DIE ERDE 134 2003 (3)

Regionaler Beitrag

S. 295-316

• Urban flora – Introduction and naturalisation of plants – Cultural history – Berlin

Herbert Sukopp (Berlin)

Flora and Vegetation Reflecting the Urban History of Berlin Flora und Vegetation als Spiegel der Stadtgeschichte Berlins With 3 Figures, 3 Tables and 5 Photos The word “urban ecology”, in German “Stadtökologie”, is used in two different ways. In the normative usage, it describes urban design programmes at the political and planning level. Within the natural sciences, however, urban ecology is used to refer to that area of biology which is concerned with urban areas. “Ecology” is understood here as the science of the relationships amongst organisms and biotic communities as well as their relationship to their environment. Urban ecology as a natural science is a young discipline. For a long time, it was thought that urban areas were not worth studying with regard to ecology. Cities were seen as anti-life. It was assumed that few plants or animals could survive in an urban setting. This view began to change over 30 years ago. Since then, ecological aspects of cities have been intensively researched. Early studies of ecology in cities shared the astonished realisation that environments created by humans provide habitats for characteristic species and that these species reoccur under similar conditions. Analyses have shown that urban areas include a wide variety of habitats, organisms and communities. The variety of species found in urban areas often exceeds that in the urban hinterland. A very good case for this phenomenon is provided by the city of Berlin. .

1. Landscape and Primeval Vegetation The Berlin region belongs to the Weichselian glaciated area of the North German lowlands and is a section of the young moraine landscape. The WarsawBerlin urstromtal with the rivers Spree and Havel subdivides the region into ground moraine plateaus. The macroclimatic character of this region is the transition from oceanic to continental with mean annual temperatures of 8.9°C and mean annual precipitation of 590 mm (Hupfer and Chmielewski 1990, Horbert 2000; see also Endlicher and Lanfer in this issue). The primeval vegetation, as reconstructed by pollen analysis (Brande 1996), analysis of soil con-

ditions, historical reports as well as recent vegetation patterns comprises three main woodland units: oak-hornbeam on arenosols, influenced by groundwater in the Urstromtal, pine-oak on dry arenosols on the sandy parts of the ground moraine plateaus, and oak with pine, hornbeam and locally beech on luvisols of the sandy till areas of those plateaus (Hueck 1963, Krausch 1965, Seidling 2000). For the number of trees and shrubs see Table 3.

2. Prehistoric and Early Historic Times Present flora and fauna and their biotic communities are the result of a long-term historic development. In Central Europe, the natural for-

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est development after the last ice age was not completed when human influence began causing disturbances on a local scale. Camps of palaeoand mesolithic hunters and gatherers were places for nitrophilous plants like Urtica dioica (Stinging Nettle, Große Brennessel), Artemisia vulgaris (Mugwort, Gewöhnlicher Beifuß) and some Chenopodiaceae. With neolithic agriculture, cultivated plants and segetal weeds appeared. Plantago lanceolata (Ribwort, Spitzwegerich) is a typical indicator species of grassland and fallowland. During the Bronze and Iron Age up to the Roman and Migration period secondary succession took place while climax forest trees, beech and hornbeam, expanded to maximum importance in the first millenium AD. The

Fig. 1

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use of wild fruit trees since early neolithic times has been proven by archaeological and palaeoethnobotanical studies (Lange 1979).

3. Middle Ages Large-scale disturbance, as shown by increasing numbers of segetal and ruderal species (Lange 1979), only began with clear-cutting of extensive areas for agriculture and with the development of the cities of Spandau, Köpenick and Berlin 800 years ago. Apart from Berlin-Cölln (Fritze 2000, Vogt 2000) archaeological studies have mainly been focused on Köpenick (Hofmann 1991) and Spandau (von Müller and von Müller-Muci

The medieval fortified settlement of Spandau Burgwall: archeological reconstruction of settlement phase 6a (AD 1000-1030), from Müller et al. 1993, with botanical sample sites, from Brande 1999, modified. 1: plant macrofossil sample series; 2: pollen analytical sample series. / Der Spandauer Burgwall im Mittelalter: archäologische Rekonstruktion der Siedlungsphase 6a (1000-1030 n. Chr.), aus Müller et al. 1993, mit Standorten der botanischen Proben, aus Brande 1999, verändert. 1: Probenserie der Großrestuntersuchungen (Samen und Früchte); 2: Probenserie der Pollenanalysen

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1999). At Köpenick dendrochronological analysis allowed the reconstruction of a settlement at the centre of the present-day town from the 12th to the 15th century (Malliaris 2000). At Spandau, a castle and a small settlement of craftsmen were built in the 8th century by Slavic people (Fig. 1). Plant remains (fruits and seeds) contain species of woodlands, grassland, fields and ruderal places (Fig. 2). Reconstruction of vegetation changes by pollen analysis of the cultural layers of the settlement has revealed intensive land-use within a restricted area of several square kilometres and its surroundings since the 11th century (Brande 1999). The Berlin area of today included arable land of three-field crop rotation, pastures and forests. The main crop was rye which was exported as far as Hamburg. Overexploitation of the forests was due to woodland pasture, charcoal burning and wood bee farming. The plague of 1348-1352, a depression of the crop market, exhaustion of sandy soils and a steady population influx into the town partly led to the abandonment of fields and villages. Increasing sheep keeping for cloth production favoured heathland expansion.

