Scholarly article on topic 'Soil seed bank in different habitats of the Eastern Desert of Egypt'

Soil seed bank in different habitats of the Eastern Desert of Egypt Academic research paper on "Biological sciences"

CC BY-NC-ND
0
0
Share paper
Academic journal
Saudi Journal of Biological Sciences
OECD Field of science
Keywords
{Desert / Egypt / "Seed bank" / "Species diversity"}

Abstract of research paper on Biological sciences, author of scientific article — Nasr H. Gomaa

Abstract The floristic composition and species diversity of the germinable soil seed bank were studied in three different habitats (desert salinized land, desert wadi, and reclaimed land) in the Eastern Desert of Egypt. Moreover, the degree of similarity between the seed bank and the above-ground vegetation was determined. The seed bank was studied in 40 stands representing the three habitats. Ten soil samples (each 25×20cm and 5cm depth) were randomly taken per stand. The seed bank was investigated by the seedling emergence method. Some 61 species belonging to 21 families and 54 genera were identified in the germinable seed bank. The recorded species include 43 annuals and 18 perennials. Ordination of stands by Detrended Correspondence Analysis (DCA) indicates that the stands of the three habitats are markedly distinguishable and show a clear pattern of segregation on the ordination planes. This indicates variations in the species composition among habitats. The results also demonstrate significant associations between the floristic composition of the seed bank and edaphic factors such as CaCO3, electrical conductivity, organic carbon and soil texture. The reclaimed land has the highest values of species richness, Shannon-index of diversity and the density of the germinable seed bank followed by the habitats of desert wadi and desert salinized land. Motyka’s similarity index between the seed bank and the above-ground vegetation is significantly higher in reclaimed land (75.1%) compared to desert wadi (38.4%) and desert salinized land (36.5%).

Academic research paper on topic "Soil seed bank in different habitats of the Eastern Desert of Egypt"

Saudi Journal of Biological Sciences (2012) 19, 211-220

King Saud University Saudi Journal of Biological Sciences

www.ksu.edu.sa www.sciencedirect.com

ORIGINAL ARTICLE

Soil seed bank in different habitats of the Eastern Desert of Egypt

Nasr H. Gomaa *

Department of Botany, Faculty of Science, Beni-Suef University, Beni-Suef, Egypt

Received 22 September 2011; revised 1 January 2012; accepted 7 January 2012 Available online 14 January 2012

KEYWORDS

Desert; Egypt; Seed bank; Species diversity

Abstract The floristic composition and species diversity of the germinable soil seed bank were studied in three different habitats (desert salinized land, desert wadi, and reclaimed land) in the Eastern Desert of Egypt. Moreover, the degree of similarity between the seed bank and the above-ground vegetation was determined. The seed bank was studied in 40 stands representing the three habitats. Ten soil samples (each 25 x 20 cm and 5 cm depth) were randomly taken per stand. The seed bank was investigated by the seedling emergence method. Some 61 species belonging to 21 families and 54 genera were identified in the germinable seed bank. The recorded species include 43 annuals and 18 perennials. Ordination of stands by Detrended Correspondence Analysis (DCA) indicates that the stands of the three habitats are markedly distinguishable and show a clear pattern of segregation on the ordination planes. This indicates variations in the species composition among habitats. The results also demonstrate significant associations between the floristic composition of the seed bank and edaphic factors such as CaCO3, electrical conductivity, organic carbon and soil texture. The reclaimed land has the highest values of species richness, Shannon-index of diversity and the density of the germinable seed bank followed by the habitats of desert wadi and desert salinized land. Motyka's similarity index between the seed bank and the above-ground vegetation is significantly higher in reclaimed land (75.1%) compared to desert wadi (38.4%) and desert salinized land (36.5%).

© 2012 King Saud University. Production and hosting by Elsevier B.V. All rights reserved.

* Tel.: +20 1287752207.

E-mail address: nhgomaa@yahoo.com

1319-562X © 2012 King Saud University. Production and hosting by Elsevier B.V. All rights reserved.

Peer review under responsibility of King Saud University. doi:10.1016/j.sjbs.2012.01.002

1. Introduction

Soil seed banks are the aggregations of viable seeds in the soil potentially capable of replacing adult plants (Baker 1989; Thompson and Grime, 1979). Most of the seeds in the seed bank come from the nearby parent plants, while the remaining seeds are contributed by plant communities a long distance away from the parent plants (Solomon, 2011). Thompson and Grime (1979) recognized two main seed bank strategies: transient types in which no seeds remain viable for more than one year and persistent types in which seeds remain viable for longer than one year. The capacity to perform persistent seed

bank allows species to survive episodes of disturbance and destruction (Thompson, 2000). Many species have this capacity and many do not (Thompson et al., 1997). Seed banks play a critical role in vegetation maintenance, succession, ecosystem restoration, differential species management and conservation of genetic variability (Harper, 1977; McGraw et al., 1991; Hills and Morris, 1992).

Desert seed banks are usually composed of very small seeds that usually lack dispersal structures (Harper, 1977; Thompson and Grime, 1979; Fenner, 1985; Chambers and MacMahon, 1994; Gutterman, 1994) and are characterized by temporal and spatial fluctuations in seed density (Kemp, 1989; Marone and Horno, 1997; Guo et al., 1998, 1999). Seed banks are a crucial component in desert ecosystems and other stressful habitats where favorable conditions for seed germination and seedling establishment are quite unpredictable both in space and time (Kemp, 1989; Nathan and Muller-Landau, 2003; Meyer and Pendleton, 2005; Koontz and Simpson, 2010). Although the seed bank is an important element in desert ecosystems, little is documented on the diversity of the soil seed bank and its relations to the above-ground vegetation in arid regions (Kemp, 1989; Al-Faraj et al., 1997; Zaghloul, 2008) and in particular in Egyptian deserts. Such information is crucial for developing programs for the conservation and habitat restoration in arid regions and in particular in the Eastern Desert of Egypt where the natural vegetation has been degraded in some areas as a result of several factors as overgrazing and excessive collection of economically important plants (Hegazy et al., 2007). Desert wadi and desert salinized land represent the main habitats that harbor the natural vegetation in the present study area.

