most part universal, it appears that for
Slovenia there is a lack of precise data on the timing of the introduction of particular
tree species as well as their planting frequency.
3 This classication system is widely used in Central-European phytogeographical studies
(e.g. Holub, Jirásek, Zur Vereinheitlichung, p. 69–113). For simplicity and compatibility with
the recent usage of the term, we use it without any relation to whether the given species arrived
accidentally or was brought in by humans. It only reects the residence time regardless of the
means of introduction. For further details and comparison with other classication systems, see
Pyšek, On the terminology, p. 72–75 and Pyšek et al., Catalogue of alien, p. 106–108.
4 Ricotta et al., Phyloecology of urban, p. 1244.
5 Vrdoljak, O mogućnosti uzgoja eukalipta, p. 143–154.
6 Vanden-Broeck et al., Natural hybridization, p. 341–349.
7 Bucharova, van Kleunen, Introduction history, p. 230–238.

Man, Nature and Environment Between the Northern Adriatic and the Eastern Alps in Premodern Times
383
The aim of our research was therefore i) to collect and compile available data
on the timing of the introduction of the main non-native tree species to present-day
Slovenia, ii) to estimate their planting frequency, iii) to compare the data with other
parts of Europe, and iv) to identify the key factors that have enabled the success
of our two most important invasive tree species.
Species introduction timing and frequency
In order to determine the timing and frequency of non-native tree species
introductions,8 we checked and analyzed the following promising sources that
might contain early mentions of their presence or cultivation. We studied the most
important published sources (Valvasor 1689,9 Scopoli 1772,10 Fleischmann 1844,11
Marchesetti 1896–1897,12 Pospichal 1897–1899,13 Stefani 1895,14 Hayek and Paulin
1907 15); edited sources;16 and published data on old gardens at Brdo pri Kranju,17
Dol pri Ljubljani 18 and the Ljubljana Botanical Garden,19 including the list of the
species grown in the Ljubljana Botanical Garden in 1812. 20
One of the early specic mentions of non-native tree species cultivation in
the territory of present-day Slovenia is connected to silkworm breeding. Culti-
vation of mulberry in the Goriška region, most likely of the species Morus alba,
was rst mentioned as early as 1565 and was highly promoted in the subsequent
few centuries.21 In general, few of the studied historical records contain specic
mentions of a considerable number of non-native tree species. One of the oldest
such records is that of Valvasor from 1689.22 The list probably does not include
all non-native species present at the time, but nevertheless we can expect that it
8 Several tree archaeophytes of Eastern Mediterranean origin, such as olive tree, g tree,
almond and Mediterranean cypress, must have already been cultivated in the Mediterranean part
of present-day Slovenia in ancient times. However, for the majority of these species reliable data
on when they were introduced could not be obtained (see Jogan et al., Tujerodne in invazivne,
p. 169–170). In this study archaeophytes were of course also treated as non-native, but it must
be pointed out that the main focus of our research was the introduction of neophytes that were
brought to our area after 1500 A.D.
9 Valvasor, Čast in slava, p. 170–354, 2146–2827.
10 Scopoli, Flora Carniolica, p. 7–496.
11 Fleischmann, Uebersicht der Flora Krain's, p. 6–133.
12 Marchesetti, Flora di Trieste, p. 1–727.
13 Pospichal, Flora des Österreichischen Küstenlandes, p. 1–574.
14 Stefani, La ora di Pirano, p. 12–196.
15 Hayek, Paulin, Flora der Sanntaler, p. 75–138.
16 Anko, Terezijanski gozdni red, p. 1–88; Anko, Gozdni red za Ilirske, p. 1–129.
17 Dobrilovič, Kravanja, Rastlinsko gradivo, p. 277–286.
18 Strgar, Rastlinstvo in vrtovi, p. 96–106.
19 Praprotnik, Inventar botaničnega, p. 38–65; Bavcon, Botanični vrt, p. 3–34.
20 AS 822, fasc. 25, p. 1–5; this previously unknown list was discovered by Nada Praprotnik
in 2009, for further details and analysis see Praprotnik, Inventar botaničnega, p. 38–65.
