Fifty stands (area = 10 × 10 m each) have been selected for sampling vegetation during 2014 as follows: 20 stands in Western Mediterranean coastal belt and 30 in inland desert (Wadi Hagul) of Egypt. In each stand, the annual and perennial species were listed. The nomenclature, identification and floristic categories of plant species were according to Tackholm [5] and up to date by Boulos [7] . Life forms were identified according to the scheme of Raunkiaer [16] .

Measuring the density of each plant species was carried out by counting randomly the number of individuals of the species [17] . The plant cover of each species in the studied stands was determined by using the line intercept method according to Canfield [18] . Relative values of cover and density were calculated and estimate of its importance value (IV = 200) for each plant species in each stand.

Three soil samples (0 - 30 cm) were collected from each stand. The soil samples were pooled together to form one composite sample. They were spread, air dried, sieved (2 mm sieves) and finally packed in plastic bags to be ready for analysis. Physical and chemical analyses of soil samples were carried out according to Piper [19] , Jackson [20] and Allen et al. [21] .

The classification technique used were the Two Way Indicator Species Analysis (TWINSPAN) and Detrended Correspondence Analysis (DCA) applied for the classification of stands into groups and ordinate stands in two- dimensional space based on the importance values of species [22] . The relation between the vegetation and soil gradients was assessed using Canonical Correspondence Analysis (CCA) [23] [24] . Data of the soil variables of the vegetation groups identified by TWINSPAN were compared by one-way ANOVA.

The recorded plant species (74) in the present study belonging to 67 genera and related to 23 families, are classified into three major groups as follows: 42 perennials (56.75%), 31 annuals (41.89%) and one species biennials (1.35%) (Table 1). The highest number of species (57) is recorded in the inland desert representing about (77.03%) of the total recorded species and the coastal desert is represented by 27 species (36.49%). Table 1 showed that, the family Asteraceae (17 species), Poaceae (9 species), Chenopodiaceae (7 species), Brassicaceae (5 species), Fabaceae (5 species) and Zygophyllaceae (5 species) are represented collectively by 48 species (64.86% of the total number of the recorded species).

According to Raunkiaer [16] , the species are grouped under six types (Figure 2(a)) as follow: Therophytes (31 species = 41.33%), Chamaephytes (18 species = 25.33%), Hemicryptophytes (14 species = 17.33%), Phanerophytes (8 species = 10.67%), Geophytes (3 species = 4.00%) and Helophytes (one species = 1.33%).

Chorological analysis of the study area revealed that, 23 species (31.08%) of the total recorded species are Saharo-Sindian. Twenty five species are Mediterranean taxa. It has been also found that, 5 species (6.76%) are Cosmopolitan, 8 species each (10.81%) are Saharo-Sindian and Sudano-Zambezian as well as Irano-Turanian and Saharo-Sindian. Pantropical, Polaeotropical, Neotropical and Sudano-Zambezian are represented by one species each (Figure 2(b)).

In the present study, the most common perennial species associated with Achillea fragrantissima communities are: Launaea nudicaulis, Lycium shawii, Artemisia judaica, Crotalaria aegyptiaca, Deverra tortuosa, Diplotaxis harra, Fagonia mollis, Farsetia aegyptia, Gypsophila capillaris and Haloxylon salicornicum. While, the most common perennial species associated with Achillea santolina are: Echinops spinosus, Launaea nudicaulis, Lycium shawii, Atractylis carduus, Convolvulus arvensis, Cynanchum acutum, Fagonia cretica.

The application of TWINSPAN classification based on the importance values of 74 plant species recorded in 50 sampled stands representing different habitat types of the study area, led to the recognition of four vegetation groups (Figure 3 and Table 1) and the soil variables are presented in Table 2.

Group A comprises 14 stands dominated by Achillea santolina which has the highest importance value of this group (IV = 42.11). The other important species which attain relatively high IV are: Cynanchum acutum (indicator species IV = 16.17), Convolvulus arvensis (IV = 14.97), Cynodon dactylon (IV = 14.47), Avena fatua (IV = 12.54) and Emex spinosa (IV = 10.77). Conyza dioscorides (IV = 11.71). The soil of this group (A) was characterized by relatively high values of sand, organic carbon, pH, bicarbonates, sulphates and extractable cations (Na⁺, K⁺, Ca⁺⁺ and Mg⁺⁺).

