Introduction

Reproduction is the most important event in a plant's life cycle (Crawley, 1997). This is especially true for monocarpic plants, which reproduce only once in their lifetime, as is the case of Heracleum mantegazzianum Sommier & Levier. This species reproduces only by seed; reproduction by vegetative means has never been observed.

As in other Apiaceae, H. mantegazzianum has unspecialized flowers, which are promiscuously pollinated by unspecialized pollinators. Many small, closely spaced flowers with exposed nectar make each insect visitor to the inflorescence a potential and probable pollinator (Bell, 1971). A list of insect taxa sampled on H. mantegazzianum (Grace and Nelson, 1981) shows that Coleoptera, Diptera, Hemiptera and Hymenoptera are the most frequent visitors.

Heracleum mantegazzianum has an andromonoecious sex habit, as has almost half of British Apiaceae (Lovett-Doust and Lovett-Doust, 1982); together with perfect (hermaphrodite) flowers, umbels bear a variable proportion of male (staminate) flowers. The species is considered to be self-compatible, which is a typical feature of Apiaceae (Bell, 1971), and protandrous (Grace and Nelson, 1981; Perglova et al., 2006). Protandry is a temporal separation of male and female flowering phases, when stigmas become receptive after the dehiscence of anthers. It is common in umbellifers. Where dichogamy is known, 40% of umbellifers are usually protandrous, compared to only about 11% of all dicotyledons (Lovett-Doust and Lovett-Doust, 1982). Although protandry has traditionally been considered to be a mechanism of avoiding or reducing selfing, it is itself unlikely to guarantee outcrossing. However, when it is strongly developed, the male and female phases of a plant may be completely separated in time so that outcrossing is assured (Webb, 1981; Snow and Grove, 1995).

© CAB International 2007. Ecology and Management of Giant Hogweed (Heracleum mantegazzianum,). (eds P. Pysek, M.J.W. Cock, W. Nentwig and H.P. Ravn)

Interspecific hybrids between H. mantegazzianum and H. sphondylium L. are reported from Great Britain (McClintock, 1975) and Germany (Ochsmann, 1996). Hybrids are found in sites where both species grow together, although they are not numerous (Grace and Nelson, 1981; Stewart and Grace, 1984). This was studied and it was found that only experimental crosses in which H. sphondylium was the female parent were successful (Stewart and Grace, 1984).

The aim of this chapter is to summarize the knowledge about the reproductive ecology of H. mantegazzianum, including the results obtained during the triennial European project GIANT ALIEN. During this project, an extensive study of flowering phenology and seed production of wild populations of H. mantegazzianum was conducted at ten localities in the Slavkovsky les Protected Landscape Area in the Czech Republic (Fig. 4.1, Table 4.1). This region is where the species was first introduced to this country in the second half of the 19th century and from where it started to spread (Pysek, 1991; see Pysek et al., Chapter 3, this volume). The rapid spread was probably facilitated by the fact that after World War II inhabitants were displaced and part of the region became a military area until the 1960s. This led to a lack of appropriate management and a specific disturbance regime; military activities are rather specific in that they occur in 'natural' parts of the landscape, which are less affected under standard

Fig. 4.1. The study area in the Slavkovsky les Protected Landscape Area, which is the region where H. mantegazzianum was first introduced into the Czech Republic. Currently the region is still heavily infested. Most of the region is in the Ore Mountains and is formed from granite. Total size of the protected area is 617 km2, altitudinal range is 373-983 m a.s.l. (Kos and Marsakova, 1997), January temperature ranges from -5.1°C (average mimimum) to -0.2°C (average maximum), July temperature from 10.5 to 21.5°C, respectively. Annual sum of precipitation is 1094 mm (Marianske Lazne meteorological station, 50-year average).

Fig. 4.1. The study area in the Slavkovsky les Protected Landscape Area, which is the region where H. mantegazzianum was first introduced into the Czech Republic. Currently the region is still heavily infested. Most of the region is in the Ore Mountains and is formed from granite. Total size of the protected area is 617 km2, altitudinal range is 373-983 m a.s.l. (Kos and Marsakova, 1997), January temperature ranges from -5.1°C (average mimimum) to -0.2°C (average maximum), July temperature from 10.5 to 21.5°C, respectively. Annual sum of precipitation is 1094 mm (Marianske Lazne meteorological station, 50-year average).

Table 4.1. Geographical location, altitude (m a.s.l.) and population size of H. mantegazzianum estimated from aerial photographs taken at ten study sites in the Slavkovsky les Protected Landscape Area, Czech Republic. Estimates were made for 60 ha sections of landscape (taken from Mullerova et al., 2005).

Table 4.1. Geographical location, altitude (m a.s.l.) and population size of H. mantegazzianum estimated from aerial photographs taken at ten study sites in the Slavkovsky les Protected Landscape Area, Czech Republic. Estimates were made for 60 ha sections of landscape (taken from Mullerova et al., 2005).

Site no.

Name

Latitude

Longitude

Altitude (m)

Population (m2)

3

Zitny I

50°03.754'

12°37.569'

787

99,121

6

Liskovec

49°59.156'

12°38.721'

541

8174

8

Potok

50°04.660'

12°35.953'

643

39,774

9

Dvorecrky

50°05.982'

12°34.137'

506

24,817

11

Arnoltov

50°06.801'

12°36.147'

575

47,170

12

Krasna Lipa I

50°05.685'

12°38.546'

597

-

13

Litrbachy

50°06.009'

12°43.777'

800

4711

14

Rajov

49°59.704'

12°54.933'

753

5198

15

Krasna Lipa II

50°06.306'

12°38.393'

596

7945

16

Zitny II

50°03.837'

12°37.304'

734

-

land use. There are still very few people in this protected landscape, which consists mainly of extensive wetlands, pastures and spruce plantations. Nowadays, the area of the Slavkovsky les is still invaded to a large extent (see Pysek et al., Chapter 3, this volume; Mullerova et al., 2005). The field study was complemented by detailed studies of flowering phenology and selfing in the experimental garden of the Institute of Botany, Pruhonice, Czech Republic (50°0.071' N, 14°33.5281' E; 310 m a.s.l.). Furthermore, information on the age at which H. mantegazzianum reproduces, gathered in both its native (Western Greater Caucasus) and invaded (Czech Republic) areas, is presented.

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