Introduction of Large Heracleum Species and their Distribution in Europe

The first known record of a giant hogweed in Europe is for England, 1817, when it appeared on the Kew Botanic Gardens, London seed list under the name of H. giganteum. It is most likely that the plant introduced under this name was H. mantegazzianum, since this was the start of the spread of this species in Europe. In 1828, the first naturalized population of H. mantegazzianum was recorded, growing wild in Cambridgeshire, England. Soon afterwards, the plant began to spread rapidly across Europe. Ranked according to the date of introduction, the UK was followed by the Netherlands, Switzerland, Germany, Ireland, Denmark and Czech Republic (Nielsen et al., 2005); in the latter country (see Pysek et al., Chapter 3, this volume) the length of period between the introduction into cultivation (1862) and the first report of a spontaneous occurrence nearby (1877) is very similar to the 11 years in England. Thus, it is reasonably certain that the pattern and historical dynamics on a national scale in various countries resembles that described in Chapter 3 for the Czech Republic. In 14 of the 19 countries for which historical data are available, it was first reported before 1900, and only in Austria, Slovakia and Iceland is the first record after 1960 (Nielsen et al., 2005). The main mechanism of introduction into Europe was as an ornamental curiosity. Seeds were planted in botanic gardens and the grounds of important estates. This fashion continued for most of the 19th century and only declined and eventually ceased after warnings about the dangers of the plant appeared in western European literature towards the end of the 20th century (Nielsen et al., 2005).

Heracleum mantegazzianum is the most widespread of the large invasive hogweed species in Europe (Fig. 1.2A). Its distribution in Europe is clearly biased towards the central and northern part of the continent and this species is virtually absent from southern Europe. This pattern of distribution can be explained by the factors determining its distribution in the Czech Republic (Pysek et al., 1998) where January temperatures and human population density most significantly affect the abundance of this species. The spread of H. mantegazzianum was fast in regions with a high human population density and limited in warmer areas with a relatively high mean January temperature (Pysek et al., 1998). Assessing the relative importance of these factors reveals that the effect of population density is stronger than that of temperature.

The under-representation of Heracleum in regions with warm January temperatures has been interpreted in terms of the germination ecology of the species (Pysek et al., 1998). The breaking of dormancy in H. mantegazzianum requires chilling (Tiley et al., 1996; Moravcova et al., 2005), which is provided by a cold period in humid soil during winter. However, the results of recent studies (see Moravcova et al., Chapter 5, this volume; Krinke et al., 2005; Moravcova et al., 2005) suggest that even areas with higher January temperatures are sufficiently cold for the stratification and breaking of dormancy. It seems that the negative effect of warm winters may not only operate by negatively affecting germination. Low winter temperatures could, at least in part, explain why the species is absent or very rare in south-eastern parts of Europe (Hungary, Romania, Bulgaria etc.), while thriving in more northerly and temperate areas, e.g. British Isles, Denmark, Sweden, Germany (Nielsen et al., 2005). The distribution of this species is unlikely to be limited by insufficient population density in southern Europe, so climatic constraints seem the most likely. Outside Europe, H. mantegazzianum is naturalized in Canada (Morton, 1978) and the USA (Kartesz and Meacham, 1999).

Heracleum sosnowskyi, originally described in 1944, was introduced into Europe as an agricultural crop and was silaged to provide fodder for livestock. Because the plant is hardy and thrives in a cold climate, it has been used as a crop in north and north-west Russia since its introduction in 1947. From the 1940s onwards, it was introduced as a crop to the Baltic countries (Latvia, Lithuania and Estonia) as well as to Belarus, Ukraine and the former German Democratic Republic. The history of planting is reflected in the species' current distribution (Fig. 1.2B). This practice was eventually abandoned in the Baltic States, partly because the anise-scented plants affected the flavour of meat and milk of the animals fed this fodder and also partly because of the health risk to humans and cattle. In parts of northern Russia, in contrast, it is still cultivated (Nielsen et al., 2005).

The history of invasion of H. persicum in Europe is the most obscure of the three species, partly because it was the first to be described (1829) and some of the subsequent records of H. persicum were probably of H. mantegazzianum or H. sosnowskyi. The only known wild populations of this plant in Europe are

Fig. 1.2. Distributions of the three main invasive Heracleum species in their invaded range in Europe: H. mantegazzianum (A), H. sosnowskyi (B) and H. persicum (C). Taken from Nielsen et al. (2005), updated. Distribution data for Norway and France are based on presence or absence at the county/department level. As a result the illustrated distribution in these areas may over-represent the actual distribution. Other countries are shaded in which the species is reported to occur but the exact distribution is unknown. Note that for Poland, the distribution data do not distinguish between H. mantegazzianum and H. sosnowskyi; the plotted distributions therefore relate to both species. The occurrence of H. persicum in the Czech Republic is based on a record in Holub (1997). Note that records of H. persicum in UK and Belgium are from cultivation.

Fig. 1.2. Distributions of the three main invasive Heracleum species in their invaded range in Europe: H. mantegazzianum (A), H. sosnowskyi (B) and H. persicum (C). Taken from Nielsen et al. (2005), updated. Distribution data for Norway and France are based on presence or absence at the county/department level. As a result the illustrated distribution in these areas may over-represent the actual distribution. Other countries are shaded in which the species is reported to occur but the exact distribution is unknown. Note that for Poland, the distribution data do not distinguish between H. mantegazzianum and H. sosnowskyi; the plotted distributions therefore relate to both species. The occurrence of H. persicum in the Czech Republic is based on a record in Holub (1997). Note that records of H. persicum in UK and Belgium are from cultivation.

in Fennoscandia (Fig. 1.2C). The earliest record of introduction again comes from the seed list of the Kew Botanic Gardens in London, where material under the name of H. laciniatum was received in 1819. Seeds from London populations of a similar plant were taken by English horticulturalists and planted in northern Norway as early as 1836 (Christy, 1837; see further below). The main mechanism of spread of this species again appears to be as an ornamental.

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