The paper provides the first estimate of the composition and structure of alien plants occurring in the wild in the European continent, based on the results of the DAISIE project (2004–2008), funded by the 6th Framework Programme of the European Union and aimed at “creating an inventory of invasive species that threaten European terrestrial, freshwater and marine environments”. The plant section of the DAISIE database is based on national checklists from 48 European countries/regions and Israel; for many of them the data were compiled during the project and for some countries DAISIE collected the first comprehensive checklists of alien species, based on primary data (e.g., Cyprus, Greece, F. Y. R. O. Macedonia, Slovenia, Ukraine). In total, the database contains records of 5789 alien plant species in Europe (including those native to a part of Europe but alien to another part), of which 2843 are alien to Europe (of extra-European origin). The research focus was on naturalized species; there are in total 3749 naturalized aliens in Europe, of which 1780 are alien to Europe. This represents a marked increase compared to 1568 alien species reported by a previous analysis of data in Flora Europaea (1964–1980). Casual aliens were marginally considered and are represented by 1507 species with European origins and 872 species whose native range falls outside Europe. The highest diversity of alien species is concentrated in industrialized countries with a tradition of good botanical recording or intensive recent research. The highest number of all alien species, regardless of status, is reported from Belgium (1969), the United Kingdom (1779) and Czech Republic (1378). The United Kingdom (857), Germany (450), Belgium (447) and Italy (440) are countries with the most naturalized neophytes. The number of naturalized neophytes in European countries is determined mainly by the interaction of temperature and precipitation; it increases with increasing precipitation but only in climatically warm and moderately warm regions. Of the nowadays naturalized neophytes alien to Europe, 50% arrived after 1899, 25% after 1962 and 10% after 1989. At present, approximately 6.2 new species, that are capable of naturalization, are arriving each year. Most alien species have relatively restricted European distributions; half of all naturalized species occur in four or fewer countries/regions, whereas 70% of non-naturalized species occur in only one region. Alien species are drawn from 213 families, dominated by large global plant families which have a weedy tendency and have undergone major radiations in temperate regions (Asteraceae, Poaceae, Rosaceae, Fabaceae, Brassicaceae). There are 1567 genera, which have alien members in European countries, the commonest being globally-diverse genera comprising mainly urban and agricultural weeds (e.g., Amaranthus, Chenopodium and Solanum) or cultivated for ornamental purposes (Cotoneaster, the genus richest in alien species). Only a few large genera which have successfully invaded (e.g., Oenothera, Oxalis, Panicum, Helianthus) are predominantly of non-European origin. Conyza canadensis, Helianthus tuberosus and Robinia pseudoacacia are most widely distributed alien species. Of all naturalized aliens present in Europe, 64.1% occur in industrial habitats and 58.5% on arable land and in parks and gardens. Grasslands and woodlands are also highly invaded, with 37.4 and 31.5%, respectively, of all naturalized aliens in Europe present in these habitats. Mires, bogs and fens are least invaded; only approximately 10% of aliens in Europe occur there. Intentional introductions to Europe (62.8% of the total number of naturalized aliens) prevail over unintentional (37.2%). Ornamental and horticultural introductions escaped from cultivation account for the highest number of species, 52.2% of the total. Among unintentional introductions, contaminants of seed, mineral materials and other commodities are responsible for 1091 alien species introductions to Europe (76.6% of all species introduced unintentionally) and 363 species are assumed to have arrived as stowaways (directly associated with human transport but arriving independently of commodity). Most aliens in Europe have a native range in the same continent (28.6% of all donor region records are from another part of Europe where the plant is native); in terms of species numbers the contribution of Europe as a region of origin is 53.2%. Considering aliens to Europe separately, 45.8% of species have their native distribution in North and South America, 45.9% in Asia, 20.7% in Africa and 5.3% in Australasia. Based on species composition, European alien flora can be classified into five major groups: (1) north-western, comprising Scandinavia and the UK; (2) west-central, extending from Belgium and the Netherlands to Germany and Switzerland; (3) Baltic, including only the former Soviet Baltic states; (4) east-central, comprizing the remainder of central and eastern Europe; (5) southern, covering the entire Mediterranean region. The clustering patterns cut across some European bioclimatic zones; cultural factors such as regional trade links and traditional local preferences for crop, forestry and ornamental species are also important by influencing the introduced species pool. Finally, the paper evaluates a state of the art in the field of plant invasions in Europe, points to research gaps and outlines avenues of further research towards documenting alien plant invasions in Europe. The data are of varying quality and need to be further assessed with respect to the invasion status and residence time of the species included. This concerns especially the naturalized/casual status; so far, this information is available comprehensively for only 19 countries/regions of the 49 considered. Collating an integrated database on the alien flora of Europe can form a principal contribution to developing a European-wide management strategy of alien species.