21 Ipavec, Murve in kavalirji, p. 25.
22 Valvasor, Čast in slava, p. 170–354, 2146–2827.

R. BRUS and D. GAJŠEK: The Introduction of Non-Native Tree Species ...
384
reects the basic structure. Through analysis of its content, we were able to identify
16 distinct non-native tree or shrub species: 23 plum (Prunus domestica), peach
(Prunus persica), apricot (Prunus armeniaca), sour cherry (Prunus cerasus), loquat
(Mespilus germanica), common quince (Cydonia oblonga), orange tree (Citrus
sinensis), lemon tree (Citrus limonia), pomegranate (Punica granatum), almond
(Prunus dulcis), mulberry (Morus alba), rosemary (Rosmarinus ofcinalis), olive
tree (Olea europaea), g tree (Ficus carica), boxwood (Buxus sempervirens) and
horse chestnut (Aesculus hippocastanum). The majority are fruit trees, and several
can be considered as archaeophytes. Among ornamental species, only boxwood and
horse chestnut are mentioned. All originate from Western Asia or the Mediterranean,
with no species from the Far East or America mentioned at that time.
For most of the 18th century new introductions were still poorly documented
in the botanical literature, indicating that they were still very rare or incidental. In
the Terezijanski gozdni red za Kranjsko (1771),24 mulberry (Morus sp.) is men-
tioned as a species used for hedges. In Scopoli’s work Flora Carniolica (1772),25
there are still only 14 non-native tree and shrub species: Rosmarinus ofcinalis,
Lonicera periclymenum, Rhamnus ziziphus (Ziziphus jujuba), Grossularia rubra
(Ribes rubrum), Aesculus hippocastanum, Rhododendron polifolium (Rhododendron
thymifolium), Punica granatum, Prunus cerasus, Mespilus pyracantha (Pyracantha
coccinea), Pyrus azarolus (Crataegus azarolus), Buxus sempervirens, Morus alba,
Quercus coccifera and Pinus pinea. It is worth noting that the work addressed the
indigenous ora, which means that some non-natives were probably overlooked.
However, it seems that the introduction of non-native species was still relatively
uncommon during that period. Up to that time no species from the Far East or
America had been mentioned.
23 Unambiguous species recognition from Valvasor’s original work, written in German,
can be difcult in certain cases. In the descriptions of domestic tree species, sometimes only the
genus but not the species can be recognized, and in several cases, very common native tree and
shrub species are not mentioned (Zwitter, Okolje na Kranjskem, p. 614–615). Altogether, nearly
one hundred plant species are mentioned in Valvasor’s monograph either with Latin, German or
partly Slovenian names (see Petkovšek, J. A. Scopoli, p. 153). For our analyses the Slovenian
translation of Valvasor's monograph was used, and since the majority of species mentioned are
well-known and relatively easily recognizable tree species, we largely relied on the translation
accuracy in this case. However, four species, named salzbaum (Valvasor, Čast in slava, p. 2314),
mangrove (Valvasor, Čast in slava, p. 2146), indijanski lešniki and indijanska vinska trta (Valva-
sor, Čast in slava, p. 2315), very likely non-native trees or shrubs, could not be unambiguously
identied, and their determination will require more detailed study of Valvasor’s monograph and
additional sources.
24 Anko, Terezijanski gozdni red, p. 49.
25 Scopoli, Flora Carniolica, p. 1–496. For more details about Scopoli’s life and work in
the territory of present-day Slovenia, see Petkovšek, J. A. Scopoli, p. 93–192.