Group B comprises 6 stands dominated by Achillea santolina which has the highest importance value (IV = 50.59). The other important species which attain relatively high IV are: Cynodon dactylon (IV = 21.25), Echinops spionousus (IV = 17.05), Convolvulus arvensis (IV = 13.78), Lycium shawii (IV = 13.01), Mesembryanthemum nodiflorum (IV = 10.67) and Bassia indica (IV = 10.45). The indicator species in this group is Imperata cylindrica (IV = 3.91). The soil was characterized by high percentages of porosity, pH, organic carbon, bicarbonates, sulphates, moderate value of chlorides and extractable cations (Na⁺ and Ca⁺⁺).

Group C comprises 21 stands codominated by Zygophyllum coccinum (IV = 23.38) and Launaea spinosa (IV = 19.33). The other important species which attain relatively high IV are: Achillea fragrantissima (IV = 14.20), Retama raetam (IV = 12.48), Ochradenus baccatus (IV = 11.74). The indicator species in this group are: Panicum turgidum (IV = 6.08), Crotalaria aegyptiaca (IV = 3.511) and Echinops spinosus (IV = 2.69). The soil of this group was characterized by high content of soil fractions (silt & clay), calcium carbonate and carbonate, while, this soil attained moderate values of electrical conductivity, chloride, sodium and SAR.

Group D comprises 9 stands dominated by Leptadenia pyrotechnica (indicator species IV = 29.48). The other important species which attain relatively high IV are: Zygophyllum coccinum (IV = 22.67), Lavandula coronopifolia (IV = 22.07) and Tamarix aphylla (indicator species IV = 18.82), Zilla spinosa (IV = 13.86), Ochradenus baccatus (IV = 10.81) and Achillea fragrantissima (IV = 10.30). The soil was characterized by high content of sand, water-holding capacity, electrical conductivity and chloride, moderate value of calcium carbonate, carbonates and sulphates.

The ordination of stands in the different habitats of the study area, given by Detrended Correspondence Analysis (DCA) is demonstrated in Figure 4. The DCA ordination of stands is shown on the plane of the first and second DCA axes. It is obvious that, the groups of vegetation obtained by TWINSPAN classification are remarkably distinguishable and having a clear pattern of segregation on the ordination plane.

Groups A and B dominated by Achillea santolina are separated at the right side of the DCA diagram and showed also superimposed intercept. On the other hand, group C codominated by Zygophyllum coccinum and Launaea spinosa is separated at the middle part of the DCA diagram. Group D dominated by Leptadenia pyrotechnica is obviously separated at the upper left side of the DCA diagram.

The variation in soil factors of the four vegetation groups of stands resulting from TWINSPAN classification indicated considerable variations in the edaphic factors among the stands of the different groups (Table 2). Electrical conductivity, pH, sulphates, bicarbonate, cations (K⁺, Na⁺, Ca⁺⁺ and Mg⁺⁺) and PAR showed significant correlations (P < 0.05) among vegetation groups. The highest percentage of coarse fractions (sand = 94.26%) was obtained in group A, but the highest percentage of silt (10.76%) and clay (1.43%) fraction were obtained in group C. Vegetation groups C and D showed higher values of electrical conductivity (383.37 and 545.25 μmohs/cm, respectively) than in groups A and B (297.38 and 222.50 μmohs/cm, respectively). Also, the percentages of sulphates and bicarbonate were relatively higher in groups A and B (0.64%, 0.68% and 1.27%, 3.53%, respectively) as compared with groups C and D (0.26%, 0.33% and 1.22%, 0.61%, respectively). pH was higher in groups A and B (8.45 each) than in groups C and D (8.0 each). Vegetation groups A and B showed values of cations and PAR which were higher than in groups C and D.

Correlations of edaphic variables with the importance values of the dominant and abundant species are shown in Table 3. It has been found that, some soil variables are positively correlated with plant species such as Achillea fragrantissima correlated significantly with silt (r = 0.392) and carbonate calcium (r = 0.415). Achillea santolina showed high (r = 0.375, 0.403, 0.633, 0.629, 0.758, 0.568, 0.753, 0.661, 0.745 and 0.814) significant correlations with sand, organic carbon, pH, SO₄, HCO₃, Na, K, Ca, Mg and PAR, respectively. Cynodon dactylon and Cynanchum acutum were correlated significantly with all edaphic factors except soil texture, porosity, WHC, organic carbon, electrical conductivity and chloride. Calcium carbonate and electrical conductivity were correlated significantly with Zygophyllum coccinum (r = 0.575) and Tamarix aphylla (r = 0.393). On the other hand, it has been also found that, some soil variables such as clay, porosity, WHC, CO₃ and SAR have negative correlated or no correlation with plant species.