The increase in the human population of Egypt necessitates the expansion of the cultivated land. This was achieved by the reclamation of many desert areas during the past few decades. Some studies were concerned with the vegetation of the reclaimed land in Egypt (e.g., El-Bakry, 1982; Shehta and El-Fahar, 2000), but knowledge on the soil seed bank and its relation with the above-ground vegetation in this habitat is very limited.

Seed distribution and storage in soil are related to soil conditions such as particle sizes, structure, and soil chemistry (Harper, 1977; Silvertown, 1981; Coffin and Lauenroth, 1989; Chambers and MacMahon, 1994).

The objectives of this study are to determine (1) the floristic composition and diversity of the germinable soil seed bank in relation to different habitats (desert salinized land, desert wadi and reclaimed land) in Eastern Desert of Egypt, (2) the similarity between the composition of the germinable soil seed bank and the above-ground vegetation in the different habitats, and (3) the associations between the edaphic factors and the composition of the soil seed bank.

2. Materials and methods

2.1. Study area

The study area is located in the Eastern Desert of Egypt between latitudes 28° 30' N to 29° 26' N and longitudes 30° 50' E to 31° 21' E. (Fig. 1). The area includes three main habitats: desert sali-nized land, desert wadi and reclaimed land. The habitat of desert salinized land is represented by areas with saline soil

supporting the growth of halophytic species such as Juncus rigidus, Tamarix nilotica, Phragmites australis and Zygophyllum album. Its salinity may be attributed to the possible shallow water table that receives drainage water from the adjacent reclaimed cultivated land. The salinity of drainage water is due to the application of fertilizers in reclaimed land. Moreover, the high evaporation rate in particular during summer season and the low rainfall contribute to soil salinity. According to Ab-rol etal. (1988) and based on the results of the present study, the soil salinity of this habitat is classified as slightly saline (electrical conductivity = 2-4 mS/cm). The study area is dissected by a number of wadis namely, Wadi Lyschayb, Wadi Bayad, Wadi Sannur, Wadi El-Fakira, and Wadi El-Sheikh. The largest of these wadis is Wadi Sannur. These wadis are drainage systems collecting water from extensive catchment areas. The vegetation of the wadis in the study area is composed of a permanent framework of perennials, the interspaces of which may be occupied by annuals after winter rains. The reclaimed land is a desert area which has been farmed from about 1to 30 years ago. The main winter crops of the reclaimed land are wheat, Egyptian clover and onion, while their main summer crops are maize, pea nut and sesame. The vegetation of the reclaimed land is a combination of desert plants and weeds.

The area of the present study is situated within a region of dry climate. The total annual rainfall is 7.8 mm with the rainy season occurring from November to April. The mean monthly air temperature ranges between 12.2 °C during January and 29.1 °C during July. The mean relative humidity varies between 35% during May and 57% during December. The mean monthly evaporation ranges from 5.5 mm day-1 during January to 20.1 mm day-1 during June (data from Ministry of Civil Aviation, 1975).

2.2. Vegetation sampling

The standing vegetation was sampled in order to compare its floristic composition with that of the soil seed bank. A total of 40 stands were identified to represent the three habitats in the study area and cover the within-habitat variations in vegetation. Some ten stands were sampled in desert salinized land, while 15 stands were sampled in each of desert wadi and reclaimed land habitats. For every stand, a floristic list was taken. Plant nomenclature and identification followed Boulos (1999, 2000, 2002, 2005, 2009). Species were categorized in terms of their life form according to Raunkiaer (1934) into therophytes, hemicryptophytes, geophytes, chamaephytes and phanerophytes.

2.3. Seed bank sampling

Ten soil samples (each 25 x 20 cm and 5 cm depth) were randomly taken per each of the 40 stands used for the determination of the floristic composition of the above-ground vegetation. To characterize the persistent seed bank, soil samples were collected in late March 2009 after seeds of most species had germinated but before the beginning of seed dispersal for most species (Thompson and Grime, 1979). The soil samples were passed through a 4 mm sieve to exclude coarse stones and plant fragments. The excluded material was examined manually for seeds and fruits. A known volume of sieved soil samples was spread as a 1 cm deep layer overlying sterilized

Figure 1 Map of the

coarse sand in 25 x 20 x 8 cm germination trays. The germination trays were placed in a greenhouse and regularly watered with tap water. Emergent seedlings were identified, counted and discarded. The seedlings which were difficult to be identified were counted, transplanted and grown separately until identification. For each stand, the species composition, mean density (number of seeds/m2) and mean relative density of the species that constitute the germinable soil seed bank were determined. After six months (December-May), the experiment was stopped as no more seedlings appeared for several consecutive weeks.

2.4. Soil analysis

Three soil samples were taken per stand, from a depth of 0to 50 cm. The samples were pooled together, forming one composite sample for each stand. The samples were air dried and sieved through a 2 mm sieve before analysis. For soil texture analysis, the soil fractions were separated by sieves. 100 g of each soil sample was passed through a series of sieves to separate gravels (>2 mm), coarse and medium sand (2-0.25 mm), fine and very fine sand (0.25-0.05 mm), and silt and clay (<0.05 mm). The percentage of CaCO3 was estimated using 1 N HCl (Jackson, 1967). Oxidizable organic carbon was

and reclaimed land (•).

determined by modified Walkley-Black method (Jackson, 1958). Soil-water extracts of 1:5 were prepared and used for the determination of electrical conductivity (E.C.) and soil reaction (pH) using a conductivity and pH meter (Jenway 4330).

2.5. Data analysis

The Detrended Correspondence Analysis - DCA (Hill, 1979) was used to ordinate the stands in two-dimensional space using the relative seed bank density of species. Data of the soil variables of the different habitats were compared by one-way ANOVA followed by Tukey's post-hoc test. The same analysis was used to compare between the different habitats regarding the diversity indices, density of the seed bank and similarity between seed bank and the above-ground vegetation. Linear correlation of soil variables with DCA axes and the seed bank density of the most abundant species in the seed bank was made to analyze the relationships between the composition of the seed bank and soil variables. The one-way ANOVA and correlation analyses were conducted using SPSS 12 for Windows.