Man, Nature and Environment Between the Northern Adriatic and the Eastern Alps in Premodern Times
385
Ailanthus altissima
Robinia pseudoacacia
Quercus rubra
Pinus strobus
Acer negundo
Aesculus hippocastanum
Pinus pinea
Ziziphus jujuba
Gleditsia triacanthos
Lonicera tatarica
Acer saccharinum
Philadelphus coronarius
Lycium barbarum
Juglans nigra
Gingko biloba
Catalpa bignonioides
Cydonia oblonga
Thuja occidentalis
Platanus ×hispanica
Rhus typhina
Amorpha fruticosa
Sophora japonica
Morus alba
Punica granatum
Mespilus germanica
Parthenocissus quinquefolia
Liriodendron tulipifera
Liquidambar styraciflua
Magnolia grandiflora
1935 (13)
1896-1897
(
12
)
1844
(
8
)
1872
(
9
)
1879
(
10
)
1889
(
11
)
1565
(
1
)
1689
(
2
)
1772
(
4
)
1781-1836
(
5
)
1790-1800
(
6
)
1771
(
3
)
1812
(
7
)
Graph 1: Early records of selected non-native tree and shrub species in the territory of present-
day Slovenia. Light grey: assumed continuous presence from the rst record on. Dark grey: later
records in subsequent sources. Example: rst recorded presence of Aesculus hippocastanum is in
1689, the second in 1772 etc. (1)–Goriška region, Ipavec, Murve in kavalirji, p. 25; (2)–Kranjska,
Valvasor, Čast in slava, p. 170–354, 2146–2827; (3)–Kranjska, Anko, Terezijanski gozdni red,
p. 49; (4)–Kranjska, Scopoli, Flora Carniolica, p. 7–496; (5)–Brdo, Dobrilovič, Začetki uva-
janja, p. 1–202, Praprotnik, Botanični vrt Karla, p. 167– 174; (6)–Dol, Strgar, Rastlinstvo in
vrtovi, p. 96–106; (7)–Ljubljana Botanical Garden, Praprotnik, Inventar botaničnega, p. 38–65;
(8)–Kranjska, Fleischmann, Uebersicht der Flora Krain’s, p. 6–133; (9)–Istria and Goriška,
Schober, Ufciali della società agraria, p. 894–897; (10)–Panovec, Betriebseinrichtung vom;
(11)–Panovec, Betriebseinrichtungs Revisionsoperat; (12)–Trieste, Marchesetti, Flora di Trieste,
p. 1–727; (13)–Panovec, Gozdnogospodarski načrt.
A large increase in the number of cultivated non-native species must have
occurred in the following years. A large number of species are mentioned for the
rst time in the period 1781–1836 in the Brdo pri Kranju Park. This garden, which
is currently a protocolar estate of the Republic of Slovenia, was established by bo-
tanist Karel Zois in 1781. Zois, who had extensive contacts with several European
botanists of that time, such as Wulfen, Host, Sternberg and Willdenow,26 built-up
an important and rich plant collection in rst few decades after the establishment
of the garden. The majority of introduced plants in Brdo came from the Nether-
lands and some from England, Italy and Austria. Most tree seedlings originated
from the renowned London Conrad Lodigges nurseries, from which in 1784 Zois
received an extensive consignment of plants. Another rich shipment was received
in 1793 from the botanist Host from Vienna. One shipment was sent to Brdo by
Captain Cook from his trip to Tahiti, but during transport most of the seedlings
unfortunately perished. The distribution and form of plants in the park was based
on soil fertility rather than related to the design of the space, which indicates the
26 Praprotnik, Botanik Karel Zois, p. 83–88.

R. BRUS and D. GAJŠEK: The Introduction of Non-Native Tree Species ...
386
scientic nature of these introductions. The abundance of plant species in the park
was larger than that in many current Slovenian nurseries. Archival material on the
Brdo garden was analyzed by Marko Dobrilovič and Nika Kravanja.27 They list a
total of 7446 introduced plant specimens. There were 165 tree species and 38 shrub
species, including indigenous ones. Among them, 156 species were non-native.
This period can be considered as a kind of milestone, for it marks the begin-
ning of the introduction of non-native species for botanical, decorative, scientic
and educational purposes and not only for their role in diet. This, among other
things, affects species which are today among most common ornamental species
from America, such as honey locust (Gleditsia triacanthos), tulip tree (Lirioden-
dron tulipifera), American sweetgum (Liquidambar styraciua), northern red oak
(Quercus rubra), white cedar or arborvitae (Thuja occidentalis), southern catalpa
(Catalpa bignonioides) and eastern white pine (Pinus strobus). Several species
are rare even today, for example, Halesia tetraptera, Zanthoxylum clava-herculis,
Gymnocladus dioicus, Callicarpa americana, Chionanthus virginicus and Stewartia
malacodendron. Zois was relatively up-to-date with planting material, but with a
certain delay compared to Western Europe where largest number of rst introduc-
tions to the European continent took place.
