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Accueil du site → Doctorat → Royaume-Uni → 1996 → The distribution and ecophysiology of Acacia species in the south western zone of Saudi Arabia

University of Edinburgh (1996)

The distribution and ecophysiology of Acacia species in the south western zone of Saudi Arabia

Arefi, I.M

Titre : The distribution and ecophysiology of Acacia species in the south western zone of Saudi Arabia

Auteur : Arefi, I.M

Université de soutenance : University of Edinburgh

Grade  : Doctor of Philosophy (PhD) 1996

Résumé
The thesis is a description of the patterns and distribution of Acacia trees in Saudi Arabia and an evaluation of the morphological and physiological responses of seedlings to limitation in soil water availability. Species examined were Acacia ehrnbergiana Hayne, Acacia asak (Forssk). Willd, and Acacia negrii Pinchi-Sermoli. The objective was to investigate the environmental factors controlling the distribution of these species in natural communities in south western Saudi Arabia.Fifteen major community types constitute the major part of the natural vegetation of the study area and are dominated by the Acacia species : A. negrii, A. gerradii, A. asak, A. seyal, A. nubica, A. homulosa, A. ehrnbergiana, A. laeta, and A. tortilis. Also are Ziziphus spina-christi, Juniperus procera, Tamarix aphylla, Hyphaen thebaica, Olea europea and Ficus salificifolia. The form of trees in the region is controlled by rainfall and by factors affecting the distribution and the availability of water, such as topography and soil texture. Acacia species showed clear differences along these environmental gradients. Juniperus procera has the highest number of trees per site whilst Tamarix aphylla has the lowest density.In the laboratory, seeds may be treated to enhance germination. Seed scarified by abrasion or acid treatment, and seed exposed to a heat shock showed stimulated germination. The duration of exposure to moist conditions required to make the testa permeable and to initiate germination of the seeds was ranked as follows : A. ehrnbergiana > A. tortilis > A. seyal > A. asak > A. negrii. The effect of drought on water relations was studied on plants grown in long soil columns for Acacia negrii, A. ehrnbergiana and A. asak. Water deficit resulted in active osmotic adjustment in leaves, with decreases in osmotic potential at full and zero turgor, and increased bulk elastic modulus and leaf dry weight to turgid weight ratio. Stomatal conductance was correlated with soil water status.

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