Motyka's similarity index (Mueller-Dombois and Ellenberg, 1974) was used to make comparisons between species

Table 1 A list of species recorded in the standing vegetation and seed bank as well as the mean density of seed bank of species (number

of seeds/m2) in the different habitats. Th, therophytes; H, hemicryptophytes; G, geophytes; Ch, chamaephytes; Ph, phanerophytes; +,

present; —, absent.

Species Life Vegetation Seed bank

form Desert Desert Reclaimed Desert Desert Reclaimed

salinized wadi land salinized land wadi land

Acacia tortilis (Forssk.) Hayne (Fabaceae) Ph — + - - - —

Achillea fragrantissima (Forssk.) Sch. Bip. Ch — + - - 1.3 —

(Asteraceae)

Alhagi graecorum Boiss. (Fabaceae) H — + - - 0.7 —

Ammi majus L. (Apiaceae) Th — + - - 1.3

Anabasis setifera Moq. (Chenopodiaceae) Ch — + - - - —

Anagallis arvensis L. (Primulaceae) Th — - + - - 7.3

Anastatica hierochuntica L. (Brassicaceae) Th — - v - 0.7 —

Artemisia judaica L. (Asteraceae) Ch — + - - - —

Avena fatua L.(Poaceae) Th — - + - - —

Avena sativa L. (Poaceae) Th — - + - - 2.0

Avena sterilis L. (Poaceae) Th — - + - - —

Beta vulgaris L. (Chenopodiaceae) Th — - + - - 5.3

Brassica nigra (L.) Koch. (Brassicaceae) Th — - + - - 7.3

Brassica tournefortii Gouan (Brassicaceae) Th — - + - - 1.3

Capsella bursa-pastoris (L.) Medik. (Brassicaceae) Th — - + - - 2.1

Centaurea scoparia Sieber ex Spreng. (Asteraceae) Ch — + - - - —

Chenopodium album L. (Chenopodiaceae) Th — - + - - 12.7

Chenopodium murale L. (Chenopodiaceae) Th — - + - - 136.7

Cichorium endivia L. (Asteraceae) Th — - + - - 0.7

Citrullus colocynthis (L.) Schrad. (Cucurbitaceae) H — + v- - 1.3 —

Convolvulus arvensis L. (Convolvulaceae) Th — - + - - 1.3

Coronopus squamatus (Forssk.) Asch. Th — - + - - 0.7

(Brassicaceae)

Cotula cinerea Delile (Asteraceae) Th — + - - 0.7 —

Cynodon dactylon (L.) Pers. (Poaceae) G — - + - - —

Cyperus laevigatus L. (Cyperaceae) H + - - - - —

Deverra tortuosa (Desf.) DC. (Apiaceae) Ch — + - - - —

Deverra triradiata Hochst. ex Boiss. (Apiaceae) Ch — + - - 1.4 —

Diplotaxis acris (Forssk.) Boiss. (Brassicaceae) Th — + - - - —

Echinops spinosus L. (Asteraceae) H — + - - - —

Emex spinosa (L.) Campd. (Polygonaceae) Th — - + - - 24.7

Erodium oxyrhynchum M. Bieb. (Geraniaceae) Th — - - - 0.7 —

Euphorbia helioscopia L. (Euphorbiaceae) Th — - + - - 14.0

Euphorbia peplis L. (Euphorbiaceae) Th — - + - - 2.0

Fagonia arabica L. (Zygophyllaceae) Ch — + - - - —

Farsetia aegyptia Turra (Brassicaceae) Ch — + + - 0.7 —

Haloxylon salicornicum (Moq.) Bunge ex Boiss. Ch — + - - - —

(Chenopodiaceae)

Heliotropium digynum (Forssk.) C. Chr. Ch — + - - - —

(Boraginaceae)

Hippocrepis multisiliquosa L. (Fabaceae) Th — + - - 1.3 —

Hordeum murinum L. (Poaceae) Th — - - - 1.3 —

Ifloga spicata (Forssk.) Sch. Bip. (Asteraceae) Th — + - - 0.7 —

Juncus rigidus Desf. (Juncaceae) H + - - 1 - —

Lactuca serriola L. (Asteraceae) Th — - + - - 1.3

Lasiurus scindicus Henrard (Poaceae) H — + - - - —

Launaea nudicaulis (L.) Hook. F. (Asteraceae) H — + + - 6.0 2.7

Lolium temulentum L. (Poaceae) Th — - + - - 6.0

Lycium shawii Roem. & Schult. (Solanaceae) Ph — + - - - v

Malva parviflora L. (Malvaceae) Th — + + - 3.3 6.0

Medicago polymorpha L. (Fabaceae) Th — - + - - 17.3

Melilotus indicus (L.) All. (Fabaceae) Th — - + v - 86.7

Nauplius graveolens (Forssk.) Wiklund Ch — + - - 0.7 —

(Asteraceae)

Ochradenus baccatus Delile (Resedaceae) Ph — + - - 1.3 —

Oligomeris linifolia (Vahl ex Hornem.) J. F. Th — + - - 0.7 —

Macbr. (Resedaceae) (continued on next page)

Table 1 (continued)

Species Life Vegetation Seed bank

form Desert Desert Reclaimed Desert Desert Reclaimed

salinized wadi land salinized land wadi land

Panicum turgidum Forssk. (Poaceae) Ch - + - - 1.3 -

Pennisetum divisum (Forssk. ex J.F. Gmel.) Ch - + - - - -

Henrard (Poaceae)

Pergularia tomentosa L. (Asclepiadaceae) Ch - + - - - -

Phalaris paradoxa L. (Poaceae) Th - - + - - 1.3

Phragmites australis (Cav.) Trin.ex Steud. G + - - - - -

(Poaceae)

Plantago amplexicaulis Cav. (Plantaginaceae) Th - - - - 1.3 -

Poa annua L. (Poaceae) Th - - + - - 4.7

Polygonum equisetiforme Sm. (Polygonaceae) H - + - - - -

Polypogon monspeliensis (L.) Desf. (Poaceae) Th + - - 2.2 - -

Pulicaria undulata (L.) C.A. Mey. (Asteraceae) Ch - + - - 0.7 -

Reichardia tingitana (L.) Roth. (Asteraceae) Th - + + - 0.7 2.0

Retama raetam (Forssk.) Webb & Berthel. Ph - + - - 0.7 -

(Fabaceae)

