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Summary1. This account presents information on all aspects of the biology of Robinia pseudoacacia L. that are relevant to understanding its ecological characteristics and behaviour. The main topics are presented within the standard framework of the Biological Flora of the British Isles: distribution, habitat, communities, responses to biotic factors, responses to environment, structure and physiology, phenology, floral and seed characters, herbivores and disease, and history and conservation. 2. Robinia pseudoacacia, false acacia or black locust, is a deciduous, broad-leaved tree native to North America. The medium-sized, fast-growing tree is armed with spines, and extensively suckering. It has become naturalized in grassland, semi-natural woodlands and urban habitats. The tree is common in the south of the British Isles and in many other regions of Europe. 3. Robinia pseudoacacia is a light-demanding pioneer species, which occurs primarily in disturbed sites on fertile to poor soils. The tree does not tolerate wet or compacted soils. In contrast to its native range, where it rapidly colonizes forest gaps and is replaced after 15-30 years by more competitive tree species, populations in the secondary range can persist for a longer time, probably due to release from natural enemies. 4. Robinia pseudoacacia reproduces sexually, and asexually by underground runners. Disturbance favours clonal growth and leads to an increase in the number of ramets. Mechanical stem damage and fires also lead to increased clonal recruitment. 5. The tree benefits from di-nitrogen fixation associated with symbiotic rhizobia in root nodules. Estimated symbiotic nitrogen fixation rates range widely from 23 to 300 kg ha À1 year À1 . The nitrogen becomes available to other plants mainly by the rapid decay of nitrogen-rich leaves. 6. Robinia pseudoacacia is host to a wide range of fungi both in the native and introduced ranges. Megaherbivores are of minor significance in Europe but browsing by ungulates occurs in the native range. Among insects, the North American black locust gall midge (Obolodiplosis robiniae) is specific to Robinia and is spreading rapidly throughout Europe. 7. In parts of Europe, Robinia pseudoacacia is considered an invasive non-indigenous plant and the tree is controlled. Negative impacts include shading and changes of soil conditions as a result of nitrogen fixation.Key-words: climatic limitation, ecophysiology, geographical and altitudinal distribution, germination, invasive, mycorrhiza, nitrogen fixation, parasites and diseases, reproductive biology, soilsFalse acacia, black locust. Fabaceae, tribe Robinieae. Robinia pseudoacacia L. is a deciduous, strongly suckering, broadleaved tree, up to 20 m high but, occasionally taller. Bark *Nomenclature of vascular plants follows Stace (2010) and, for nonBritish species, Flora Europaea.

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Roadsides are preferential migration corridors for invasive plant species and can act as starting points for plant invasions into adjacent habitats. Rapid spread and interrupted distribution patterns of introduced plant species indicate long-distance dispersal along roads. The extent to which this process is due to species' migration along linear habitats or, alternatively, to seed transport by vehicles has not yet been tested systematically. We tested this by sampling seeds inside long motorway tunnels to exclude nontraffic dispersal. Vehicles transported large amounts of seeds. The annual seed rain caused by vehicles on the roadsides of five different tunnel lanes within three tunnels along a single urban motorway in Berlin, Germany, ranged from 635 to 1579 seeds/m(2)/year. Seeds of non-native species accounted for 50.0% of the 204 species and 54.4% of the total 11,818 seeds trapped inside the tunnels. Among the samples were 39 (19.1%) highly invasive species that exhibit detrimental effects on native biodiversity in some parts of the world. By comparing the flora in the tunnel with that adjacent to the tunnel entrances we confirmed long-distance dispersal events (>250 m) for 32.3% of the sampled species. Seed sources in a radius of 100 m around the entrances of the tunnels had no significant effect on species richness and species composition of seed samples from inside the tunnels, indicating a strong effect of long-distance dispersal by vehicles. Consistently, the species composition of the tunnel seeds was more similar to the regional roadside flora of Berlin than to the local flora around the tunnel entrances. Long-distance dispersal occurred significantly more frequently in seeds of non-native (mean share 38.5%) than native species (mean share 4.1%). Our results showed that long-distance dispersal by vehicles was a routine rather than an occasional mechanism. Dispersal of plants by vehicles will thus accelerate plant invasions and induce rapid changes in biodiversity patterns.

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The role of alien species in urban vegetation is reviewed. Representation of aliens is compared with that of native species by taking into account both historical and spatial aspects. On a long-term time scale (more than a century), the importance of aliens, especially neophytes, is increasing. The representation of alien species, if expressed in quantitative terms, shows a close relationship to the spatial structure of the city, and decreases from the city center towards the outskirts. In many cases, this trend is still valid at the level of particular species, as exemplified by an alien tree Ailanthus altissima. The relationship between species richness and the level of man-induced disturbance supports the intermediate disturbance theory only if the native species are considered; aliens, both archaeophytes and neophytes, are encouraged in sites subjected to high disturbance levels. At the level of phytosociological alliances, the proportion of alien species was closely correlated with the disturbance level, although the relationship was rather variable between particular alliances. As regards the role of alien species in succession on urban waste land, it appears that alien species may persist as dominants in succession for a long period. The following factors promoting the success of alien species in urban environment are discussed: availability of specific urban niches, high level of disturbance in urban environment, and the minor isolation of seed sources.

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