Another important introduction gateway was the garden in Dol pri Ljubljani,
established shortly after Brdo by Joseph Kalasanc Erberg in the period 1790–1800.
The period after the year 1790 can be considered the peak of the garden, when the
entire outdoor area of plantations was gradually included into planning, creating a
large and comprehensive park area. The majority of non-native tree species were
planted around the year 1795, mostly in the grove and arboretum part of the garden.
The garden is no longer preserved, but it was probably the richest regional plant
collection at that time. Analysis of the published list28 of its plant collections from
the archives revealed the cultivation of 206 non-native tree and shrub species in
Dol. This gure is considerably larger than that of the Brdo collection. The design
and the botanical and horticultural wealth of the Dol plant collections and gardens
were in many respects (especially with respect to the number of different taxa) ahead
of comparable Central European gardens. The richness of the plant collections also
reects Erberg’s good relations and connections with his botanical contemporaries
from other European countries, which resulted in his expert knowledge of the
botanical novelties of the time.
The introduction of non-native tree species to the territory of present-day Slo-
venia over time has not increased linearly, as one might expect, since, for example,
towards the end of the 18th century the tree species collections in the Brdo and Dol
parks were still considerably richer than collections that were analyzed several
decades later. In the inventory of the Ljubljana Botanical Garden from 1812,29 only
27 Dobrilovič, Kravanja, Rastlinsko gradivo, p. 277–286; Dobrilovič, Začetki uvajanja, p.
1–202.
28 Strgar, Rastlinstvo in vrtovi, p. 96–106.
29 Praprotnik, Inventar botaničnega, p. 38–65; ARS, AS 822, Hladnik, Inventar Botaničnega,
p. 1–5.

Man, Nature and Environment Between the Northern Adriatic and the Eastern Alps in Premodern Times
387
21 non-native tree or shrub species are listed, and only 48 non-native tree species
are included in Fleischmann’s work.
30
This indicates that the exchange of species
had not yet been well established at that time. From all this we can conclude that,
in general, the introduction of non-native tree species to the territory of present-day
Slovenia is poorly documented, and the known data do not necessarily reect the true
pace of introductions. However, it seems quite clear that in most cases the planting
frequency of introduced species was very low and for the most part limited to special
collections or gardens rather than large-scale afforestation or frequent planting. This
low planting frequency, besides the widely accepted and implemented sustainable
forest management that promotes a closed canopy, is one of the important reasons
that several tree species that are invasive in other European areas have not spread
across large areas of present-day Slovenia. However, there are a few exceptions to
this rule that will be discussed later in this paper.
Comparison with Europe
Due to the poorly documented introduction of species to the territory of present-
day Slovenia, it is difcult to make a reliable comparison with the earliest known
introductions to other parts of Europe, particularly Western Europe,
31
with respect
to the timing. We compared the existing Slovenian data with those compiled for all
of Europe from several authors.
32
In Graph 2 it can be seen that for the selected tree
species there was a certain time lag between the rst introduction to Europe and
the rst introduction to present-day Slovenia. As a rule, introduction to Western
Europe was always earlier; however, the time lag was not uniform and not always
substantial. Considering the species studied in Graph 2, the average time lag was
around 70–90 years, but with signicant variation; for example, in the case of Ro-
binia pseudoacacia there was a delay of more than 150 years, while the delay for
Ailanthus altissima was shorter at around 50 years. The only exception to this rule
that we found was goji (Lycium barbarum) – its presence in present-day Slovenia
was documented earlier than in Europe as a whole (see Graph 2). Generally, delayed
introduction to the territory of present-day Slovenia seems logical since Slovenian
naturalists did not participate in large research expeditions around the globe. To be
up-to-date, Slovenian gardeners depended greatly on their connections with other,
mainly Western European, nurseries and botanists. Direct shipments from expedi-
tions were an exception, and planting material was normally imported
from various
well-known nurseries from around Europe and the Western World.33
30 Fleischmann, Uebersicht der Flora Krain's, p. 6–133.
31 The main purpose was a comparison of the introduction time to the territory of present-
day Slovenia and the rst known introduction to the European continent. The largest number of
rst introductions of neophytes took place in present-day Western European countries (e. g. Great
Britain, France, Germany, the Netherlands, Spain and Italy) and the comparison of present-day
Slovenia with Europe is in large part, but not exclusively, a comparison with Western Europe.