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4. Early Modern Times Outside the city walls, especially along the river Spree, various manufactures like slaughterhouses, tanneries and timberyards were set up. Parts of the common land were transformed into cabbage and tree gardens. In the year 1565, 70 vineyards, 26 vine gardens, 1 hop garden and 236 tree and vegetable gardens belonged to BerlinCölln. There were 5 sheep farms and 17 dairies outside the city. The number of inhabitants was estimated at 7000-8000. The nitrophilous flora of the surroundings of Berlin has shown a strong decline since 1850, caused by the cessation of cattle-breeding in the former villages around the city. Complete loss is known for Xanthium strumarium (Cocklebur, Gewöhnliche Spitzklette; “in ruderatis pagisque frequens”, Schlechtendal 1823/24: 425), Chenopodium urbicum (Upright Goosefoot, Straßen-Gänsefuß), Ch. vulvaria (Stinking Goosefoot, Stink-Gänsefuß), Coronopus squamatus (Swine-cress, Gewöhnlicher Krähenfuß), Marrubium vulgare (White Horehound, Gewöhnlicher Andorn), Malva pusilla (Small Mallow, Kleinblütige Malve), Anthemis cotula (Stinking Mayweed, Stink-

Fig. 2 Plant species numbers (fossil seeds and fruits) from the sample sites (in Fig. 1) of Spandau Burgwall, from Brande et al. 1987, modified / Anzahl der Pflanzenarten (fossile Samen und Früchte) aus den Proben vom Spandauer Burgwall (in Fig. 1), aus Brande et al. 1987, verändert

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Hundskamille) and Pulicaria vulgaris (Small Fleabane, Kleines Flohkraut). Some villages are now completely urbanised (Schöneberg, Lietzow, Schmargendorf, Steglitz), only few (e.g. Gatow, Lübars) have still hosted typical village plants all through the 20th century (Sukopp and Köster 1986). Medicinal plants used in prehistoric and medieval times have run wild and are now part of the ruderal flora and vegetation: Hyoscyamus niger (Henbane, Schwarzes Bilsenkraut), Datura stramonium (Thornapple, Stechapfel), Leonurus cardiaca (Motherwort, Herzgespann).

4.1 Cultivated flora in gardens, greens and parks The first survey of the plant stock of German gardens was conducted by Conrad Gesner (in Cordus 1561). Introduced plants dominated over native ones. Whereas medieval gardens were often largely composed of herbal and medicinal plants,

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ornamental gardens have prevailed since the 17th and 18th century (Tab. 1). Among other species introduced by horticultural trade are some neophytes which have become frequent today: Solidago canadensis (Common Golden-rod, Kanadische Goldrute), Erigeron annuus (Tall Fleabane, Feinstrahl-Berufkraut), Aster spec. (Aster, Aster) and Oenothera spec.(Evening primrose, Nachtkerze). The history of introductions of ornamental trees and shrubs in Berlin is described by Kowarik (1992). Plants introduced intentionally for horticultural use were first planted in monastery and peasant gardens and have spread spontaneously to the surrounding landscapes. The effect of escaped ornamentals on the flora around Berlin has been documented by Kunick (1991), Kowarik (1992) and Maurer (2002). Trees in cities – in Central Europe essentially Tilia ssp. (Linden tree, Linde) – were mainly found on major squares, in front of churches or town halls (Hennebo 1978). Since the 19th century trees have been appreciated as elements of urban planning

Tab. 1 Periods of introduction of garden plants into Central Europe (according to Kraus 1984, Wein 1943, Krausch 2003, adapted) / Phasen der Einführung von Gartenpflanzen nach Mitteleuropa (verändert nach Kraus 1984, Wein 1943, Krausch 2003) Period

Cultural period

Leading country

Main sources of garden plants

Before 1560

Middle Ages

Italy

Mediterranean, Europe

1560-1620

Renaissance

Italy

Greece to Persia via Constantinople, Vienna, Venice, Padua

1620-1740

Baroque

France, The Netherlands

North America, Portugal

The Netherlands

Ceylon, Java, Cape, Curacao

1687-1772 1715-1850

Rococo

England The Netherlands

China, Japan, East Sibiria

Before 1750

Enlightenment

England

North America

England, France

Australia, France

Russia

Caucasia

Diverse

All countries

1772-1825 1800-1830 1825-1943

Start of the World Trade Climax of the World Trade

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(Stübben 1890). Around 1900 all big cities used tree plantations along the streets. Plants are able to exercise a positive influence on both the climate and the air in the surrounding area. The influence of areas covered by vegetation on the urban climate has been investigated by Stülpnagel (1987). He found a reduction in temperature not only in the green area itself but also up to 1.5 km away. This climatic influence increases with the size of a green area but is reduced where the area is divided by a road. The first cemeteries were built outside the city walls during the medieval plague epidemics. A survey on ecological investigations of cemeteries and an analysis of flora and vegetation of the Berlin cemeteries is given by Graf (1986).

4.2 Wild flora Due to favourable microclimatic conditions and the existence of ruderal sites, plants and animals from warmer regions have been spreading in urban areas of Central European cities. In Berlin, three fifths of all non-native plants have come from such regions; this is true for archaeophytes as well as neophytes (Scholz 1960). Ruderal plants grow in places strongly disturbed by human activities, but not cultivated, e.g. on rubble. Classical ruderata (from Lat. rudus: rubble, ruins; Linnaeus 1751) are ruins and waste lands, walls and pavements. Willdenow’s “Florae Berolinensis Prodromus” (1787) describes the flora at the early time of modern urban development. In 1786, Berlin housed 110 000 inhabitants, 30 000 soldiers, 5 000 refugiés and 3372 Jews and was a bilingual city (German and French , with Jewish trilingual). The city wall surrounded only 1343 ha, with 410 ha dedicated to agricultural and horticultural use (Hofmeister 1985). The first printed description of the urban flora was not restricted to the area intra muros, but comprised areas far from the

