Ecologia mediterranea 1999-25(2)

Abd EI-Ghani Soil variables affecting the vegetation ofinland western desert ofEgvpt
each), Zygophyllaceae (11.7%), Caryophyllaceae,
Compositae and Leguminosae (10.0% each), while the
other 14 families share 31.7%. Chamaephytes are the
most abundant life form and constituted 40.0% of the
total flora of the study area, followed by therophytes
(33.3%), hemicryptophytes (15.0%) and phanero­
phytes (11.7%). The most common perennials re­
corded were: Prosopis farcta, Tamarix nilotica,
Fagonia arabica, Zygophyllum coccineum, Salsola
imbricata subsp. imbicata, Cornulaca monacantha,
Alhagi graecorum, Atriplex leucoclada, Randonia af­
ricana, Deverra tortuosa and Capparis spinosa var.
aegyptia may be considered as leading dominants and
characteristic species. Each of these species attains a
maximum importance value (IV) of more than 140
(out of 300 for all species in a stand), and a mean of
more than 60. Common but less important perennials
are Phoenix dactylifera, Pulicaria crispa, Anabasis
articulata, Zilla spinosa subsp. spinosa, Stipagrostis
plumosa and Pulicaria incisa. Common annuals in­
clude: Trigonella stellata, Zygophyllum simplex, Co­
tula cinerea, Eremobium aegyptiacum, Schouwia
thebaica and Paronychia arabica subsp. arabica.
The 144 stands were classified into eight vegeta­
tion groups according to TWINSPAN technique (Fig­
ure 2). The first level of the dendrogram separates all
the stands into two main groups. The first group com­
prises 57 stands found mainly in the northern transect,
and the second comprises 87 stands sampled from
both transects. Table 3 summarises the mean values
and the standard deviations of the measured soil vari­
ables in the eight groups derived from TWINSPAN.
Generally, pH shows the least variation among
groups. It can also be noted that whereas levels of
lime and fine materials attain their highest values in
the groups of the northern transect, the organic matter
content reaches its highest levels in those of the south­
ern transect.
The identified vegetation groups are named after
the characteristic species as follows: Prosopis farcta­
Tamarix nilotica (lower part of the southern transect
in the vicinity of the lowlands of Dakhla Oasis);
Tamarix nilotica-Alhagi graecorum (southern tran­
sect, high salinity levels favour the growth of some
halophytic species, e.g. Nitraria retusa and Zygo­
phyllum album); Zygophyllum coccineum-Salsola imbricata
subsp. imbricata (in runnels and depressions
of the southern transect); Cornulaca monacantha­
Fagonia arabica (larger catchment areas of the north-
178
ern transect, some species of this group do not pene­
trate into other groups of the southern transect);
Atriplex leucoclada (lower part of the northern tran­
sect); Randonia africana-Deverra tortuosa (the silty
runnels, and occupying a distance of about 20 km of
the middle part of the northern transect); Deverra
tortuosa-Capparis spinosa var. aegyptia (occupying a
distance of about 30 km in the upper stretches of the
middle part of the northern transect); Capparis spi­
nosa var. aegyptia-Zilla spinosa subsp. biparmata (in
the low depressions between km 185 and km 198
along the lower part of the northern transect).
The four DCA axes explain 11.7%, 7.0%, 5.3%
and 3.4% of the total variation in the species data, re­
spectively. This low percentage of variance explained
by the axes is attributed to the many zero values in the
vegetation data set. Table 4 shows that the eigenvalue
for the first DCA axis was high indicating that it cap­
tured the greater proportion of the variation in species
composition among stands, but the species­
environment correlation coefficients were low for
DCA axes.
DCA ordination of the perennial species (Figure 3)
shows that species with high positive scores on axis I
are found mainly in stands of the southern transect,
and ordinated close to the right end-point: Prosopis
farcta, Phoenix dactylifera, Nitraria retusa, Sarcocor­
nia fruticosa and Imperata cylindrica. The species po­
sitioned on the other end of this axis include:
Capparis spinosa var. aegyptia (negative end), De­
verra tortuosa, Randonia africana, Ephedra alata and
Helianthemum lippii. These and many other species
are commonly found in the upper and middle parts of
the northern transect. In the centre of axis I there are
many species found throughout the investigated area
with no preference to any geographical aspect (e.g.
Zygophyllum coccineum, Tamarix nilotica, Alhagi
graecorum, Fagonia arabica and Cornulaca monacantha).
Along axis 2, species with high positive
scores include: Atriplex leucoclada and Zilla spinosa
subsp. biparmata. These species are recorded from the
lower part of the northern transect, whereas species on
the other end are of common occurrence in the middle
and upper parts. To compare the classification and
DCA ordination results, the recognised eight
TWINSPAN groups are superimposed. Significance
correlations of soil variables with the first four DCA
axes (results not shown) revealed greater correlations
along axis I than the higher order axes.
ecalogia mediterranea 25 (2) - 1999