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CLAUDE J. GRENOT
populations of Arthrophytum. However, after a rain one can see in the
rocky debris a group characterized by annual species; the group of Ammosperma cinereum and Volutaria saharae. (2) In the northwestern
Sahara, two parts are equally distinct: in the northern region a new type
of vegetation that develops in the beds of the sandy or non-salt-type rock
wadis and where the tropical infiltrations are already apparent (important
populations of Acacia raddiana) (see Fig. 16). Several associations can
be distinguished: Acacia raddiana-Ziziphus lotus, Acacia raddiana-Rhus
tripartitum, and Acacia raddiana-Panicum turgidum-Foleyola billotii.
However, on the hamadas and rock work, other types of vegetable groups,
remarkable for their richness in endemics, form a uniform phytosociologic
complex (Lemée, 1953; Guinet and Sauvage, in Joly et al, 1954; Guinet,
1958; Quézel and Simonneau, 1960). One can distinguish an association
of Anabasis aretioides, where the pseudosteppe of Anabasis aretioides develops so well on the surface of the hamadas, and an association of
Whitania adpressa-Linaria sagittata var. heterophylla. A meridional region: the increasing of the aridity of the climate toward the south causes
a considerable decrease in the flora and new types of vegetation appear.
The association Plantago ciliata-Ormeonis lonadioides (characteristic of
the dayas and hamadas of the Saoura, Guir, South-marocan, and
Tademait) represent an aspect of the increasing poverty of the groups of
the beds of the wadis of Acacia. The associations of Enartrophyton chevallier i-Tourneuxia variifolia colonize the inclined rock work of the hamadas.
The steppes of Salsolaceae live parallel to those which are individualized
in the northeastern region of the northern Sahara.
c. The Central Sahara. This region is provided with vegetation that is
well known, thanks to the works of Gram (1935) on Mouydir, of Maire
(1933, 1940) on the Saharan central massif, of Maire and Monod (1950)
on Tibesti, of Leredde (1957) on Tassili, and of Quézel (1954a,b, 1956,
1958) on the Hoggar, the Tefedest, and the Tibesti.
This sector (the high mountains excluded) is characterized by the development of a flora of sahelian origin, of which Acacia-Panicum of the
desert savannah colonizes the wadis of the massifs. On the contrary, a
vegetation of Saharo-sindian predominates on the sandy and rocky substrates outside the beds of the wadis.
d. High Mountain. A high mountain Saharan vegetation particularly rich
in endemic elements extends up about 1800 meters. The vegetation then
becomes diffuse. The characteristic trees of the lower regions disappear
and are replaced by other types of trees: Olea laperrini, Myrtus nivellei,
and Erhetia obtusifolia. A steppe of chamephytes (sage brush and
Ephedrd) appears most often at the higher summits. Also present are
Acacia seyal and A. laeta.
CLAUDE J. GRENOT
populations of Arthrophytum. However, after a rain one can see in the
rocky debris a group characterized by annual species; the group of Ammosperma cinereum and Volutaria saharae. (2) In the northwestern
Sahara, two parts are equally distinct: in the northern region a new type
of vegetation that develops in the beds of the sandy or non-salt-type rock
wadis and where the tropical infiltrations are already apparent (important
populations of Acacia raddiana) (see Fig. 16). Several associations can
be distinguished: Acacia raddiana-Ziziphus lotus, Acacia raddiana-Rhus
tripartitum, and Acacia raddiana-Panicum turgidum-Foleyola billotii.
However, on the hamadas and rock work, other types of vegetable groups,
remarkable for their richness in endemics, form a uniform phytosociologic
complex (Lemée, 1953; Guinet and Sauvage, in Joly et al, 1954; Guinet,
1958; Quézel and Simonneau, 1960). One can distinguish an association
of Anabasis aretioides, where the pseudosteppe of Anabasis aretioides develops so well on the surface of the hamadas, and an association of
Whitania adpressa-Linaria sagittata var. heterophylla. A meridional region: the increasing of the aridity of the climate toward the south causes
a considerable decrease in the flora and new types of vegetation appear.
The association Plantago ciliata-Ormeonis lonadioides (characteristic of
the dayas and hamadas of the Saoura, Guir, South-marocan, and
Tademait) represent an aspect of the increasing poverty of the groups of
the beds of the wadis of Acacia. The associations of Enartrophyton chevallier i-Tourneuxia variifolia colonize the inclined rock work of the hamadas.
The steppes of Salsolaceae live parallel to those which are individualized
in the northeastern region of the northern Sahara.
c. The Central Sahara. This region is provided with vegetation that is
well known, thanks to the works of Gram (1935) on Mouydir, of Maire
(1933, 1940) on the Saharan central massif, of Maire and Monod (1950)
on Tibesti, of Leredde (1957) on Tassili, and of Quézel (1954a,b, 1956,
1958) on the Hoggar, the Tefedest, and the Tibesti.
This sector (the high mountains excluded) is characterized by the development of a flora of sahelian origin, of which Acacia-Panicum of the
desert savannah colonizes the wadis of the massifs. On the contrary, a
vegetation of Saharo-sindian predominates on the sandy and rocky substrates outside the beds of the wadis.
d. High Mountain. A high mountain Saharan vegetation particularly rich
in endemic elements extends up about 1800 meters. The vegetation then
becomes diffuse. The characteristic trees of the lower regions disappear
and are replaced by other types of trees: Olea laperrini, Myrtus nivellei,
and Erhetia obtusifolia. A steppe of chamephytes (sage brush and
Ephedrd) appears most often at the higher summits. Also present are
Acacia seyal and A. laeta.