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centre. Willdenow (1787) mentioned only 8 species “in urbe ipsa” (of a total of 822 species). The number of publications in urban ecology – even those depicting well-known groups like birds and mammals – increased only slowly during the 19th century, but the increase accelerated after 1900 and numbers have been growing rapidly after 1950. Early publications used the term “migration of plants” for phenomena later described as “introduction and naturalisation” of non-native plants or “biotic invasion”. In the foundation of plant geography (Willdenow 1792, Chapter on “Geschichte der Gewächse” in his “Grundriss der Kräuterkunde”) any reference to urban conditions is missing. But already Schouw (1823), in the first textbook on plant geography, introduced the term plantae urbanae for plants living near cities and villages, e.g. Onopordum acanthium (Scotch Thistle, Eselsdistel), Xanthium strumarium (Rough Cocklebur, Gewöhnliche Spitzklette). He added : “In most cases foreign origin is the cause why these plants are located only near cities and villages” (Schouw 1823: 160f.). In particular, he named wall plants, ruin plants, roof plants, rubble plants and garden weeds. Chamisso (1827) described conditions and effects of anthropogenic changes of flora and fauna for settlements: “Wherever man settles, the face of nature is changed. His domesticated animals and plants follow him; the woods become sparse; and animals shy away; his plants and seeds spread themselves around his habitation; rats, mice and insects move in under his roof; many kinds of swallow, finch, lark and partridge seek his care and enjoy, as guests, the fruits of his labour. In his gardens and fields a number of plants grow as weeds among the crops he has planted. They mix freely with the crops and share their fate. And where he no longer claims the entire area his tenants estrange themselves from him and even the wild, where he has not set foot, changes its form.” This quotation is taken from an instructional work “Botany for the non-botanists” which Chamisso wrote for the Ministry of Culture. The subtitle was

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“A survey of the most useful and harmful plants, whether wild or cultivated, which occur in Northern Germany. Including views of botany and the plant kingdom”, a book in the tradition of natural history. The paragraph quoted above can be translated into modern terminology as follows: the introduction of non-native species, changes of biotopes, synanthropy, hemerochory, apophyty and agriophyty. Introduction and naturalisation of non-native plants, the so-called adventive plants, have often been studied since Ascherson (1864).

Fig. 3

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5. The Modern Metropolis: A New Type of Environment Modern cities consist of a mixture of near-natural areas in urban forests, parks and nature reserves, remnants of agro-ecosystems (“encapsulated countryside”), of densely settled areas in the historic centres and of partly built-up areas with gardens. Urban ecosystems differ from non-urban ones in a number of ways. Although most of the factors which affect urban

Ecological zonation of Berlin (former West Berlin), defined by the floristic composition, from Kunick 1974, modified. Plant lists in Kunick 1982 / Ökologische Zonierung Berlins (ehemaliges West-Berlin), gemäß der Florenzusammensetzung, nach Kunick 1974, verändert. Pflanzenliste in Kunick 1982

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ecosystems also operate in non-urban areas, the combination of these factors means that unique urban ecosystems evolve, with species combinations peculiar to urban areas. Since the second half of the 19th century, the landscape of the Middle Ages and Early Modern Times and its rural character has totally changed with urbanisation. Today, about 3.3 mill. people live in the city of Berlin, on 889 km2, of which 57% are covered by buildings or transport networks, whereas 43% of the Berlin city area are not built on. To understand the plant-environment relationship it is necessary to see the present biotopes as a result of the historic development. From the beginning, towns have been places of shelter against nature and its dangers, and nature was only allowed in them in an ornamental fashion. Historically, cities have fought back nature, creating an artificial environment as opposed to the more natural environments prevailing outside the city. In the course of urban development site conditions have been altered by humans – intentionally or unintentionally. Thus urban open spaces of today represent modifications of older ones. The similarity between former and present site conditions decreases with time and along a gradient from the periphery to the centre. Within the built-up area the original ecosystems have been destroyed and many species have become extinct, but simultaneously, new organisms and new biotic communities have become established in all zones of the city.

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On the basis of differing ecological features Berlin has been divided into four concentric zones: (1) densely built-up and (2) partly built-up central areas, (3) inner and (4) outer suburbs (Fig. 3). In any city, these zones are, of course, not strictly concentric, and their distribution as well as their individual share of the total area vary with peculiarities in the history of the individual city. The landscape of the city is divided into zones of closed and open urbanisation. Small gardens, railway areas and rubble hills as well as vast fields are typical for the inner margin zone, forest areas and large parks for the outer margin zone. Air pollution and warming, changes of the groundwater level and backfills of great expanse are the consequences of urban construction and business. As the volume of imported construction materials, raw materials for finished products and foodstuffs is larger than that of the waste materials which have been taken away, in the course of time the ground level of the city has risen several metres in specific areas. Eutrophication of many locations and compaction or sealing of the soil are linked with the extent of the depth of the cultivated layer. Eutrophication by urban waste does not just concern the waste dumps and sewage fields (used for the filtration of waste water in order to treat it biologically), but influences the composition of biotic communities of almost all watercourses. The floristic composition of these zones reflects the abiotic factors of plant growth. The sealing of surfaces with asphalt, concrete or buildings, for example, varies clinally from 85-100% in Zone 1

Tab. 2 Characteristics of floristic city zones in Berlin (Kunick 1982) Merkmale der floristischen Stadtzonen in Berlin (Kunick 1982) Zone Total vegetation cover (in %) 2

Vascular plant species / km 2

Rare species / km

Non-native plant species (in %)

