introduction - Ecologia Mediterranea

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Size structure and dynamics of the common shrubs in Omayed Biosphere Reserve in the western Mediterranean coast of Egypt Structure et dynamique d’arbustes communs de la Réserve de Biosphère d’Omayed, côte méditerranéenne occidentale de l’Égypte Yassin M. Al-Sodany Biological and Geological Department, Faculty of Education at Kafr El-Sheikh, Tanta University, Kafr El-Sheikh, Egypt. Abstract The population structure of 15 common shrubby species of the Omayed Biosphere Reserve (Egypt) was evaluated in terms of size distribution. These species are 3 phanerophytes (Lycium europaeum, Phlomis floccosa and Thymelaea hirsuta) and 12 chamaephytes (Anabasis articulata, Artemisia herba-alba, Artemisia monosperma, Arthrocnemum macrostachyum Deverra tortuosa, Echiochilon fruticosum, Gymnocarpos decander, Haloc-nemum strobilaceum, Helianthemum lippii, Limoniastrum monopetalum, Noaea mucronata and Suaeda vera). The size estimations were used to classify the population of each species into 9 size classes: the first and second classes were chosen to represent the seedling and juvenile stages. The absolute and relative frequency of individuals and mean height, diameter and height to diameter ratio per individual in each size class were also determined. The present study indicated the size structures of the investigated species were belonged to 6 types of size distributions: more or less inverse J-shaped distribution (e.g. Halocnemum strobilaceum, Echio-chilon fruticosum and Helianthemum lippii), positively skewed distributions towards the small (i.e. young) individuals (e.g. Lycium europaeum, Arthrocnemum macrostachyum and Anabasis articulata), more or less J-shaped distribution (e.g. Noaea mucronata and Artemisia monosperma), approximately symmetrical (i.e. bell shaped) distribution (Phlomis floccosa), bimodal size distribution (e.g. Thymelaea hirsuta, Limoniastrum monopetalum and Suaeda vera) and more or less stationary size distribution (e.g. Gymnocarpos decander, Deverra tortuosa and Artemisia herbaalba). Key-words Population dynamic, population structure, distribution types, desert, juvenile, competition, regeneration. ecologia mediterranea, tome 29, fascicule 1, 2003, p. 39-48 Résumé La structure des populations de 15 arbustes communs de la Réserve de Biosphère d’Omayed (Égypte) a été évaluée en terme de distribution de taille. Les espèces étudiées sont 3 phanérophytes (Lycium europaeum, Phlomis floccosa et Thymelaea hirsuta) et 12 chamaephytes (Anabasis articulata, Artemisia herba-alba, Artemisia monosperma, Arthrocnemum macrostachyum, Deverra tortuosa, Echiochilon fruticosum, Gymnocarpos decander, Halocnemum strobilaceum, Helianthemum lippii, Limoniastrum monopetalum, Noaea mucronata et Suaeda vera). Les estimations de taille ont permis de classer les populations de chaque espèce dans 9 classes de taille ; les première et deuxième classes ont été choisies pour représenter les plantules et les étapes juvéniles de ces arbustes. La fréquence absolue et relative des individus et de leur taille ainsi que leur diamètre et le rapport de la taille moyenne sur le diamètre par individu ont été également déterminés dans chaque classe de taille. Cette étude a permis de mettre en évidence différentes structures de taille appartenant à 6 types de distributions : (i) proche d’une distribution “J” inverse (par exemple Halocnemum strobilaceum, Echiochilon fruticosum et Helianthemum lippii), (ii) distribution modifiée comparativement à une distribution “J” formée d’individus juvéniles (par exemple Lycium europaeum, Arthrocnemum macrostachyum et Anabasis articulata), (iii) proche d’une distribution symétrique (par exemple Noaea mucronata et Artemisia monosperma), (iv) distribution “en cloche” (par exemple Phlomis floccosa), (v) distribution bimodale (par exemple Thymelaea hirsuta, Limoniastrum monopetalum et Suaeda vera) et (vi) proche d’une distribution stationnaire (par exemple Gymnocarpos decander, Deverra tortuosa et Artemisia herba-alba). Mots-clés Dynamique des populations, structure des populations, types de distribution, désert, juvéniles, compétition, régénération. 39
