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Accueil du site → Doctorat → États-Unis → 1981 → VEGETATION TYPES OF SEMI-ARID RANGELANDS IN NORTHWESTERN LIBYA, NORTH AFRICA

University of Wyoming (1981)

VEGETATION TYPES OF SEMI-ARID RANGELANDS IN NORTHWESTERN LIBYA, NORTH AFRICA

SHAWESH, OTHMAN MOHAMED

Titre : VEGETATION TYPES OF SEMI-ARID RANGELANDS IN NORTHWESTERN LIBYA, NORTH AFRICA

Auteur : SHAWESH, OTHMAN MOHAMED

Université de soutenance : University of Wyoming

Grade : Doctor of Philosophy (PhD) 1981

Résumé
A study was conducted on the Assa Range Management Research Area in the semi-arid rangeland of northwestern Libya in 1976 to obtain quantitative data on the composition and structure of the vegetation types, obtain information on the soil associated with the major vegetation types, obtain quantitative data on the interrelationships of the vegetation and soil properties and determine and evaluate the soil factors that significantly affected the various vegetation parameters. Soil physical and chemical characteristics are reported and discussed in relation to height, diameter and density of perennial plants and frequency and percent cover of all vegetation life forms. Six plant communities representing most of the major vegetation and edaphic variations were recognized on the basis of changes in the floristic composition, relative dominance of leading species and changes in the edaphic features. These were identified as follows : the Artemisia campestris community, the Thymelaea-Helianthemum-Echiochilon community, the Lygeum-Artemisia community, the Suaeda-Halocnemum community, the sand-dune areas of broken relief community and the blowout pavement wind eroded community. Each community was found to be associated with distinct groups of major species showing presence values and ecological amplitude characteristics different from the others. Step-wise multiple regression analysis showed that 76.5% of the variation of the annual forb cover was derived from the relationship to only three independent soil variables. These were organic matter, phosphorus and manganese. Sand, total carbonate and manganese explained 77.6% of the variation in annual grass cover. Clay, phosphorus and manganese accounted for 76.0% of the variation in perennial grass cover. Total carbonate and calcium were apparently the most important soil factors affecting shrub cover. Judging by the value of R(’2), 74.7% of the variation in shrub cover was explained by the amount of calcium and total carbonate. Of the 18 soil factors measured, sand, moisture equivalent and electrical conductivity were the only factors found by step-wise multiple regression analysis to have no significant effects on the dominant species studied in this investigation. These species were : Artemisia campestris, Thymelaea microphylla, Lygeum spartum, Echiochilon fruticosum, Heliantheum lippii and Suaeda vermiculata. Many plant species and vegetative life forms showed different responses to various soil factors. Soil properties which influence the moisture regime, soil structure and fertility showed a significant effect on the growth and distribution of the plants. Soil factors were also found to have interacting relationships, which means that any effect of a factor may be related to differing levels of other factors. Silt, clay, organic matter, organic carbon and salt content of the soil are correlated closely with moisture holding capacity. The moisture regime of the soils was influenced by such factors as an increase in organic matter content and the amount of fine material (silt and clay). Either or both of these factors improve the physical, chemical and water holding capacity of the soil, especially in the sandy area in which the average percentage of sand ranged from 83.45 to 96.49%. Organic matter and organic carbon not only increased water holding capacity of the soil, but also encouraged soil granulation and provides plants with nutrients such as nitrogen.

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