Rumex dentatus L. (Polygonaceae) Th - - + - 0.7 2.7

Rumex vesicarius L. (Polygonaceae) Th - + - - - -

Schismus barbatus (L.) Thell. (Poaceae) Th - + - - 11.3 -

Senecio glaucus L. (Asteraceae) Th - - + - 2.7 1.3

Sisymbrium irio L. (Brassicaceae) Th - - + - - 7.3

Sonchus oleraceus L. (Asteraceae) Th - + + - 0.7 77.3

Tamarix aphylla (L.) H. Karst. (Tamaricaceae) Ph - + - - - -

Tamarix nilotica (Ehrenb.) Bunge (Tamaricaceae) Ph + + - 8.0 0.7 -

Trichodesma africanum (L.) R. Br. (Boraginaceae) Ch - + - - 2.7 -

Trifolium resupinatum L. (Fabaceae) Th - - + - - 5.3

Trigonella hamosa L. (Fabaceae) Th - + + - - 17.1

Trigonella stellata Forssk. (Fabaceae) Th - + - - 4.0 -

Vicia sativa L. (Fabaceae) Th - - + - - 1.3

Zilla spinosa (L.) Prantl (Brassicaceae) Ch - + + - 12.7 0.7

Zygophyllum album L. F. (Zygophyllaceae) Ch + - - 3.3 - -

Zygophyllum coccineum L. (Zygophyllaceae) Ch + + + 12.0 102.0 11.3

Zygophyllum simplex L. (Zygophyllaceae) Th - - + 2.2 6.7 0.7

composition of the soil seed bank and the above-ground vegetation in each stand: similarity index (%) = 2c/(a + b) x 100, where a and b are the numbers of all species in sample A (ger-minable soil seed bank) and all species in sample B (above-ground vegetation), respectively, and c is the number of species common to the both samples.

Three indices were applied for measurement of the diversity of the germinable seed bank in each stand (Pielou, 1975; Zhang, 1995): Species Richness = S

Shannon — index of diversity : H — — Y1 pi lnpi

Evenness index : E — lnp^j =lnS

where pi = Ni/N, Ni is the number of seeds of species i, N is the total number of seeds of all species present, and S is the number of species present.

3. Results

3.1. Floristic composition of above-ground vegetation

A total of 77 species belonging to 22 families and 66 genera were recorded in the standing vegetation. Asteraceae has the

highest contribution to the total flora (14 species) followed by members of Poaceae (13 species), Fabaceae (10 species) and Brassicaceae (8 species). The recorded species include 42 annuals (54%) and 35 perennials (Table 1).

3.2. Floristic composition and structure of seed bank

At seed bank level, some 61 species belonging to 21 families and 54 genera were recorded (Table 1). The largest families were Asteraceae (12 species) followed by Fabaceae (9 species), Poaceae and Brassicaceae (8 species for each). The recorded species include 43 annuals (70%) and 18 perennials. The most frequent life forms in the seed bank are therophytes (70%) and chamaephytes (18%), while hemicryptophytes and phanero-phytes represent only 7% and 5%, respectively (Table 1).

The most abundant species in the germinable soil seed bank of salinized land are Zygophyllum coccineum, T. nilotica and Z. album (mean seed bank density = 12.0, 8.0 and 3.3 m~2, respectively, Table 1). Z. coccineum, Schismus barbatus and Zilla spinosa are the most abundant species in the soil seed bank of desert wadi (102.0, 11.3 and 12.7 m-2, respectively), while Chenopodium murale, Melilotus indicus and Sonchus oleraceus are the most abundant species in the soil seed bank of the reclaimed land (136.7, 86.7 and 77.3 m~2, respectively) (Table 1).

Figure 2 DCA ordination of the 40 stands based on the mean relative density of seed bank of species. A = desert salinized land, B = desert wadi, C = reclaimed land.

Table 2 Linear correlation coefficients (r) between edaphic factors and the first two DCA axes.

Edaphic parameter DCA axis 1 2

CaCO3 (%) -0.652*** -0.323*

HCO3- (%) 0.209 -0.093

pH 0.267 0.032

Electrical conductivity (mS/cm) -0.653*** -0.378*

Organic carbon (%) 0.459** -0.110

Gravels (%) 0.542*** 0.015

Coarse and medium sand (%) -0.444** 0.331*

Fine and very fine sand (%) 0.039 -0.421**

Silt and clay (%) 0.449** -0.021

* P < 0.05. ** P < 0.01. *** P < 0.001.

3.3. Seed bank-soil relationships

Correlation analysis indicates that DCA axis 1 is negatively correlated with CaCO3 (r = -0.652, P < 0.001), electrical conductivity (r = -0.653, P < 0.001) and coarse and medium sand (r = -0.444, P < 0.01) and positively correlated with organic carbon (r = 0.459, P < 0.01), gravels (r = 0.542, P < 0.001), silt and clay (r = 0.449, P < 0.01) (Table 2). DCA axis 2 shows significant negative correlations with CaCO3 (r = -0.323, P < 0.05), electrical conductivity (r = -0.378, P < 0.05) and fine and very fine sand (r = -0.421, P < 0.01) and positive correlation with coarse and medium sand (r = 0.331, P < 0.05) (Table 2).

Edaphic characteristics of the three habitat types are summarized in Table 3. With the exception of HCO3-, pH, fine and very fine sand, the measured soil parameters show significant differences among habitats. The correlations of the seed bank density of the most abundant species in the seed bank with soil variables are shown in Table 4. Except, S. oleraceus, all the tested species show significant correlations with at least one of the measured edaphic parameters. C. murale and M. indicus are negatively correlated with CaCO3 (r = -0.653, P < 0.001; r = -0.499, P < 0.01, respectively) and positively correlated with organic carbon (r = 0.492, P < 0.01; r = 0.364, P < 0.05, respectively), silt and clay (r = 0.374, P < 0.05; r = 0.396, P < 0.05, respectively). T. nilotica shows significant positive correlations with electrical conductivity (r = 0.780, P < 0.001) and fine and very fine sand (r = 0.454, P < 0.01) and significant negative correlation with gravels (r = -0.515, P < 0.01). Z. coccineum displays significant positive correlation with CaCO3 (r = 0.580, P < 0.001) and significant negative correlations with pH (r = -0.515, P < 0.01) and organic carbon (r = -0.568, P < 0.001).

