Sinkers and floaters – vive la difference
There are several well-known ecological and
physiological features that we might list in connection with a plant's ability
to become a successful invader. However, it is doubtful whether seed buoyancy
would be included in our lists if we were considering terrestrial plants. Such
blinkered thinking is challenged by the work of Fumanal
et al., Dijon, France (pp. 305–313), which suggests strongly that this feature contributes much to the
success of Ambrosia artemisiifolia as an
invader in France.
This ruderal species arrived in Europe from the USA
about 100 years ago but is still extending its range. For ruderal
species, the ability to reach and to establish quickly on disturbed land is
likely to be important and for this reason the authors have focussed
on achenes, the main units of dispersal. They found achene mass to vary two-fold; further, many of the lighter achenes were also buoyant and remained afloat in agitated
water. Although most of these floating achenes had
sunk after about 24 hours, achenes from one
population remained afloat for up to 90 hours. However, buoyancy for only a few
hours is long enough for the achenes to float
considerable distances in flowing water, thereby enhancing the probability of 'landfall'
on an exposed bank. Other features associated with buoyancy included deeper
dormancy than in non-floating achenes, although after
dormancy breakage they germinated faster than non-buoyant achenes
(albeit with lower germination percentages). Further, despite their smaller
size, buoyant achenes produced taller plants with
greater shoot dry mass and higher relative growth rates. This variability in
seed mass, dormancy, germinability and subsequent
plant growth helps explain the propensity for A. artemisiifolia
to colonize a range of disturbed habitats, with the ability to float (hydrochory) facilitating long-distance dispersal via rivers
such as the Dordogne, Loire and Rhone.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
