Revue internationale d'écologie méditerranéenne International ...

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◆ M. A. SHEIKH, M. E. MADY & K. H. SHALTOUT
1973; Danin, 1983). Therefore, there is a high species
richness, especially in annual plants, and this pattern is the
product of relatively rapid development of plants under
stress conditions induced by drought (Naveh & Whittaker,
1979; Kadmon & Danin, 1999).
The Arthrocnemum macrostachyum and Mesembryanthemum
crystallinum units which inhabit the saline humid
habitat (sand sheets and bank of mouth pond) have low
species diversity and high values of salinity and cations. The
ensuing spatial variation in soil moisture has significant
effects on diversity and production in arid zones. The
most humid sites have the highest shrub abundance and
the lowest diversities, and the most xeric sites have the
lowest shrub abundance and the highest diversities. The
over-all trend is one of increasing diversity with increasing
drought (see Noy-Meir, 1973; Naveh & Whittaker, 1979).
The halophytic species growing in this habitat have high
internal osmotic potentials and efficient mechanisms for
salt uptake, transport and secretion (Noy-Meir, 1973).
The salinity is the stress factor for growth of many species
and hence relatively poor vegetation. A similar conclusion
was depicted by Shaltout and El-Ghareeb (1992) in their
study on salt marsh plant communities in the western
Mediterranean region of Egypt.
The Alhagi graecorum - Xanthium spinosum unit that
inhabits the wadi bed had the highest fine texture, carbonate
and low vegetation cover. The high percentage of
clay and carbonates often leads to impermeable layers in
the soil profile, which modify the vertical distribution by
causing accumulation of water above them. However, in
hard rocks and calcified horizons there are enough cracks
to allow (or even facilitate) the passage of water and roots,
thus seemingly shallow soils on such substrates in arid
zones may in fact be deep soils ecologically (Noy-Meir,
1973). The fact that fine texture in Wadi bed is common
results from their depositions from the soil surface of
the wadi terrace and slope by the wind action. These
fine deposit reduce the evaporation of water and create
favorable microsites for support many plant species. The
dominance of Tamarix in the Tamarix nilotica – Arundo
donax unit (inhabiting the bank of the mouth pond) is
caused by the elevation of the wadi banks, allowing for an
accumulation of underground waters near the banks (see
Noy-Meir, 1973; Zohary, 1973; Kadmon & Danin, 1999).
The present study indicates that the Tamarix nilotica unit
has no clear distribution. This could be interpreted in the
view of the wide ecological amplitude of Tamarix (it grows
on both saline and non saline soils) and/or its interspecific
heterogeneity (see Zohary, 1973; Baum, 1978; Batanouny,
1994; Springuel, 1997).
Correlation analysis in the present study indicates that
the species diversity (relative evenness) increases with the
increase of total cover. Similar results were reported in Kutiel
and Danin (1987). In their study, Naveh and Whittaker
(1979) obtained the same correlation in open woodlands. In
such cases, the woodland with its mosaic of light and partial
shade, relatively abundant light for the undergrowth, and less
dense occupation of soil by tree roots than in forest, offers
a great range of resources that may be divided among plant
niches as a basis of understorey diversity. Shaltout and El-
Ghareeb (1992) noted an inverse relation between diversity
and productivity. The existing literature contains conflicting
evidence on the relationship between species diversity and
phytomass (or plant cover). The direction of correlation
between both community properties appears to depend on
the position of a given stand on the production scale (Grime,
1979; Kutiel & Danin, 1987). On the other hand, concentration
of dominance (Simpson index) correlated positively
with nitrates (fertility). Therefore the diversity decreases
as the fertility increases. However, when the environment
becomes more favorable, competitive exclusion occurs and
therefore the number of species is reduced. Similar conclusions
were attained by Grime (1979) and Tilman (1982).
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ecologia mediterranea, tome 29, fascicule 2, 2003