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