The ecology of the Caucasian herbaceous perennial Heracleum mantegazzianum Somm. et Lev. (Giant Hogweed) in cultural ecosystems of Central Europe
Otte, Annette; Franke, René
Heracleum mantegazzianum Somm. et Lev. (Giant Hogweed) is a herbaceous perennial from the subalpine zone of the western Caucasus Mountains. It was first introduced to Central Europe around 1850 as a horticultural plant, and has subsequently attained a wide distribution through wildling spread (ergasiophygophyte). Heracleum has firmly established itself in Artemisietea phytosociological communities (nitrophytic herbaceous perennial communities), on fresh to moist sites with good to very good nutrient levels, in lowland to submontane zones (epecophyte and agriophyte). These sites include those formerly used agriculturally or horticulturally, eutrophied forest or copse edge ecotones, as well as extensively or no longer managed road or railway verges. These are all sites where nutrient levels have been greatly altered or where eutrophication has occurred, through anthropogenic influence. The documentation of the phenological development of a Heracleum stand on a river floodplain site, near the city of Giessen in the lowland zone of Central Hesse, has shown that the species is at least biennial, usually however triennial to perennial. Heracleum mantegazzianum spreads exclusively in a generative fashion by seeds, individually contained within fruiting bodies (mericarps), which are almost always found within a 250 cm radius around the senesced mother plant (passive autochory). The diaspores germinate predominantly in early spring (flowering period of Tussilago farfara), germination being possible only after the maturation of the embryo during a two to three month cold period. By the end of the growth period, only a few individuals have developed from the several thousand germinated seedlings, namely those which have accumulated sufficient nutrient reserves in the roots. This enables Heracleum mantegazzianum to develop a new vegetative body quickly and independently of weather conditions during the following growth period. Heracleum mantegazzianum has the property of being able to draw the vegetation point in the hypocotyl up to 10 cm into the soil by vertical contraction of the central tap-root. This property facilitates rapid sprouting in spring. Heracleum belongs to those perennial species with a juvenile phase, flowering and bearing fruit only once before senescing. This senescence is caused by the depletion of nutrient and energy reserves, the reproductive organs competing with the vegetative organs for the remaining nutrients. During the flowering and seed ripening phase, the vegetative structures shrivel up and the energy of the carbohydrates in the roots is exhausted. Regenerative growth is possible during the vegetative growth phase, because the plant is able to draw on considerable nutrient reserves in the roots. The regenerative capacity during the generative growth phase is restricted however; only those plants disturbed before full flowering are able to develop new inflorescences with lesser flower and seed quantities.