3.4. Diversity and density of seed bank

Ordination of the 40 stands by DCA (Fig. 2) indicates that the stands of the three habitats show a clear pattern of segregation on the ordination planes. The stands of the habitats are clearly distinguished and distributed mainly along axis 1 from left to right in the order: desert salinized land (A), desert wadi (B) and reclaimed land (C). The eigenvalues for the first two DCA axes are 0.919 and 0.574, respectively. The high eigenvalue for DCA axis 1 indicates that it captures the greater proportion of the variation in species composition among stands.

Species richness varies significantly among habitats (P < 0.05). The reclaimed land has the highest species richness (9.3) followed by desert wadi (4.7) and desert salinized land (2.2). Shannon index is significantly higher (P < 0.05) in reclaimed land (1.57) than in desert salinized land (0.74) and desert wadi (1.12), while evenness is significantly higher (P < 0.05) in salinized land (0.95) compared to desert wadi (0.73) and reclaimed land (0.71) (Table 5). The density of the seed bank differs significantly among habitats (P < 0.05). It is maximum in reclaimed land (471.3 seeds m-2) followed by

Table 3 Means ± SD of edaphic factors of the different habitats.

Edaphic variable Habitat

Desert salinized land Desert wadi Reclaimed land

CaCO3 (%) 18.4a ± 3.0 22.2b ±3.1 12.5c ± 2.1

HCO3- (%) 0.06a ± 0.01 0.05a ± 0.01 0.06a ± 0.01

pH 7.8a ± 0.1 7.7a ± 0.2 7.8a ± 0.3

Electrical conductivity (mS/cm) 3.02a ± 0.18 0.78b ± 0.17 0.74b ± 0.07

Organic carbon (%) 0.59a ± 0.14 0.29b ± 0.11 0.65a ± 0.14

Gravels (%) 5.6a ± 1.8 8.3a ± 3.6 11.3b ± 3.5

Coarse and medium sand (%) 45.9ab ± 10.5 45.7a ± 10.4 35.9b ± 5.6

Fine and very fine sand (%) 39.6a ± 9.9 36.7a ± 7.0 37.4a ± 3.7

Silt and clay (%) 9.9a ± 3.7 8.6a ± 4.6 19.0b ± 8.0

Values in a raw sharing the same letter are not significantly different at the 0.05 level of probability.

Table 4 Linear correlation coefficients (r) between edaphic factors and the seed bank density of the most abundant species in the different habitats.

Species Edaphic variable

CaCO3 HCO3- pH Electrical Organic Gravels Coarse and Fine and Silt and

conductivity carbon medium sand very fine sand clay

Chenopodium murale -0.653*** 0.261 0.371* -0.375* 0.492** 0.294 -0.253 -0.088 0.374*

Melilotus indicus -0.499** 0.095 0.358* -0.302 0.364* 0.245 -0.324* 0.030 0.396*

Schismus barbatus 0.389* -0.344* -0.268 -0.168 -0.224 0.148 -0.031 0.131 -0.165

Sonchus oleraceus 0.231 0.066 -0.009 -0.120 -0.156 0.039 0.044 -0.023 -0.073

Tamarix nilotica 0.165 - 0.072 0.207 0.780*** 0.206 -0.515** 0.028 0.454** -0.228

Zygophyllum album 0.042 0.183 0.117 0.531*** -0.012 -0.181 0.124 -0.100 -0.035

Zygophyllum coccineum 0.580*** 0.072 -0.515** -0.278 -0.568*** -0.032 -0.075 -0.013 0.007

Zygophyllum simplex 0.223 -0.201 -0.253 -0.091 -0.380* -0.057 0.304 -0.157 -0.263

* P < 0.05. ** P < 0.01. *** P < 0.001.

Table 5 Means ± SD of diversity indices and density of seed bank as well as the similarity between seed bank and the above-ground vegetation in the different habitats.

Diversity index Habitat

Desert salinized land Desert wadi Reclaimed land

Species richness Shannon index Evenness Density of seed bank (seeds m-2) Motyka's similarity index 2.2a ± 0.4 0.74a ± 0.16 0.95a ± 0.06 28.0a ± 9.2 36.5a ± 3.7 4.7b ± 1.4 1.12a ± 0.36 0.73b ± 0.18 174.7b ± 83.6 38.4a ± 10.3 9.3c ± 2.3 1.57b ± 0.49 0.71b ± 0.20 471.3c ± 177.0 75.1b ± 5.0

Values in a raw sharing the same letter are not significantly different at the 0.05 level.

desert wadi (174.7 seeds m 2) and desert salinized land (28.0 seeds m~2) (Table 5).

3.5. Similarity between seed bank and above-ground vegetation

Motyka's similarity index between seed bank and above-ground vegetation is significantly higher (P < 0.05) in reclaimed land (75.1%) compared to desert wadi (38.4%) and desert salinized land (36.5%) (Table 5).

Of the 77 species recorded in the standing vegetation, only 57 species (74%) were present in the seed bank (Table 1). Four species Anastatica hierochuntica, Erodium oxyrhynchum, Hord-eum murinum and Plantago amplexicaulis were recorded only in the seed bank and not found in the standing vegetation. The species that were present only in the standing vegetation are mainly perennials (85%), whereas all the species which were recorded only in the seed bank are annuals.

4. Discussion

4.1. Floristic composition and structure of seed bank

A total of 77 species were found in the above-ground vegetation including 42 annuals and 35 perennials, while some 61 species were recorded in the germinable soil seed bank including 43 annuals and 18 perennials. The separation of the stands of the different habitats on the ordination planes of DCA analysis demonstrates that the species composition of the seed

bank differs among the habitats. Such variations in floristic composition can be explained by variation in environmental conditions among habitats. Environmental variations among habitats are indicated by differences in soil characteristics among the different habitats demonstrated in the present study. The variations among habitats also include differences due to the crop management practices applied in the reclaimed land. The stands of salinized soil are scattered along DCA axis one while those of the desert wadi are scattered along axis two. This pattern can be related to the within-habitat variations in species composition among stands. These differences are not major but this pattern appears due to the low species richness per stand in these habitats. So, any difference even in only one species between two stands will result in a significant distance between them in the ordination plot.