1

2

3

4

32

55

75

95

380

424

415

357

17

23

35

58

49.8

46.9

43.4

28.5

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Photo 1 Zone 1: Rubble soils and pioneer vegetation 1964. Mugwort (Artemisia vulgaris) along the street on more eutrophicated site. Central part poor in nutrients with grasses (mainly Flattened Poa, Poa compressa). Ritter-/Alexandrinenstr., Sept. 1964 / Schuttböden und Pioniervegetation 1964. Gewöhnlicher Beifuß (Artemisia vulgaris) in eher eutropher Situation am Straßenrand. Innere Grundstücksbereiche arm an Nährstoffen und von Gräsern bestanden (vorwiegend Flaches Rispengras, Poa compressa). Ritter-/Alexandrinenstr., Sept. 1964

to 0-15% in Zone 4. Also, the heat island effect is more pronounced in Zones 1 and 2 than in 3 and 4 (Sukopp 1990). Some characteristics of the different city zones are given in Table 2, examples of flora and vegetation in the four zones are documented by Photos 1-4.

5.1 Climate, soils and waters Cities have particular climatic characteristics (Kratzer 1937, 1956, Hupfer and Chmielewski 1990, Horbert 2000; see also Endlicher and Lanfer in this issue). The ecologically most important effect of the urban climate is a raised air tempera-

ture. Cities are “heat islands” or “hot spots” on the surface of the earth. The temperature difference between the city of Berlin and its surroundings can reach 9°C on clear days, or 0.5 to 1.5°C in the mean annual temperature (Horbert 2000). Climatic conditions within the city can vary considerably, depending on the type of construction, the paving, the location in the city and, especially, the distance to large vegetated areas. According to these changes different climatic zones, usually more or less concentric, can be distinguished. The city heat island usually covers the same area as the built-up area, but changes in wind direction can raise air temperature in other areas, too. Air pollu-

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Photo 2 Berlin-Steglitz: Densely built-up zone (Zone 1) on the left, partly built-up zone (Zone 2) on the right, with Botanical Garden. Photo: D. Biewald, Gesellschaft für Erdkunde, Sept. 2003 / Berlin-Steglitz: Dicht bebaute Zone (Zone 1) links, teilweise bebaute Zone (Zone 2) rechts, mit Botanischem Garten. Photo: D. Biewald, Gesellschaft für Erdkunde, Sept. 2003

tion is caused by traffic, heating of buildings, power stations and industry. Trace gases, except lowlevel ozone, occur at concentrations five to fifty times higher than usual (Kuttler 1998). Two soil types are characteristic for the urban areas of Berlin: deeply cultivated garden soils (hortisols) with high nutrient content and high waterholding capacity, and anthropogenic rendzinas formed on rubble (e.g. from demolished buildings). The latter are alkaline, dry and well-aerated in the beginning, but develop further as a consequence of humus accumulation. They are usually low in nitrogen but have moderate to high contents of phosphorus, calcium, potassium and

other nutrients (Blume et al. 1989). These latter soils can support a specific ruderal vegetation with a succession leading to forest of dominating Robinia pseudoacacia (Acacia, Robinie) and Acer platanoides (Norway Maple, Spitzahorn). Berlin is the first city in which urban soils have been systematically investigated (Runge 1975, Blume 1981, Alaily et al. 1986). A classification of soils in urban agglomerations is given by Blume (1989), and information on their protection by Blume (1992). Most cities are located near rivers. Many urban waters have changed by the development of run-off following urbanisation of the catchment, by increase of the impervious surface cover, canalisation, pol-

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Photo 3 Zone 2: Berlin-Zehlendorf. Original pine forest with 1920s housing, birch trees and Bent-Fescue grassland (Agrostis tenuis, Festuca ovina) / Zone 2: Berlin-Zehlendorf. Ursprünglicher Kiefernforst mit Wohnbebauung der 1920er Jahre, Birken und Trockenrasen mit Rotem Straußgras und Schafschwingel (Agrostis tenuis, Festuca ovina).

lution and decline in richness of biotic communities (Köhler et al. 2002). Drinking water supply and waste water treatment were necessary rather early in urban agglomerations. Kolkwitz and Marsson (1902) worked out a water quality standard by biological examination whose further revision led to the saprobic system expressed by plant and animal indicator species according to their capability to survive under different saprobic conditions. Under Kolkwitz’s direction, the first successful rehabilitation of a lake was carried out. Kolkwitz (1909, 1914) had investigated the mass growth of algae in Berlin’s Lietzensee lake and came to the conclusion that the decisive factor for algae pro-

duction lay in the almost continuous introduction of nutrients from the mud at the bottom of the lake. He developed a method to remove the nutrient-rich mud-layers at the lake bottom with nutrient-poor water – a method which proved to be a success. Changes of the historical flora in Lake Tegel (Berlin) and their causes were described by Sukopp and Brande (1996) and by Geissler and Kies (2003). The beginning of woodland clearance activities during the middle to late neolithic age in the vicinity of the lake prompted an increase in primary production in the littoral and pelagic zones. The anthropogenic influences, which had been effective since that time in connection with relatively