32 Goeze, Liste, p. 130–197; Johnson, The International Book of Trees, p. 254–255; Wein,
Die Erste Einführung, I, p. 137–163; Wein, Die Erste Einführung, II, p. 95–154.
33 Dobrilovič, Kravanja, Rastlinsko gradivo, p. 281–282.

R. BRUS and D. GAJŠEK: The Introduction of Non-Native Tree Species ...
388
Ailanthus altissima
Robinia pseudoacacia
Quercus rubra
Pinus strobus
Acer negundo
Aesculus hippocastanum
Pinus pinea
Ziziphus jujuba
Gleditsia triacanthos
Lonicera tatarica
Acer saccharinum
Lycium barbarum
Juglans nigra
Gingko biloba
Catalpa bignonioides
Thuja occidentalis
Platanus ×hispanica
Rhus typhina
Amorpha fruticosa
Sophora japonica
Morus alba
Parthenocissus quinquefolia
Liriodendron tulipifera
Liquidambar styraciflua
Magnolia grandiflora
1850
1800
1700
1772 (3)
1781-1836 (4)
1790-1800 (5)
1500
1565 (1)
1600
1689 (2)
Graph 2: Comparison of the rst known documented introductions of selected tree species to
Europe and specically to the territory of present-day Slovenia. Light grey: assumed continuous
presence from the rst record on in Europe. Dark grey: assumed continuous presence from the rst
record on in the territory of present-day Slovenia. Example: rst recorded presence of Robinia
pseudoacacia in Europe is in 1600 and in the territory of present-day Slovenia in the period
1790–1800. Data for Europe based on Goeze, Liste, p. 130–197; Johnson, The International Book
of Trees, p. 254–255; Wein, Die Erste Einführung, I, p. 137–163; Wein, Die Erste Einführung, II,
p. 95–154. Data for the territory of present-day Slovenia: (1)–Ipavec, Murve in kavalirji, p. 25;
(2)–Valvasor, Čast in slava, p. 170–354, 2146–2827; (3)–Scopoli, Flora Carniolica, p. 7–496;
(4)–Brdo, Dobrilovič, Začetki uvajanja, p. 1–202, Praprotnik, Botanični vrt Karla, p. 167–174;
(5)–Strgar, Rastlinstvo in vrtovi, p. 96–106.
Invasive tree species
One of the possible consequences of non-native species introduction is
invasiveness; invasive species are among the most intensively studied groups of
introduced plants. In Slovenia the two most important invasive trees are black lo-
cust (Robinia pseudoacacia) and tree of heaven (Ailanthus altissima). For both, we
wished to establish their introduction history and planting frequency and attempted
to identify the key factors that promoted their later invasiveness.
Black locust originates from North America and was rst introduced to Europe
in 1600 in Paris.34 We found its rst mention in Dol pri Ljubljani Park at the end of
the 18th century35 as well as in most of the other plant collections (Ljubljana Botanical
Garden 1812,36 Fleischmann 184437). In addition to these collections, its use was
often recommended or reported in the afforestation of degraded karst soils as early
34 Brus, Drevesne vrste, p. 268.
35 Strgar, Rastlinstvo in vrtovi, p. 96–106.
36 ARS, AS 822, Hladnik, Inventar Botaničnega, p. 1–5.
37 Fleischmann, Uebersicht der Flora Krain's, p. 6–133.

Man, Nature and Environment Between the Northern Adriatic and the Eastern Alps in Premodern Times
389
as in 1810.38 In 1858 it was still recommended for its multiple uses but for the rst
time its high persistence in the natural environment was recognized as a potential
problem.39 In addition, in 1872 as many as 1.3 million black locust seedlings in-
tended for afforestation were grown in two Mediterranean nurseries in Srmin and
Gorica/Görz/Gorizia.40 From these data we can clearly conclude that the introduc-
tion of black locust was relatively early and, more importantly, the frequency of
its planting in the 19th century was high. Another important factor promoting the
species’ invasiveness is its spread rate: this is not only accelerated by its durable
seeds which can survive for decades, but also by its successful propagation from
root and stump suckers. Black locust’s articial range size is extensive and a single
tree can grow up to 30 meters, which can be considered tall.41 It is therefore clear
that all of the important conditions that promote naturalization success42 are fullled.
