Science Facing Aliens - Invasive Alien Species in Belgium - Belgian ...

Comparing Fallopia japonica, F. sachalinensis and their hybrid F.
xbohemica in Belgium: population ecology, functional traits and
invasiveness
Basile HERPIGNY 1, 2 , Grégory MAHY 3 & Pierre MEERTS 1
1 Laboratoire d’Ecologie Végétale et Biogéochimie, Université Libre de Bruxelles, Belgium.
2 Corresponding author ; e-mail : bherpign@ulb.ac.be
3 Laboratoire d’Ecologie, Faculté Agronomique de Gembloux, Belgium
Introduction
In the course of the nineteenth century two Fallopia (knotweed) species were introduced to
Europe from Asia. F. japonica (2n=88) has become one of the most proliferous invasive
plants in Europe (Beerling et al, 1994). F. sachalinensis (2n=44) is much less invasive and
still rare in Western Europe. Though F. japonica is clonal and male sterile in Europe, it can be
pollinated by F. sachalinensis. This cross produces F. ×bohemica, a hybrid that is said to
spread even more rapidly than its parents (Bailey & Wisskirchen, 2006; Gammon et al, 2007;
Mandak et al, 2004). The biological reasons for this high invasiveness are unknown. The
physiology of the hybrid has been less studied than the physiology of its parents (Adachi et al,
1996; Marigo & Pautou, 1998; Price et al, 2001), but the cause could be genetic. Recent data
(Tiébré et al, 2007) show that the hybrid is represented in Belgium (and other European
countries: Mandak et al, 2003) by at least three cytotypes (2n= 44, 66, 88) and that it could be
more variable than its parents. It is possible that the hybridisation in the japonicasachalinensis
complex is a factor promoting quick evolution. Hybridisation and
polyploidisation are well-known evolutionary mechanisms and their importance in the
evolution of invasiveness has been shown in other polyploidy complexes (Lee, 2002;
Ainouche et al, 2003).
Our objective is to test if the three taxa have contrasting values of key functional traits
that might explain their contrasting invasiveness. We also examine if the hybrid is
intermediate between its parents or, alternatively, if it shows transgressive variation in some
traits.
Study outline and preliminary results
In the course of 2008, we monitored, from April to August, the following traits in populations
from six sites where two or three taxa coexist in sympatry: shoot height, number of leaves and
ramifications (secondary and tertiary), number and length of internodes, leaf area. That the
taxa coexist allows us to assume that phenotypic variation originates solely from genetic
differences and is not environmentally induced. The first results reveal interesting differences
among the three taxa. F. ×bohemica is intermediate between its parents for height (fig 1),
number of internodes and ramifications, length of the internodes, and leaf area. It shows
transgressive variation for number of leaves (Figure 2), outperforming both parental taxa.
Other functional traits were measured once or twice during the year, on populations of
the three taxa in a site called “Verrewinkel graveyard” (Uccle, Brussels). Specific leaf area
(SLA) was measured during biomass peak (July). Leaf and shoot water, nitrogen and carbon
content were measured during biomass peak and leaf senescence (October). Results show that
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