4.2. Seed bank-soil relationships

Linear correlation of soil variables with DCA axes and seed bank density of the most abundant species in the seed bank indicates significant associations between the floristic composition of the seed bank and the edaphic factors such as CaCO3, electrical conductivity, organic carbon and soil texture. It was reported previously that edaphic factors influence soil seed banks. The contribution of edaphic factors as soil texture (Chambers et al., 1991; Rundel and Gibson, 1996; Goodson et al., 2002), moisture (Leckie et al., 2000) and fertility (Kitajima and Tilman, 1996) to seed bank composition has been reported.

4.3. Diversity and density of seed bank

Significant differences were found in species diversity and density of seed bank among habitats. The reclaimed land has the highest diversity (as measured by species richness and Shannon index) and density of the seed bank. This may be related to the fact that the seed bank of reclaimed land consists of a mixture of desert plants seeds already present in the soil before land reclamation and the seeds dispersed from surrounding natural desert vegetation in addition to the seeds of weeds which grow associated with the cultivated crops in the reclaimed land. In general, weeds are characterized by the production of large numbers of long-lived seeds. This allows the presence of the large and relatively constant weed seed bank in cultivated land (Radosevich et al., 1997). The mean ± SD density of the seed bank varies between 28.0 ± 9.2 seeds m-2 in salinized land and 471.3 ± 177.0 seeds m-2 in reclaimed land. These values are comparable to those reported by Zaghloul (2008) in his study on soil seed banks of Sinai (overall mean ± SD = 118.4 ± 226.4 seeds m-2).

4.4. Similarity between seed bank and above-ground vegetation

Of the 77 species recorded in the standing vegetation, only 57 species were present in the seed bank. Four species were recorded only in the seed bank and not found in the above-ground vegetation. At habitat level, seven species were recorded in the above-ground vegetation in salinized land. Five of them are present in the seed bank. Out of 42 species recorded in the above-ground vegetation of desert wadi, 24 species were found in the seed bank. Some 38 species were recorded in the above-ground vegetation of reclaimed land. Out of these species, 34 are present in the seed bank. The species which were recorded only in the standing vegetation and were absent in the seed bank are mostly long-lived perennials. A similar pattern was reported by Guo et al. (1999) and Marone et al. (1998) who indicated the scarce presence of perennials in the seed bank of desert ecosystems in comparison with the short-lived species. The large seeds of long-lived species are more likely to suffer predation in the soil than are small seeds of short-lived plants (Bertiller, 1998). Additionally, the lower seed production, higher seed retention in the canopy, and scarce presence in the seed rain of long-lived species, may also contribute to their infrequent occurrence in the soil (Kemp, 1989; Jimenez and Armes-to, 1992; Bertiller, 1998). The proportion of the annual species in the seed bank is high (70%) when compared to their proportion in the standing vegetation (54%). Moreover, the four species which were present only in the seed bank and absent in the above-ground vegetation are annuals. These results are consistent with those of Assaeed and Al-Doss (2002) who reported that annuals are over-represented in the soil seed bank of desert rangelands in Saudi Arabia. Annuals have only one chance to reproduce, and if an environmental stress such as drought in deserts or weed management practices in reclaimed land results in the death of all individuals before they have a chance to produce seeds, the seed bank will provide the source for recruitment of the species in subsequent years. So, the only way for an annual species to survive in such risky environments is to have a persistent seed bank (Ungar, 1988; Baskin and Baskin, 1998; De Villiers et al., 2003).

The similarity between seed bank and above-ground vegetation is high in the reclaimed land in comparison with the desert

wadi and desert salinized land which show little similarity. This agrees with the results of other studies that reported a poor relationship between existing vegetation and underground seed reserves in desert communities (Khan, 1993; Aziz and Khan, 1996). This poor correspondence between seed bank and above-ground vegetation may be due to seed predation (Baskin and Baskin, 1998; Crowley and Garnett, 1999; Marone et al., 2000), reliance on vegetative reproduction (Baker, 1989) and lack of dormancy mechanisms (Esmailzadeh et al., 2011). The high similarity between the seed bank and above-ground vegetation in the reclaimed land agrees with the findings of Shaukat and Siddiqui (2004) who reported high similarity between above-ground vegetation and seed bank in arable land. The similarity between seed bank and above-ground vegetation may be determined by the perennial/annual species ratio (Peco et al., 1998). In annual dominated communities like those of the reclaimed land, there are high similarities between the seed bank and vegetation composition, while in communities dominated by perennials like those of the desert wadi and salinized land in the present study, there are low similarities.

4.5. Implications for conservation

The vegetation of desert wadi and desert salinized land in the present study area suffers from human activities as overgrazing and collection of plants for medicinal purposes. This may lead to habitat degradation and vegetation destruction in some areas. Knowledge of the soil seed bank in natural communities is a useful tool for conservation and restoration efforts (Hegazy, 1996; Bakker and Berendse, 1999; Funes et al., 2001; Wassie and Teketay, 2006; Satterthwaite et al., 2007). In years with severe drought conditions, or following severe disturbance, a persistent seed bank might reduce the chance of extinction for a population on a site (Strykstra et al., 1998; Bakker and Berendse, 1999; Funes et al., 1999). Of the 77 species that constitute the standing vegetation, some 20 species (26%) are not represented in the seed bank (see Table 1). So, they are vulnerable to elimination from the standing vegetation and not to be replaced by individuals of their own species (Fenner, 1985; O'Connor, 1991). The restoration of these species would be difficult and slow to accomplish if they are destroyed. Such species must receive attention to protect them. On the other hand, other species such as Z. coccineum, S. barbatus and Z. spinosa have a better chance of recovery since they have relatively large persistent soil seed banks. If there is a high similarity between seed bank and the standing vegetation, the seed bank will be an effective method for the restoration of the vegetation. On the other hand, if the vegetation has low similarity with the soil seed bank, seed bank alone will therefore not be helpful for restoration of the destructed vegetation (Halassy, 2001; De Villiers et al., 2003). The low similarity between seed bank and above-ground vegetation in the desert wadi and salinized land of the present study area suggests that soil seed bank cannot be used effectively for restoration of the degraded vegetation in these habitats.