40 u Y. M. AL-SODANY INTRODUCTION The natural vegetation in the desert has to adapt to growth conditions which are characterized by high summer temperature that coincides with shortage of water with the alternation of more humid winter period (El- Kenany, 1995). Desert communities are especially sensitive to natural and human interventions. Many shrubs and trees are of structural and economic importance in the arid regions (Crisp & Lange, 1976).They play an important role in soil protection and stabilization against movement by wind or water, provide a source of forage for animals and fuel for local inhabitants, and have medicinal and potential industrial values (Thalen, 1979). The structure of a population of plants can be described in terms of ages, sizes and forms of the individuals that compose it (Harper & White, 1974). Since the fecundity and survival of plants is often much more closely related to size than to age (Harper, 1977; Watkinson & White, 1985; Caswell, 1986;Weiner, 1986; Shaltout & Ayyad, 1988), some authors (e.g. Werner & Caswell, 1977; Kirkpatrick, 1984; Caswell, 1986) argued that it is better to classify the life history of plants by size rather than age which is the most widely used classification for unitary organisms. Size differences may be caused directly or through differences in growth rates due to age differences, genetic variation, heterogeneity of resources, herbivory, and competition (Weiner, 1985). The studies of Ayyad (1976), Abdel Razik (1976), Shaltout (1983), Abdel Razik (1988), Shaltout (1985), Shaltout and Ayyad (1994), and Ayyad and Fakhry (1996) are examples for the studies dealt with the vegetation analysis and species diversity of the western Mediterranean desert in general, and the Omayed Biosphere Reserve in particular.The studies like that of El-Kady (1980), Ayyad and El-Kady (1982), Shaltout and El-Ghareeb (1985) and Abdel Razik et al. (1988a, b) dealt with the effect of protection and controlled grazing on the primary productivity of the ecosystem of non-saline depressions in this region. Other studies dealt with the vegetation and landuse using aerial photographs (e.g. Ayyad & Le Floc’h, 1983; Hammouda, 1988) or remote sensing (e.g. Salem, 1989; El-Kenany, 1995). Little studies has been carried out on the size structure of the shrubby populations in Egypt, in general, and in the western Mediterranean desert, in particular. The study of Shaltout and Ayyad (1988) on the structure and standing crop of Egyptian Thymelaea hirsuta populations could be considered the pilot study in this domain. The present study aims at studying the size structure and dynamics of 15 common shrubby populations, in terms of height, diameter, volume and density, in different habitats in Omayed Biosphere Reserve in the western Mediterranean desert of Egypt.These species are: 3 phanerophytes (Lycium europaeum, Phlomis floccosa and Thymelaea hirsuta) and 12 chamaephytes (Anabasis articulata, Artemisia herba-alba, Artemisia monosperma, Arthrocnemum macrostachyum Deverra tortuosa,Echiochilon fruticosum, Gymnocarpos decander, Halocnemum strobilaceum,Helianthemum lippii,Limoniastrum monopetalum and Noaea mucronata and Suaeda vera). STUDY AREA The study area, the Omayed Biosphere Reserve, is located in the western Mediterranean coastal region of Egypt, 80 km west of Alexandria. Its N-S landscape is differentiated into a northern coastal plain and a southern inland plateau.The coastal plain is characterized by alternating ridges and depressions running parallel to the coast in E-W direction. This physiographic variation leads to the distinction of six types of habitats.These habitats are more or less arranged in the same sequence from the northern Mediterranean coast to the south as follows (fig. 1): coastal ridge which composed mainly of snow-white oolitic calcareous rocks, and overlain by dunes; saline depressions with brackish water and saline calcareous deposits (i.e. salt marshes); non-saline depressions with a mixture of calcareous and siliceous deposits of deep loess; inland ridges which formed of limestone with a hard crystallized crust, and less calcareous than the coastal ridge; inland plateau that characterized by an extensive flat rocky surface and shallow soil; and inland siliceous deposits, sporadically distributed on the inland plateau and occasionally forming dunes, specially in more inland sites. Climate of this region belongs to the warm coastal desert climate (Meigs, 1973): the warmest summer month (August) has a mean temperature less than 30 °C, and the coldest winter month (January) has a mean temperature above 10 °C; occasional short rainstorms occur mainly in winter (see also Daget, 1977). The ratio of annual precipitation to annual evaporation is between 0.03 and 0.2 (UNESCO, 1977).The comparison of metereological records in two stations, one near the Mediterranean coast (Burg El-Arab) and the other about 40 km to the south (Dammanhur), demonstrates the N-S climatic gradient in this region (fig. 2). These records indicate the increase in environmental aridity and thermal continentality from the north to the south. ecologia mediterranea, tome 29, fascicule 1, 2003
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