Today, black locust is the most widespread non-native tree species in Slovenia. Its
0.6% share in the total growing stock is not dramatic but is forecasted to increase
– perhaps even double – by the end of the century.43 Black locust’s present status
is controversial, however. Due to its multiple uses, such as durable wood and rich
honey production, as well as its easy cultivation, it is clearly a desired and pro-
moted species among forest owners. In addition, its drought resistance might be
important in the altered climatic conditions in the future. But, at the same time, this
once highly-promoted species is now considered to be the most important invasive
plant species in Slovenia due to its potentially negative effects on biodiversity44
and the difculty in suppressing it. In this context it is probably the right time to
reconsider black locust’s present and future status in the country.
Another tree species on which we have focused is tree of heaven (Ailanthus
altissima), which was brought to Europe in 1751 from China. Like black locust, it
was rst mentioned in Dol pri Ljubljani Park around 1795 and also cultivated in
other collections (Ljubljana Botanical Garden 181245, Fleischmann 184446). Not
only was it promoted for its beauty in gardens, but it was also promoted and used
for the afforestation of degraded lands such as Kras. In 1863, for instance, the Rodik
priest Josip Schöpf proudly reported on the planting success of 2000 seedlings of
black pine and tree of heaven,47 and in 1872, 72,100 tree of heaven seedlings were
raised in the Srmin nursery near Koper/Capodistria.48 In the karst region along the
Adriatic coast, it was still highly promoted in the mid-20th century.49 Like black
38 Anko, Gozdni red za Ilirske, p. 1–129.
39 Anon. O seji akacije, p. 1.
40 Schober, Ufciali della società agraria, p. 894–897.
41 Brus, Drevesne vrste, p. 268–269.
42 Bucharova, van Kleunen, Introduction history, p. 230–238.
43 Kutnar, Kobler, Sedanje stanje razširjenosti robinije, p. 21–30.
44 Jogan, Neoti, p. 35–36; Zelnik, Razširjenost tujerodnih invazivnih vrst, p. 60.
45 ARS, AS 822, Hladnik, Inventar Botaničnega, p. 1–5.
46 Fleischmann, Uebersicht der Flora Krain's, p. 6–133.
47 Anko, Kresničke iz gozdarske zgodovine, p. 75–76.
48 Schober, Ufciali della società agraria, p. 894–897.
49 Giperborejski, Vrste drveta, p. 390–400.

R. BRUS and D. GAJŠEK: The Introduction of Non-Native Tree Species ...
390
locust, tree of heaven is also a species with an early introduction and high plan-
ting frequency. Its range size is large, and a single tree can grow up to 30 meters,
which is again tall.50 One of the most important promoting factors is the spread
rate of a given species: in the research conducted in Kras, it was established that
tree of heaven was not only promoted by seeds (47% of the young trees were of
this origin), but also by propagation from root suckers (41%) and stump suckers
(12%).51 All of the important conditions that promote naturalization success are
therefore fullled, and tree of heaven has proved to be an excellent pioneer for
afforestation with high resistance to drought and high potential future value in
changing climatic conditions. However, the species has limited use and is conside-
red undesirable by forest owners; moreover, after successfully establishing itself in
harsh environments, it is recognized as one of the most dangerous invasive species
and is thought to have negative effect on the biodiversity of local ecosystems.52
Both species clearly illustrate how human interference in natural ecosystems can
generate long-term processes that normally cannot be stopped after a sudden change
in social perspectives and values.
Conclusions
Our study was based on data from different sources that we are currently aware
of and that can provide a basic idea on when and for what purpose non-native tree
species were introduced to the territory of present-day Slovenia. However, this
knowledge is fragmented and far from complete; future analysis of new, probably
yet undiscovered historical sources (as was the case with the plant register of the
Ljubljana Botanical Garden found in 2009), will probably contribute to a more
comprehensive understanding of the tree species introductions.
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