introduction - Ecologia Mediterranea

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size structure of the sampled species in order to avoid extrapolation. As many woodlands are heterogenous, misleading results may be obtained if small samples are measured (Shaltout & El-Ghareeb, 1985; Shaltout & Ayyad, 1990). Size differences in plant populations may be caused directly or through differences in growth rates due to age difference, genetic variation, heterogeneity of resources, herbivory, and competition (Weiner, 1985). In the present study, some species had inverse J-shaped distribution (Halocnemum strobilaceum,Echiochilon fruticosum and Helianthemum lippii) or positively skewed (Lycium euro- ecologia mediterranea, tome 29, fascicule 1, 2003, p. 39-48 SIZE STRUCTURE OF THE COMMON SHRUBS IN OMAYED BIOSPHERE RESERVE… u Fig 3b. Size-frequency distribution for the populations of 7 shrublets in Omayed Biosphere Reserve. The ranges of size classes are: 1: 30 dm 3. paeum, Arthrocnemum macrostachyum and Anabasis articulata) size distributions towards the small (i.e. young) individuals. These may represent rapidly growing populations with high reproductive capacity. Such distributions may indicate also a high juvenile mortality (Harper, 1977), but nevertheless, they seem to represent long-term stability, since in most stable populations one would except an excess of juvenile over mature individuals (Crisp & Lange, 1976; Goldberg & Turner, 1986; Shaltout & Ayyad, 1988). Moreover, some of these species (Echiochilon fruticosum and Helianthemum lippii) were indicated as the most palatable in the study area, where Helianthemum lippii contri- 45
46 u Y. M. AL-SODANY buted 19 % and Echiochilon fruticosum 13 % to the diet of grazing animals (Abed-Razik et al. 1988a, b).The commulative effect of grazing is evidenced by signs such as pale unnatural color of herbage, reduced height of growth and reduced volume and biomass of individual plant and plant life per unit area. Furthermore, Gray (1975) reported that the positively skewed distribution is indicative of a self-perpetuating species, with markedly more frequency of the smaller (younger) size classes. Similar conclusion was made by Shaltout and Ayyad (1988). The bell-shaped size distribution of Phlomis floccosa indicated comparable representation of the juvenile and mature individuals. If current situation continues, a reduction in population size of this species is expected in the future. Similar results were reported by Shaltout and Mady (1993) in their study on the size distribution of Lycium shawii in Central Saudi Arabia.The J-shaped distribution of Noaea mucronata and Artemisia monosperma indicated the dominance of mature individuals over the juvenile ones. This distribution characterizes a declining populations, because the population has a large proportion of larger individuals than smaller ones (i.e. limited regeneration capacity).This may indicate that the recruitment of this species is rare which may be related to hyperaridity and low fertility. Also the soil of the inland plateau and inland siliceous deposits, in which these species are dominant, had the lowest contents of almost soil variables (Shaltout, 1983). Bimodal size distribution may result from initially unimodal size distribution when there is discontinuous variation in exponential growth rates among individuals. Normally distributed variation in exponential growth rates will not produce bimodality. Sources of discontinuous variation may be genetic and/or environmental heterogeneity, or dominance-and- suppression competition. Such competition may be considered asymmetric because the resulting negative effects are experienced only by the smaller plants (Huston, 1986). In the present study, three species had a bimodal size distribution (Thymelaea hirsuta, Limoniastrum monopetalum and Suaeda vera). These species contribute more than 77 % of the total plant cover in this area (Shaltout & El-Ghareeb, 1992) which leads to dominance- and- suppression competition among individuals in which large plants preempt resources such as water in deserts and suppress the growth of small plants. Also, Shaltout and Ayyad (1988) described a discontinuous plant size distribution in Thymelaea hirsuta and attributed this to the annual variability of precipitation, which may cause variable input of new seedling to the population each year. as a further support for this, El- Keblawy et al. (1997) reported that the population growth rates of this species varied significantly by habitat and over time. The height/diameter ratio gives an idea about the growth habit of the plant. In the present study, this ratio is less than unity for most species which means that the individual diameter exceeds, in average, its height and hence individuals of these species tends to expand horizontally rather than vertically. This may be a strategy of the desert trees and shrubs in order to provide safe sites for their self-regeneration, as the horizontal expansion usually provides shade, which leads to decrease the severe heating effect and increase the soil moisture (Shaltout & Mady, 1993). Such conclusion needs further verification on several desert woody populations in different regions. Artemisia monosperma in the area of the present study has the highest average density than the other species. Although this species was recorded in the earlier studies in this region (Abel-Razik, 1976; Shaltout, 1983), but it was not so dominant as in the present days. It usually inhabits the deep semi-stabilized sand that has been accumulated in the study area due to some natural and manmade activities. The recent man-made activities in this region include the digging of new canal which lead to move a huge amount of sand that become a fresh start for the population of Artemisia monosperma in this region. Other activities include the annual multinational maneuver which lead to disturb the stabilized sandy formations in the study area. CONCLUSION In Omayed Biosphere Reserve, the population of Artemisia monosperma has the highest density in the habitat of inland siliceous deposits and non-saline depressions, while the population of Phlomis floccosa had the lowest density in the habitat of inland ridges. Six types of size distributions of the investigated species were indicated in the present study: more or less inverse J-shaped, positively skewed distributions towards the small (i.e. young) individuals, more or less J-shaped, approximately symmetrical (i.e. bell shaped), bimodal and more or less stationary size distributions. The study area needs further studies for assessing the regeneration capacity of the populations of some noteworthy, rare and threatened species in terms of natality, mortality, survival and growth rates. ecologia mediterranea, tome 29, fascicule 1, 2003
Page 1 and 2: Tome 29 Fascicule 1, 2003 ISSN 0153
Page 3 and 4: Rédacteur en chef • Managing edi
Page 5 and 6: Ce fascicule a bénéficié du sout
Page 7 and 8: 6 u G. GARFÌ, S.FICARROTTA INTRODU
Page 9 and 10: 8 u G. GARFÌ, S.FICARROTTA The inv
Page 11 and 12: 10 u G. GARFÌ, S.FICARROTTA The me
Page 13 and 14: 12 u G. GARFÌ, S.FICARROTTA other
Page 15 and 16: 14 u G. GARFÌ, S.FICARROTTA FRITTS
Page 17 and 18: 16 u T. LA MANTIA, S.CULLOTTA & G.
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Page 89 and 90: 88 u MOHAMED FENNANE INTRODUCTION L
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Page 93 and 94: 92 u MOHAMED FENNANE Barbéro, Bena
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96 u MOHAMED FENNANE Au Moyen Atlas
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98 u MOHAMED FENNANE * Frankenio la
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100 u MOHAMED FENNANE * Spartinetum
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102 u MOHAMED FENNANE précisées.
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104 u MOHAMED FENNANE KAABECHE M.,
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106 u MOHAMED FENNANE TÜXEN R. & E
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108 u FAITS DE CONSERVATION EN MÉD
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116 Perspectives pour une géobiolo
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118 u ANALYSES D’OUVRAGES recensa
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120 Ecologia Mediterranea publishes
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introduction - Ecologia Mediterranea

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