Acknowledgment

The author appreciates the reviewers for their valuable comments and suggestions which significantly improved the manuscript.

References

Abrol, I.P., Yadour, J.S.P., Massoud, F.I., 1988. Salt affected soils and their management. FAO Soil Bulletin, No. 39. FAO, Rome, p. 131.

Al-Faraj, M.M., Al-Farhan, A., Al-Yemeni, M., 1997. Ecological studies on Rawdhat system in Saudi-Arabia I- Rawdhat khorim. Pak. J. Bot. 29, 75-88.

Assaeed, A.M., Al-Doss, A.A., 2002. Soil seed bank of a desert range site infested with Rhazya stricta in Raudhat al-Khafs, Saudi Arabia. Arid Land Res. Manag. 16, 83-95.

Aziz, S., Khan, M.A., 1996. Seed bank dynamics of a semi-arid coastal shrub community in Pakistan. J. Arid Environ. 34, 81-87.

Baker, H.G., 1989. Some aspects of the natural history of seed banks. In: Leck, M.A., Parker, V.T., Simpson, R.L. (Eds.), Ecology of Soil Seed Bank. Academic Press, San Diego, pp. 9-24.

Bakker, J.P., Berendse, F., 1999. Constraints in the restoration of ecological diversity in grassland and heathland communities. Trends Ecol. Evol. 14, 63-68.

Baskin, C.C., Baskin, J.M., 1998. Seeds: Ecology, Biogeography, and Evolution of Dormancy and Germination. Academic Press, New York.

Bertiller, M.B., 1998. Spatial patterns of the germinable soil seed bank in northern Patagonia. Seed Sci. Res. 8, 39-45.

Boulos, L., 1999. Azollaceae-Oxalidace. In: Flora of Egypt, vol. 1. Al Hadara Publishing, Cairo.

Boulos, L., 2000. Geraniaceae-Boraginaceae. In: Flora of Egypt, vol. 2. Al Hadara Publishing, Cairo.

Boulos, L., 2002. Verbenaceae-Compositae. In: Flora of Egypt, vol. 3. Al Hadara Publishing, Cairo.

Boulos, L., 2005. Monocotyledons: Alismataceae-Orchidaceae. In: Flora of Egypt, vol. 4. Al Hadara Publishing, Cairo.

Boulos, L., 2009. Flora of Egypt Checklist. Revised Annotated Edition. Al Hadara Publishing, Cairo.

Chambers, J.C., MacMahon, J.A., 1994. A day in the life of a seed: movements and fates of seeds and their implications for natural and managed systems. Annu. Rev. Ecol. Syst. 25, 263-292.

Chambers, J.C., MacMahon, J.A., Haefner, J.H., 1991. Seed entrapment in alpine ecosystems: effects of soil particle size and diaspore morphology. Ecology 72, 1668-1677.

Coffin, D.P., Lauenroth, W.K., 1989. Spatial and temporal variation in the seed bank of a semiarid grassland. Am. J. Bot. 76, 53-58.

Crowley, G., Garnett, S., 1999. Seeds of the annual grasses Schiz-achyrium spp. as a food resource for tropical granivorous birds. Austral Ecol. 24, 208-220.

De Villiers, A.J., Van Rooyen, M.W., Theron, G.K., 2003. Similarity between the soil seed bank and the standing vegetation in the Strandveld Succulent Karoo, South Afriqa. Land. Degrad. Dev. 14, 527-540.

El-Bakry, A.A.H., 1982. Studies on plant life in the Cairo - Ismailia region. M. Sc. thesis. Faculty of Science, Cairo University, Egypt.

Esmailzadeh, O., Hosseini, S.M., Tabari, M., Baskin, C.C., Asadi, H., 2011. Persistent soil seed banks and floristic diversity in Fagus orientalis forest communities in the Hyrcanian vegetation region of Iran. Flora 206, 365-372.

Fenner, M., 1985. Seed Ecology. Chapman & Hall, London.

Funes, G., Basconcelo, S., Diaz, S., Cabido, M., 1999. Seed bank dynamics of Lachemilla pinnata (Rosaceae) in different plant communities of mountain grassland in central Argentina. Ann. Bot. Fenn. 36, 109-114.

Funes, G., Basconcelo, S., Diaz, S., Cabido, M., 2001. Edaphic patchiness influences grassland regeneration from the soil seed-bank in mountain grasslands of central Argentina. Austral Ecol. 26, 205-212.

Goodson, J.M., Gurnell, A.M., Angold, P.G., Morrissey, I.P., 2002. Riparian seed banks along the lower River Dove, UK: their structure and ecological implications. Geomorphology 47, 45-60.

Guo, Q., Rundel, P.W., Goodall, D.W., 1998. Horizontal and vertical distribution of desert seed banks: patterns, causes, and implications. J. Arid Environ. 38, 465-478.

Guo, Q., Rundel, P.W., Goodall, D.W., 1999. Structure of desert seed banks: comparisons across four North American deserts. J. Arid Environ. 42, 1-14.

Gutterman, Y., 1994. Strategies of seed dispersal and germination in plants inhabiting deserts. Bot. Rev. 60, 373-425.

Halassy, M., 2001. Possible role of the seed bank in the restoration of open sand grassland in old fields. Community Ecol. 2, 101-108.

Harper, J.L., 1977. Population Biology of Plants. Academic Press, London.

Hegazy, A.K., 1996. Effects of cement-kiln dust pollution on the vegetation and seed-bank species diversity in the Eastern Desert of Egypt. Environ. Conserv. 23, 249-258.

Hegazy, A.K., Lovett-Doust, J., Hammouda, O., Gomaa, N.H., 2007. Vegetation distribution along the altitudinal gradient in the northwestern Red Sea region. Community Ecol. 8 (2), 151-162.

Hill, M.O., 1979. DECORANA- A FORTRAN program for detr-ended correspondence analysis and reciprocal averaging. Cornell University, Ithaca, NY.