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intensive land-use in the early Bronze and Iron age, led to further inputs of allochthonous mineral substances and nutrients into the lake. Further considerable changes in late medieval times consist in an increase of the water level as a consequence of the construction of the Spandau mill barrage at about AD 1232 and the extensive clearance activities at that time (Brande 1988) which both correspond to the establishment of Tegel village in around 1230 (first mentioned in 1322) on the north-eastern bank of the lake near the estuary of the Tegel brook. The degree of change from an oligotrophic (up to 1850) to a hypertrophic lake by the discharge of percolating water from the North Berlin irrigation fields since the beginning of the 20th century is indicated by the extinction of a vegetation with many Charophyta (Stoneworts, Armleuchteralgen), Potamogeton (Pondweed, Laichkraut) and Najas (Naiad, Nixkraut) species and the only freshwater Phaeophyta (Brown algae, Braunalgen) species, Pleurocladia lacustris (Cushion brown alga, Kissen-Braunalge). Now water blooming caused by cyanobacteria is dominant. Within these changes the extinction of organisms as well as the introduction and dispersal of neophytes and neozoa in Lake Tegel are well documented for the last 200 years. Elodea canadensis (Canadian pondweed, Kanadische Wasserpest), native in temperate North America, has been known in Central Europe since 1859. Its dispersal began in botanical gardens on the North German lowlands. The waterweed was released from the Berlin Botanical Gardens in 1859 into the ditches near Charlottenhof at PotsdamSanssouci. The plant already existed outside the cultivated areas, for example at Lake Tegel (Bolle 1865). The intensive growth of the Canadian waterweed, the “Green Ghost”, led to all kinds of difficulties for shipping, water management and fishing. However, after some years the mass expansion of Elodea canadensis declined. Human influence on the development of reed beds is similarly strong. From 1962 to 1987

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reed areas along Berlin’s Havel river diminished from 40% of the shoreline to 12%. The main causes were mechanical erosion of organic soil, eutrophication of water and artifical recharge of ground water at the shore. For protection and recultivation of reed belts it is necessary to take into account the combination of these three factors (Sukopp 1991).

5.2. Introduction and naturalisation of non-native organisms The altered climate, soil and water conditions in cities have a corresponding impact on the composition of biotic communities occurring in urban areas (Wittig 1991, 2002, Klausnitzer 1993). These changes increase with size of the urban area and with proximity to the city centre. Comparisons between urban and rural areas and between different cities as well as city margin to inner city gradient studies have been established as methods of analysing these differences. The number of fern and flowering plant species per area unit is higher in cities with more than 50 000 inhabitants than in the surrounding area. In Central Europe, the number of fern and flowering plant species correlates closely with population size and/or density. In small and mediumsized towns, between 530 and 560 species are usually found; 650-730 species are found in cities with 100 000 to 200 000 inhabitants, 900 to 1000 species in older cities with a population of between 250 000 and 400 000; and in cities with over a million inhabitants, the number of species usually exceeds 1300 (Klotz 1990, Pysek 1993). There are mainly two reasons for this: • Urban areas are heterogeneous, consisting of a variety of settlement structures, land-uses and small-scale habitats (Böcker et al. 1998). This creates many specific and even unusual ecological conditions. • The introduction of-non native organisms.

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Photo 4 Zone 3: Former railway area with spontaneous vegetation. Schöneberger Südgelände. 1985 Zone 3: Ehemaliges Bahngelände mit spontaner Vegetation. Schöneberger Südgelände, 1985

Historically, species which have been introduced into an area by human activity have usually begun their dispersal in urban areas and therefore occur there most frequently. With increasing settlement size, trade and traffic in and out of the city grow, and thus the proportion of non-native species in the flora increases, too. The number of species by immigration is chiefly due either directly to the activity of man, as in the case of ornamental plants, or indirectly, e.g. when impurities get into the transported materials or seeds. These plant species are called hemerochores. Today the number of naturalised plants by far exceeds that of extinct species, thus being one of the causes for a high number of species in urban areas. This is explained not only by considerable habitat heterogeneity, as mentioned above, and the role of big cities as centres of species immigra-

tion but also by the better adaptation of alien species to man-made perturbations (Sukopp and Trepl 1987; Kowarik 1990, 1995). Due to industrialisation, most introduced species reached Central Europe in the second half of the 19th century (Jäger 1991). Besides changes in species number, there were also shifts in species frequency. A comparison of species frequency in Berlin with data from the 19th century (Ascherson 1864) reveals differences between native and non-native species. Two-thirds of the species which have become more frequent during the last 120 years established themselves after 1500 AD (Kutschkau 1982). One major question is whether the process of man-caused species migrations has passed beyond its zenith yet. If not, a further enormous in-

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Photo 5 Zone 4: Green roof at Berlin-Tegel Waterworks, after 80 years of undisturbed development of soil and vegetation. 1978. Photo: Senatsverwaltung für Stadtentwicklung Berlin Zone 4: Grünes Dach im Wasserwerk Berlin-Tegel, nach 80 Jahren ungestörter Boden- und Vegetationsentwicklung. 1978. Photo: Senatsverwaltung für Stadtentwicklung Berlin

crease of species richness in some areas, in particular urban ones, seems to be possible in the future. But we do not know enough about the mechanisms of invasion processes to make even rough prognoses (Trepl and Sukopp 1993). Recent weed evolution after species introduction is evident in many cases (Sukopp and Scholz 1997). There are two concepts about the origin of non-native weeds: a) Introduction and naturalisation of plants; b) Evolution of new taxa on secondary habitats of the cultural landscape: anecophytes (Zohary 1962) – processes frequently associated with genetic changes taking place in the plants and permitting them to spread over large areas. Recent weed evolution after

species introduction is evident in many cases: Oenothera (Evening primrose, Nachtkerze), Corispermum (Bug-seed, Wanzensame); Xanthium (Cocklebur, Spitzklette), Fallopia (Knotweed, Staudenknöterich). The creation of cultivated ecosystems and changes within cultivated landscapes result in faster evolutionary changes (Timoféeff-Ressovsky et al. 1975). Consequently, “we must realize that most naturalised aliens will persist and become part of our more permanent flora” and “create new ecological communities”. More important, “we would be wise also to recall that it is the widely dispersed and fairly abundant species that are most likely to survive stresses causing extinction, and