The invasion of Austria by the alien vascular plant Ambrosia artemisiifolia (Asteraceae) is analysed in detail, based on a survey of available records. In total, 697 records were collated. The first record for Austria is a herbarium specimen collected in 1883. Up to the end of the 1940s, records were rare and only of casual populations resulting from long-distance dispersal. Since the 1950s, the number of records has increased exponentially, and more than one third of all records (242) were collected in the last 5-year period (2001–2005) included in the survey. The first naturalized population was recorded in 1952, nearly 70 years after the first record of a casual population. Recently, the number of naturalized populations increased considerably faster than that of casual populations. Several pathways (contaminated crops and bird seed, agricultural machines, transport of soil) have contributed to the high levels of propagule pressure and this successful invasion. Ambrosia artemisiifolia has undergone a niche expansion during the invasion process. Up to 1950, most records were from sites along railway routes, whereas in the period 1950–1974 itwas mostly ruderal habitats, not associated with traffic infrastructure, which were colonized. Since the 1970s, records from roadsides have increased strongly and now dominate. Fields were colonized first in the 1970s and since then have gained in importance. The distribution of naturalized populations was related to environmental and climatic variables by means of a generalized linear model. Their distribution in Austria is closely related to temperature. Landscape variables, describing aspects of habitat availability (topography, land use, major street density) also significantly explain the current distribution of A. artemisiifolia. Suitable habitats currently occur mainly in the eastern and southeastern lowlands. We conclude that global warming will disproportionally enhance the invasion success of A. artemisiifolia in Austria, even if there is only a slight increase in temperature, as significant areas of agricultural land in Austria are currently only slightly too cool for A. artemisiifolia. The widespread occurrence of this species will have serious consequences for human health and agriculture.
The invasion of Paulownia tomentosa (Paulowniaceae), a new alien tree species in Central Europe, native to China, is analysed. By using its distribution in Austria, the invasion of this country is analysed in detail. The first reports of P. tomentosa in Austria were in the 1960s in Vienna. Since then, the number of sites has increased exponentially, with a total of 151 sites in 27 grid cells of the Floristic Mapping project of Austria. The number of sites per grid cell is strongly positively correlated with the minimum residence time in grid cell, which explains 86% of the deviance in the general linear model (GLM). The localities are confined to warm lowland areas (below 450 m altitude) and are concentrated in cities, with 90% of all localities recorded in cities with > 100,000 inhabitants. Paulownia tomentosa typically occurs in small populations of less then 10 individuals (83% of all records) and behaves as a pioneer species colonizing mainly disturbed urban habitats. Near-natural habitats, e.g. forest clearings and riparian shrubberies are rarely colonized. In extremely disturbed areas, the average number of vascular plant species is low (8.9 species), as is total plant cover (17%). As P. tomentosa is currently mostly confined to synanthropic habitats in urban areas, the invasion is not yet a nature conservation issue. In the future, predicted climate change might allow P. tomentosa to spread beyond its current distribution. The habitat preference in the eastern USA indicates that further spread of P. tomentosa in Central Europe might be accompanied by a switch to more natural habitats, e.g. forest clearings and forest margins. Thus, the future spread of this species should be closely monitored.