Hills, C.S., Morris., M.D., 1992. The function of seed banks in northern forest ecosystems: a literature review. Ontario Ministry of Natural Resources, Forest Research Information Paper No. 107, 1-25.

Jackson, M.L., 1958. Soil Chemical Analysis. Constable and Co. Ltd., London.

Jackson, M.L., 1967. Soil Chemical Analysis-Advanced Course. Washington Department of Soil Sciences, USA.

Jimenez, H.E., Armesto, J.J., 1992. Importance of the soil seed bank of disturbed sites in Chilean matorral in early secondary succession. J. Veg. Sci. 3, 579-586.

Kemp, P.R., 1989. Seed Banks and vegetation processes in deserts. In: Leck, M.A., Parker, V.T., Simpson, R.L. (Eds.), Ecology of Soil Seed Banks. Academic Press, San Diego, pp. 257-281.

Khan, M.A., 1993. Relationship of seed bank to plant distribution in saline arid communities. Pak. J. Bot. 25, 73-82.

Kitajima, K., Tilman, D., 1996. Seed banks and seedling establishment on an experimental productivity gradient. Oikos 76, 381-391.

Koontz, T.L., Simpson, H.L., 2010. The composition of seed banks on kangaroo rat (Dipodomys spectabilis) mounds in a Chihuahuan Desert grassland. J. Arid Environ. 74, 1156-1161.

Leckie, S., Velland, M., Bell, G., Waterway, M.J., Lechwicz, M.J., 2000. The seed bank in an old-growth, temperate deciduous forest. Can. J. Bot. 78, 181-192.

Marone, L., Horno, M.E., 1997. Seed reserves in the central Monte Desert, Argentina: implications for granivory. J. Arid Environ. 36, 661-670.

Marone, L., Rossi, B.E., Horno, M.E., 1998. Timing and spatial patterning of seed dispersal and redistribution in a South American warm desert. Plant Ecol. 137, 143-150.

Marone, L., Lopez de Casenave, J., Cueto, V.R., 2000. Granivory in southern South American deserts: conceptual issues and current evidence. Biosciences 50, 123-132.

McGraw, J.B., Vavrek, M.C., Bennington, C.C., 1991. Ecological genetics variation in seed banks. I. Establishment of a time transect. J. Ecol. 79, 617-625.

Meyer, S.E., Pendleton, B.K., 2005. Factors affecting seed germination and seedling establishment of a long-lived desert shrub (Coleogyne ramosissima: Rosaceae). Plant Ecol. 178, 171-187.

Ministry of Civil Aviation, 1975. Climatological Normals for the Arab Republic of Egypt up to 1975. Meteorological Authority, Cairo.

Mueller-Dombois, D., Ellenberg, H., 1974. Aims and Methods of Vegetation Ecology. Wiley, New York, NY.

Nathan, R., Muller-Landau, H.C., 2003. Spatial patterns of seed dispersal, their determinants and consequences for recruitment. Trends Ecol. Evol. 15, 278-285.

O'Connor, T.G., 1991. Local extinction in perennial grasslands: a life-history approach. Am. Nat. 137, 753-773.

Peco, B., Ortega, M., Levassor, C., 1998. Similarity between seed bank and vegetation in Mediterranean grassland: a predictive model. J. Veg. Sci. 9, 815-828.

Pielou, E.C., 1975. Ecological Diversity. Wiley, London.

Radosevich, S., Holt, J., Ghersa, C., 1997. Weed Ecology: Implications for Management, second ed. John Wiley & Sons, New York.

Raunkiaer, C., 1934. The Life Forms of Plants and Statistical Plant Geography. Oxford Univ Press, Oxford.

Rundel, P.W., Gibson, A.C., 1996. Ecological Communities and Processes in a Mojave Desert Ecosystem. Cambridge University, Cambridge.

Satterthwaite, W.H., Holl, K.D., Hayes, G.F., Barber, A.L., 2007. Seed banks in plant conservation: case study of Santa Cruz tarplant restoration. Biol. Conserv. 135, 57-66.

Shaukat, S.S., Siddiqui, I.A., 2004. Spatial pattern analysis of seeds of an arable soil seed bank and its relationship with above-ground vegetation in an arid region. J. Arid Environ. 57, 311-327.

Shehta, M.N., El-Fahar, R.A., 2000. The vegetation of reclaimed areas in Salhya region. Proceeding of the 1st International Conference on Biological Sciences, Faculty of Science, Tanta University 1, pp. 315-332.

Silvertown, J.W., 1981. Seed size, life span and germination date as co-adapted features of plant life history. Am. Nat. 118, 860-864.

Solomon, T.B., 2011. Soil seed bank dynamics in relation to land management and soil types in the semi-arid savannas of Swaziland. Afr. J. Agric. Res. 6, 2494-2505.

Strykstra, R.J., Bekker, R., Bakker, J.P., 1998. Assessment of dispersule availability: its practical use in restoration management. Acta Bot. Neerl. 47, 57-70.

Thompson, K., 2000. The functional ecology of soil seed banks. In: Fenner, M. (Ed.), Seeds: the Ecology of Regeneration in Plant Communities, second ed. CAB International, Wallingford, pp. 215-235.

Thompson, K., Grime, J.P., 1979. Seasonal variation in the seed banks of herbaceous species in ten contrasting habitats. J. Ecol. 67, 893-921.

Thompson, K., Bakker, J.P., Bekker, R.M., 1997. Soil Seed Banks of North West Europe: Methodology, Density and Longevity. Cambridge University Press, Cambridge.

Ungar, I.A., 1988. A significant seed bank for Spergularia marina (Caryophyllaceae). Ohio J. Sci. 88, 200-202.

Wassie, A., Teketay, D., 2006. Soil seed banks in church forests of northern Ethiopia: implications for the conservation of woody plants. Flora 201, 32-43.

Zaghloul, M.S., 2008. Diversity in soil seed bank of Sinai and implications for conservation and restoration. Afr. J. Environ. Sci. Techn. 2, 172-184.

Zhang, J.-T., 1995. Quantitative Methods in Vegetation Ecology. China Science and Technology Press, Beijing.