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be the founder stocks for new diversification” (Scholz 1997: 144). For Berlin, the change of vegetation, species composition and site conditions from the last glaciation until the present has been thoroughly investigated. The number of fern and flowering plant species for Berlin (today’s area: 889 km2) was 822 in the mid-18th century (Willdenow 1787), 1130 in the mid-19th century (Ascherson 1864) and 1392 by the year 2000 (Prasse et al. 2001). Altogether 2178 species have been registered since 1787 (1015 of them neophytes), but 786 never could establish themselves. In the same time period 202 species have become extinct. These numbers refer to spontaneously occurring species only. In addition, there is a large number of cultivated plants in parks, gardens and churchyards, on small patches of ornamental green, as street trees or even on balconies or in flower pots. Their number – in species as well as in individuals – far exceeds that of the spontaneous plants. The urban green enhanced by city dwellers had either the function to symbolise the superiority of man over nature (as in baroque gardens) or to provoke the dream of natural rural life (landscape gardens). In both cases, “green” stands for the contrast between nature and city. Although cultivated plants serve important functions in cities, e.g. serving as living space for animals, the investigations have usually focused on spontaneous plants. Spontaneous urban vegetation – as a type of nature adapted to the specific urban conditions and capable to exist under them – symbolises the city. This gives a chance for a greening design that does not accentuate the contrast between nature and city as it used to be. Green roofs of Berlin (Darius and Drepper 1985), which became a well-known feature of the city by the end of the 19th century, gave inspiration to many architects. Plants growing on buildings influence the climate in a positive way.

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Extensive investigations confirm the considerable benefits of climbing plants on walls to microclimate and air-hygiene. A cover of plants on roofs can reduce the surface temperature, filter the air, fix harmful substances and reduce heat losses in winter (Darius and Drepper 1985). An unexpected result of early studies on plants in cities was the fact that they do not co-occur accidentally, but form distinct patterns of co-occurrence and plant communities in the same way as plants in more natural environments. Typical urban vegetation of Berlin is represented by the Prickly Lettuce association (Conyzo-Lactucetum serriolae) and the Viper’s Bugloss-Melilot association (Echio-Melilotetum). Characteristic plant species of the centre of Berlin are e.g. Ailanthus altissima (Tree-ofHeaven, Götterbaum), Amaranthus albus (White Pigweed, Weißer Amarant), A. blitoides (Postrate Pigweed, Westamerikanischer Amarant), Chaenorhinum minus (Small Toadflax, Klaffmund), Chenopodium botrys (Sticky Goosefoot, Klebriger Gänsefuß), C. strictum (Striped Goosefoot, Gestreifter Gänsefuß), Commelina communis (Spiderwort, Commeline), Potentilla supina (Prostrate Cinquefoil, Niedriges Fingerkraut), also forming characteristic plant communities. One of the characteristic pioneer colonisers is the summer annual Chenopodium botrys (Sukopp 1971). This southern Eurasian-Mediterranean plant has expanded its range due to human influence into broad areas of central and western Europe, North America and Australia. Introduced to Berlin in 1889, it became a characteristic and specific ruderal plant for the heat island of inner Berlin after World War II. Natural habitats of the plant are sandy and stony soils near river banks; i.e. special habitats with little competition. Accordingly, roadsides, cultivated areas and derelict land are colonised as secondary habitats. Under natural conditions, the area covered by such open, low-competition habitats is quite small in Central Europe. But the open calcium-rich sandy to gravelly habitats which have been cre-

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ated by man have made the appearance of Chenopodium botrys possible. Large and lasting populations of this species, however, have been found north of the Alps only in Berlin, Mannheim, the Ruhr area and Lille, especially after 1945. The colonies in Stuttgart, Saarbrücken and Leipzig are unstable or have disappeared. Among the woody plants on inner-city derelict land in Berlin, Robinia pseudoacacia stands cover the greatest area. On rubble-mortar substrate, a calcium-containing loose syrosem develops into a pararendzina under Robinia in the course of vegetation and soil genesis. Among the numerous neophytic woody plants, not only the amount of species but, in particular, their lush development is surprising. Besides Robinia pseudoacacia, the Tree-of-heaven (Ailanthus altissima) has spread on inner-city derelict land (Kowarik and Böcker 1984, see also Endlicher and Lanfer in this issue). This vigorous drought-tolerant tree settles on extreme habitats such as railway land and places adjacent to buildings, but is also frequent in green open places. In derelict areas Ailanthus is able to establish large polycormons through suckering. A particularity of urban floras is the fact that they are richer in non-native species than the outskirts of cities. The proportion of non-natives in the flora regions of the world is between 5 and 25%; on islands it is often higher (Jäger 1988). In Berlin, the proportion of non-natives varies from 28.5% in the outer fringe (Zone 4) to close to

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50% in the inner city (Zone 1, Kunick 1974, 1982) with an average of 41% for the whole city (Kowarik 1990). These numbers contrast to only 20-25% of non-natives in surrounding districts. Among these species especially the neophytes are more frequent than in the environs, whereas the proportion of archaeophytes is not significantly higher than in rural areas. Native and naturalised alien woody species in Berlin are given in Table 3. A well-known factor for the introduction and spread of plants by trade are certain economic sectors and is the wool industry in the beginning of the 19th century, when diaspores of plants travelled with wool from other continents into Europe and were found growing in the surroundings of the embarking points of wool mills.

6. From World War II to Reunification In a historical context, sites bombed during World War II have played an important part in the development of the urban flora and its investigation. Only few years after the bombing various plants had colonised the ruined houses. In many cities the war damage and its effects gave rise to special studies of the rubble flora and fauna (Düll and Werner 1955/56, Scholz 1960 and others). During the colonisation of rubble, which offers warmer and drier conditions than natural habitats, plants and animals from warmer regions of the world found suitable conditions. Many plants that had been rather rare have become permanent members of the urban flora in war-damaged Berlin.

