Revue internationale d'écologie méditerranéenne International ...

SAMI YOUSSEF, ERROL VELA, ALEX BAUMEL, THIERRY TATONI
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Introduction
Plant communities are complex assemblages
of species filtered by the environment from
the species pool available after historical
events. In the Mediterranean region, the
mountain top environments impose both harsh
summer drought and cold winter condition
that severely limit plant reproduction and
growth (Cavieres et al. 2005, 2007; Ramirez
et al. 2006; Giménez-Benavides et al. 2007).
In these extreme conditions, forest establishment
is prevented by high constraints and
chamaephytous plant species find their “ecological
niche” in low competitive habitats
(Mota et al. 1991; Crawford 2008). The natural
landscape is thus a mosaic of plant communities
with low-density of high shrubs,
thorny scrubs, grasslands and cushion-like
communities (Davis 1951; Quézel 1971; Barbero
1972; Barbero et al. 1972; Loisel 1976;
Barbero & Bonin 1980; Nimis 1981; Penas et
al. 2001; Crawford 2008). Among the highest
Mediterranean mountains, characterized by
sharp altitudinal and ecological gradient, the
Sierra Nevada, the Maritime and Ligurian
Alps or the Corsica mountains shelter numerous
endemic species (Verlaque et al. 1995;
Médail & Quézel 1997; Blanca et al. 1999;
Casazza et al. 2008).
Besides the high and large mountains, the
Mediterranean basin also possesses numerous
hills of modest elevation. They are isolated or
connected to higher belt of mounts such as
Alps or Pyrénnées in France and have an
important proportion of cliff, rupicolous or
more generally open rocky habitats where
narrow endemics may also be encountered. As
reported by different authors (Médail & Verlaque
1997; Lavergne et al. 2004) and underlined
by Thompson (2005), the occurrence of
Mediterranean endemics in open, rocky habitats
with low competition pressures could be
both related to the historical isolation necessary
for differentiation from widespread congeners,
but also to the stability of such ecosystem
promoting persistence of endemic plants.
A good illustration of an ecosystem with
endemics or rare plants are the calcareous
hills of the southern part of the “Provence”,
called “Basse Provence” (S-E France).
Although this region is affected by important
changes, combination of urbanisation (Aix-
Marseilles urban area), land abandonment and
habitat fragmentation (Tatoni et al. 2004;
Tatoni 2007; Dumas et al. 2008), very few
numerical analysis of its floristic variability
have been done. In two recent papers, the distribution
and ecology of Arenaria provincialis
Chater & Halliday, a small winter annual
Caryophyllaceae restricted to the calcareous
hills surrounding the city of Marseilles has
been reported (Véla et al. 2008; Baumel et
al. 2009). Ranging along a wide altitudinal
range, A. provincialis occurs principally in
open patches where there is little competition
but in association with different plant communities.
In the same geographical and landscape context,
this paper deals with the distribution and
associated floristic variability of Genista
lobelii DC. (Fabaceae) a rare thorny cushion
perennial plant. In comparison to A. provincialis,
G. lobelii is less abundant, apparently
even more ecologically restricted, but is more
widely distributed on Bouches du Rhône and
Var counties. Considering the G. lobelii “fruticose
Chamaephytes” communities in Basse
Provence, Molinier (1934) described the
“Genistetum lobelii” association, restricted to
summits at 1000 m above level (a.s.l.) and
characterized by the dominance of G. lobelii
and others Mediterranean mountains plants
species such as Anthyllis montana L., Serratula
nudicaulis (L.) DC. and Valeriana
tuberose L. However, the habitats of G. lobelii
remain poorly described, for example Loisel
(1976) and Charles (2001) noticed that
G. lobelii belongs to a heterogeneous group
plant associations not restricted to Mediterranean
mountains. Indeed, Charles (2001)
presented three plant species, previously cited
by Molinier (1934), Scorzonera austriaca
Willd., Iberis saxatilis L. and Arenaria aggregata
(L.) Loisel. as representative of the
“Genistetum lobelii” association. Finally, we
note that these studies do not include all
G. lobelii stations along its distribution range
and fail to structure their floristic diversity due
to absence of numerical analysis. Our purpose
is then to update the geographical distribution
map of G. lobelii and to analyse the floristic
composition of its stations. We also set
hypothesis on the relationship between marginality
and population size of G. lobelii.
ecologia mediterranea – Vol. 36 (1) – 2010