Ecologia mediterranea 1999-25(2)

Abd El-Ghani Soil variables affecting the vegetation ofinland western desert ofEgypt
INTRODUCTION
Desert vegetation in Egypt is by far the most important
and characteristic type of the natural plant life.
It covers vast areas and is formed mainly of xero­
phytic shrubs and subshrubs. Monod (1954) recog­
nised two types of desert vegetation, namely
contracted and diffuse. Both types refer to permanent
vegetation which can be accompanied by ephemeral
(or annual) plant growth depending on the amount of
precipitation in a given year. Kassas (1966, 1971)
added a third type "accidental vegetation" where pre­
cipitation is so low and falls so irregularly that no
permanent vegetation exists. It occurs mainly as con­
tracted patches in runnels, shallow depressions, hol­
lows, wadis and on old dunes with coarse sand.
Accidental vegetation consists of species which are
able to perform an annual life cycle: potential annuals
(sensu Haines, 1951), or potential perennials (sensu
Bornkamm, 1987), but can likewise continue growing
as long as water persists in the soil. Thomas (1988)
identified these plants as those with episodic growth
strategies linked to immediate water availability. Re­
cently, Springuel (1997) classified the accidental
vegetation in south eastern Egypt into three groups:
run-off dependent vegetation in the main wadi chan­
nels, run-on dependent vegetation of playa formation,
and rain dependent vegetation on levelled plains of
sand sheets.
In a survey of the vegetation units in the Western
Desert of Egypt, outside the Oases, Bornkamm and
Kehl (1990) distinguished five desert zones along a
precipitation gradient. Besides the well known
semidesert and full desert zones in the very north,
three zones of extreme desert show a significant differentiation
(Figure I). Both extreme desert zones III
and IV support the growth of accidental vegetation,
where the precipitation in the former amounts to 5-10
(20) mm year I whereas in the latter is 1-5 mm year-I.
On the other hand, extreme desert V in the very south
is practically void of vegetation where precipitation is
proposed to be less than 1 mm year-I. Typical acci­
dental vegetation types in the Western Desert of Egypt
are: Zygophyllum coccineum with Salsola imbricata
subsp. imbricata, Stipagrostis acutiflora with Zilla
spinosa as well as stands of Salsola imbricata subsp.
imbricata with Fagonia arabica. However, ground­
water-dependent vegetation in all the three extreme
desert zones exists too: Zahran (1972) and Abd EI-
174
Ghani (1981, 1985) in large oases (Siwa, Bahariya and
Farafra), and in small oases and depressions (Bir Saf­
saf, Bir EI-Shab, Bir Tarfawi and Qara): EI-Hadidi
(l980b), Bornkamm (1986), Abd EI-Ghani (1992).
Although our knowledge of the growth of acci­
dental vegetation in Egypt has considerably increased
during the last two decades (Alaily et aI., 1987;
Bornkamm, 1987; Springuel et aI., 1990), much less is
known about this vegetation in quantitative terms. The
present study aims at describing the floristic composi­
tion of the accidental type of vegetation growing in
parts of the Western Desert of Egypt and analysing the
distribution of species in relation to certain environ­
mental factors by applying the muItivariate analysis
techniques.
STUDY AREA
The present study has been conducted in two con­
secutive extreme desert zones (sensu Bornkamm &
Kehl, 1990), where the accidental type of vegetation
exists. Data is from two transects: the northern one
extends for a distance of about 150 km; half-way
along Siwa Oasis-Mersa Matruh road, and represents
the extreme desert zone III (Figure 1). This transect is
principally located in the inland part of the Middle
Miocene plateau that rises to about 100 m above the
depression floor (reaches 20 m below sea level). The
southern transect extends for a distance of about 140
km, along the Dakhla-Farafra road and represents the
extreme desert zone IV. It is located in the middle
limestone plateau (500 m above sea level). The north­
ern transect lies in the Libyan Desert while the southern
one in the Nubian Desert (EI-Hadidi, 1980a). In
general, the landscape of the northern transect is a part
of the Central Sahara, whereas the southern transect is
a part of the Southern Sahara (Schiffers, 1971).
According to WaIter and Breckle (1984) the study
area lies in the zone of subtropical arid deserts. The
temperature regime is characterised by mild winters
and very hot summers. Whereas average January tem­
perature remains rather constant between 12°C and
14°C, the July mean rises to approximately 31°C. The
absolute maxima of the southern region of the study
area may reach 49°C. Precipitation is erratic, variable
and unpredictable with frequent long dry periods. Zahran
and Willis (1992) reported that the mean annual
rainfall ranges from 9.6 mm year -1 in Siwa Oasis (the
nearest station to the northern transect) to nearly I mm
ecologia mediterranea 25 (2) - 1999