Tab. 3 Native and naturalised alien woody species in Berlin (Kowarik 1992) Heimische und eingebürgerte Baumarten in Berlin (Kowarik 1992) Natives

Introduced aliens

Established aliens

Trees

30

68

25

Shrubs

57

109

29

Woody climbers Sum

2

5

2

89

182

56

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After World War II an intensive and spontaneous development of vegetation began on the rubble, proceeding in more or less rapid succession from short-lived and perennial stages of herbaceous vegetation to shrub and forest-like stands. The variety of species on such inner-city waste land is surprisingly large. Thus a site at Lützowplatz in Berlin-Tiergarten accommodated 140 species of seed plants and at least 200 taxa of insects (Weigmann et al. 1978). The carefully maintained lawns and bushes of the nearby Tiergarten Park have not more than a quarter as many insect species on the same area. This natural succession was generally disrupted by site clearing and the (re)construction of buildings. The areas cleared of rubble or destined for redevelopment, which had often been recolonised by woody plants, have gradually disappeared in the course of ongoing construction. Dispersal of organisms and development of vegetation can be studied on rubble sites on a large scale and in an environment that differs considerably from previously known ruderal places. As Pfeiffer (1957) wrote: “The recolonization of rubble, created in many cities due to the activity of bombers in the last war, has unintentionally become a tremendous natural experiment, which with respect to its size, must be compared to the colonisation of new habitats created by volcanic activity”. The studies on the dynamic character of vegetation development on rubble have led to a first peak in urban ecology studies. Differing from adventive floristics the studies concentrated on dispersal strategies of individual species, on succession depending on site qualities and on the formation of new plant communities. But also these investigations were essentially based on rubble sites, not on the city as a whole. A major reason for the (relative) unpredictability of succession in urban ecosystems is the high degree to which these systems are subject to invasions of “aliens”. The biogeographical spec-

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trum of species composition of cities is very different from that of the surrounding countryside. The main causes obviously lie (a) in the conditions of naturalisation, i.e. in the high invasibility of the biocoenoses, and (b) in the conditions of dispersal (introduction, transportation). Disturbances generally increase invasibility and urban ecosystems are disturbed ones. Towns are open to invasions of alien species, their number is unforeseeable (Trepl 1994). Plants spreading during wars have been called polemochores. Thellung (1917) has given the name stratiobotany (polemobotany) to war botany comprising the destructive influence of war, the development of new plant communities with a characteristic flora and the changes in cultivated formations. Polemochores of World War II in Berlin are two Chenopodiaceae of eastern origin (Salsola collina, Artemisia scoparia; Saltwort, Wormwood; Hügel-Salzkraut, Besen-Beifuß) and one grass species of North American origin (Panicum lindheimeri, Lindheimer’s Millet, Lindheimers Hirse). The hypothesis that disturbance favours the establishment and spread of alien species is accepted in most studies on biological invasions. In urban environments, it is mainly the man-made components of disturbance which affect species composition and promote non-natives. Kowarik (1995) has shown that naturalised non-natives are enhanced on sites that are subject to a high level of disturbance. The highest percentages of alien species are found in ruderal vegetation units with annuals as the dominating life form. The changes of the flora have proved to be reflections of economic and cultural history: The development of the ruderal flora “essentially runs parallel to the size and the intensity of trade and industry; it is a direct standard of the technical culture” (Naegeli and Thellung 1905). Within the last 150 years 202 species of flowering plants have disappeared from Berlin (Sukopp 1968, Prasse et al. 2001) and many further species de-

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creased considerably. On the other hand, many more new species have been established and several have expanded. In addition to numerous nonnative species, also native organisms may profit from urban habitats, especially if the conditions are similar to their original habitats. In the natural landscape, these plants and animals originally occurred on open sites such as gravel or mud banks along rivers and streams, on open places within forests, or on ground disturbed by fire. As most large cities are located at large rivers, erosion and sedimentation act as important initial habitats for urban apophytes (native plants colonising anthropogenic habitats). Native plants which have greatly expanded in Berlin during the last 50 years are Cardamine hirsuta (Hairy Bitter-cress, Viermänniges Schaumkraut), Veronica sublobata (Hedge speedwell, Hecken-Ehrenpreis), Elytrigia repens (Couch-grass, Quecke), Calamagrostis epigeios (Bushgrass, Land-Reitgras), Calystegia sepium (Larger Bindweed, Zaunwinde), Humulus lupulus (Hop, Hopfen), Rorippa sylvestris (Creeping yellow-cress, Wilde Sumpfkresse), Acer platanoides (Norway Maple, Spitzahorn). Based on 6000 vegetation relevés made in West Berlin, the hemeroby concept (Jalas 1955) was used to construct spectra of hemeroby for each species of the urban flora (Kowarik 1990). This approach is particularly valuable to relate a species’ response to the complex measure of human influence. Human impact consists of many partial environmental factors, some of which (stress, disturbance) cannot be measured directly (Sukopp 1968). The current distribution of plants in urban areas is determined by their relationship to urban land-use. Wittig et al. (1985) have distinguished urbanophilous, urbanoneutral and urbanophobous species (see also Korsch 1999). Historically urban districts of different ages were investigated with respect to the distribution of plants. Saarisalo-Taubert (1963) has shown that the distribution of the “flora accompanying old settlements” (“Begleitflora alter Siedlungen”) is determined by favourable edaphic and mi-

