The situation of Humulus japonicus in France
Humulus japonicus (= H. scandens) (Cannabaceae) is an annual climber vine originating from East Asia. In Europe, it is only recorded in France, Hungary and Italy, and this species is registered on the EPPO Alert List.
In France, Humulus japonicus was observed as naturalized in Russan (municipality of Sainte-Anastasie) in the Gard Department in September 2004 in a degraded riparian habitat near the Gard River. It is thought to have escaped from a garden. The Conservatoire Botanique Méditerranéen de Porquerolles found the species 2 km upstream (in the municipality of Dions) on the Gardon which is tributary to the Gard River, and several kilometres downstream. In all these locations, the species colonizes dense nitrophilous fallowlands composed of high grasses.
In October 2009, surveys have been done on the location in Russan, highlighting different levels of invasion:
- in a stand of 140 m², the species covered 100% of the stand;
- in a stand of 220 m², H. japonicus covered 80% of the stand, where the diversity ranged from 0 to 3 plants species per m², the other two species being the nitrophilous species Chenopodium album or the invasive species Artemisia verlotiorum. In comparison, non invaded neighbouring areas contained 8 to 10 plant species per m²;
- in another stand, plants of H. japonicus were dispersed, emerging from a thick litter made of the aerial parts of the plant in the previous years;
- in a stand of 100 m², the plant only covered 10% and seemed to be outcompeted by Parthenocissus inserta.
H. japonicus has exhibited a high competitive ability, outcompeting tall species such as Sorghum halepense or even Arundo donax. Nevertheless, the invaded communities do not present a high floristic interest as they are mostly composed of ruderal and nitrophilous species (Atriplex prostrata, Chenopodium album, Galium aparine, Torilis arvensis), or other invasive species (Ambrosia artemisiifolia - EPPO List of Invasive Alien Plants, Artemisia annua, Artemisia verlotiorum, Helianthus tuberosus - EPPO List of IAP, Parthenocissus inserta). However, although being annual, H. japonicus forms an important litter that can modify the subtrate for many years. The potential impact of this species on riparian habitats should therefore not be overlooked.
Personal communication with Guillaume Fried, LNPV Station de Montpellier, SupAgro (firstname.lastname@example.org)