Ecologia mediterranea 1999-25(2)

Abd El-Ghani Soil variables affecting the vegetation ofinland western desert ofEgypt
The relative advantage of shrubs over grasses when
water is limiting, as in the study area, can be explained
by their extensive root systems which are capable to
utilise water stored in different soil depths, whereas
grasses utilise the transient water stored in the upper
soil synchronic with precipitation pulses. The upper
dry layer of the surface deposits acts as a protective
layer, moisture is stored in subsurface layers, and the
underlying sandstone provides added water storage
capacity. The presence of a subsurface layer that is
permanently wet is well-known phenomenon in the
Egyptian Deserts (Kassas & Batanouny, 1984). As
presented in the results, the dominance of shrubby
plant species over the grasses is evident.
Chamaephytes constitute 40% of the floristic compo­
sition, followed by therophytes. The dominance of
both chamaephytes and therophytes over other life
forms seem to be a response to the hot dry climate and
human and animal interferences. A comparison of the
life-form spectrum of the same 5° of the northern
latitude in the corresponding Eastern Desert of Egypt
(25°N - 30 0
N), Abd EI-Ghani (1998) showed more
therophytes (38.3%) and hemicryptophytes (22.0%)
and less chamaephytes (29.0%).
The vegetation that characterise the study area can
be divided, according to TWINSPAN technique, into
eight vegetation groups: Prosopis farcta-Tamarix nilotica,
Tamarix nilotica-Alhagi graecorum, Zygo­
phyllum coccineum-Salsola imbricata subsp.
imbricata, Cornulaca monacantha-Fagonia arabica,
Atriplex leucoclada, Randonia africana-Deverra tor­
tuosa, Deverra tortuosa-Capparis spinosa var. aegyp­
tia and Capparis spinosa var. aegyptia-Zilla spinosa
subsp. biparmata. Some species: Atriplex leucoclada,
Anabasis articulata, Capparis spinosa var. aegyptia
and Randonia africana common to the western Medi­
terranean coastal belt (Ayyad & Ammar, 1974; Abdel­
Razik et aI., 1984) are found in the less arid sites of
the northern transect where the silty soil decreases
water infiltration. A group of salt-tolerant plants in­
cluding Nitraria retusa, Tamarix nilotica, Sarcocornia
fruticosa and Alhagi graecorum are found in the dry
saline sites of the southern transect, and form phyto­
genic mounds of variable size. Alhagi graecorum is a
widely distributed species that seems to grow in dif­
ferent habitats (Kassas, 1952). It is also considered as
a groundwater-indicating plant (Girgis, 1972). According
to Kassas and Girgis (1965), the growth of the
desert scrub Nitraria retusa represents the highest tol-
182
erance to soil salinity conditions, and a penultimate
stage in the successional development. The plant
reaches its northernmost limit of distribution around
Qara Oasis on the south-western edge of Qattara Depression
(Abd EI-Ghani, 1992) as well as in Bahariya
Oasis (Abd EI-Ghani, 1981). Nitraria retusa, how­
ever, has not been recorded beyond Latitude 28°N in
Egypt (M. Kassas, pers. comm.). Further studies con­
cerning the distribution of this plant in the country is
recommended. Prosopis farcta, Imperata cylindrica
and Salsola imbricata subsp. imbricata are commonly
found in the dry sandy plains along the southern tran­
sect. Restriction of Imperata to the high sandy plains
is apparently due to the inability of this species to
reach the capillary fringe of the groundwater which is
fairly close to the surface (Rikli, 1943). The species is
considered as facultative halophyte mainly occurring
on sandy soil with slight salt content. Thus, this habi­
tat may represent a transitional zone between the less
arid and dry saline habitats. The xero-psammophytes
Fagonia arabica, Cornulaca monacantha, Zilla spi­
nosa subsp. spinosa, Calligonum polygonoides subsp.
comosum, Pulicaria incisa, Citrullus colocynthis and
Heliotropium digynum were found in dry non-saline
sandy sites along both transects where infiltration is
higher and water accumulated in deeper layers, and
the soil is highly fertile. This group of species are
more widely distributed in Egypt and neighbouring
countries (Batanouny, 1979; Zahran & Willis, 1992;
Frankenberg & Klaus, 1980; Wojterski, 1985).
Ayyad (1976) pointed out that physiographic and
edaphic factors have greatly affected the distribution
of plant communities in the Western Desert of Egypt.
DCA and CCA analyses of the vegetation and soil
data in the present study indicated the relative posi­
tions of species and sites along the most important
ecological gradients. DCA axis 1 may represent a
geographical trend in the floristic data set. The main
reason for this may be a gradient in the local harsh
climate within the study area. Both ordination tech­
niques emphasised that salinity, fine sediments, or­
ganic matter and moisture content are the significant
factors controlling the distribution of the vegetation in
this region. This has been reported in other studies
such as those of EI-Ghareeb and Hassan (1989), El­
Demerdash et al. (1995) and Shaltout et aI., (1997).
The soil texture gradient in arid desert environments
results in a corresponding gradient of available soil
moisture. Therefore, moisture content is probably one
ecologia mediterranea 25 (2) - 1999