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croclimatic conditions. They can be natives as well as introduced plants (Aey 1990). The Berlin Wall (1961-1989) also led to differences in the distribution of plants within Berlin. Surprisingly, Iva xanthiifolia (Marsh-elder, Spitzkletten-Rispenkraut), of North American origin, was mainly found in East Berlin. During the 19th and 20th century this plant was introduced to Eastern Europe and Western Asia, spreading into available land. With imported wheat from Ukraine and Kazachstan it became frequent along railway systems in Eastern Germany. Passarge (1996) has described the Iva xanthiifolia vegetation at Potsdamer Platz in central Berlin. Plant communities in areas along the borderline between East and West Berlin were significantly influenced by neighbouring habitats. A typical example are sites adjacent to allotments showing an increased occurrence of wild-growing ornamental plants (Schmitz 1998). With the end of the division of East and West Berlin in 1989, the city of Berlin has faced a new situation concerning its ecological development. Species introduced as ornamentals will remain by far the largest group from which neophytes will emerge. In this regard changing preferences are important. Predicting which species will become naturalised depends largely on their preference by the public as well as their dispersal mechanisms. Choice of the locations from which species will be drawn will continue to be widespread (Mack 2001). Over the last 30 years, Senecio inaequidens (Narrow-leaved ragwort, Schmalblättriges Geiskraut) has become one of the most successful alien plants on ruderal sites in Central Europe. This originally South African plant was repeatedly introduced by wool transports. After initial colonisation in 1896 the plant successfully spread from seven locations of introduction throughout Europe. In 1993 it was found in Berlin for the first time. Most of its settlements are located along motorways, a great number also on railway areas. An expansion of Senecio inaequidens towards

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the north and the east has not yet been finished, and the inner colonisation of the area will continue (Bornkamm 2002).

7. Conclusion Urban biocoenoses are an extreme example of communities produced by successive invasions and not by co-evolutionary development. In principle, the historic uniqueness of urban situations, i.e. the combinations of environmental factors and organisms, differentiates urban ecosystems from most natural ones, even those subject to strong disturbance. Concerning plant introductions societal awareness of the hazards of the indiscriminate global dispersal of plants is growing. Understanding human behaviour in relation to plant dispersal will prove as important as understanding the biology of dispersed species.

8. Acknowledgements Sincere thanks for the help in the preparation of this article and with the literature are due to Arthur Brande, Hildemar Scholz, Stephan Sukopp and Ulrich Sukopp – and to colleagues, many of them cited in the text, for cooperation for decades.

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Summary: Flora and Vegetation Reflecting the Urban History of Berlin The composition and the changes of the flora and the vegetation have proven to be reflections of the economic and cultural history of Berlin. The evolution of the ruderal flora essentially runs parallel to the size and the intensity of trade and industry and forms a good indicator for the state of the technical culture. To understand the plant-environment relationship it is necessary to see the present habitats as a result of the historical evolution. Urban biocoenoses are extreme examples of communities produced by successive invasions and not by co-evolution. Generally, the historic uniqueness of urban situations, i.e. the combinations of environmental factors and organisms, differentiates urban ecosystems from most natual ones, even those subject to strong natural disturbance. Understanding human behaviour in relation to plant dispersal is as important as understanding the biology of the plant species, as it contributes essentially to the individuality of metropolitan Berlin, including through changing fashions in horticulture.

Zusammenfassung: Flora und Vegetation als Spiegel der Stadtgeschichte Berlins Die Veränderungen von Flora und Vegetation spiegeln die Wirtschafts- und Kulturgeschichte Berlins wider. Die Entwicklung der siedlungsbegleitenden Ruderalflora verläuft parallel zu Umfang und Intensität von Handel, Gewerbe und Industrie und ist ein Gradmesser der Zivilisation. Zum Verständnis des Verhältnisses von Umwelt und Pflanzen ist es notwendig, die gegenwärtigen Lebensräume als Ergebnis der historischen Entwicklung

DIE ERDE

zu sehen. Städtische Lebensgemeinschaften entstehen fortwährend durch Einbürgerungen neuer Arten ohne lange Koevolution ihrer Bestandteile. Die historische Einmaligkeit einer Großstadt in Standorten und Organismenbestand unterscheidet urbane Ökosysteme von natürlichen, selbst denen mit starken natürlichen Störungen. Zur Individualität der Großstadt Berlin tragen menschliche Verhaltensweisen – auch durch wechselnde Moden im Gartenbau – wesentlich bei.

Résumé: Flore et végétation, miroirs de l’histoire de Berlin L’évolution de la flore et de la végétation reflète l’histoire de l’économie et de la civilisation berlinoises. La flore rudérale évolue avec le processus d’urbanisation et parallèlement au volume et à l’intensité du commerce et des activités industrielles et permet ainsi de mesurer les effets civilisateurs. Afin d’apprécier les rapports entre l’environnement et les espèces végétales, il est indispensable de considérer les habitats d’aujourd’hui comme le résultat de leur évolution historique. Les biocoenoses urbaines se créent continuellement à travers de nouvelles espèces apparaissant sans que pour autant leurs composants n’accusent une co-évolution prolongée. L’unicité historique d’une grande ville quant aux sites et à l’inventaire des organismes différencie les écosystèmes urbains de leurs homologues naturels même lorsque ces derniers sont affectés par d’importantes perturbations d’origine naturelle. Les comportements humains et parmi eux les modes changeantes de l’horticulture contribuent à l’individualité de la métropole berlinoise.

Prof. em. Dr. Dr. h.c. Herbert Sukopp, Rüdesheimer P latz 10, 14197 Berlin, Germany, [email protected]

Manuskripteingang: 18.6.2003 Annahme zum Druck: 22.11.2003