Distributional Pattern of Thymelaea hirsuta (L.) Endl. and Associated Species Along the Mediterranean Coast of Egypt

Abstract

This study is a quantitative analysis of the distributional pattern of Thymelaea hirsuta (L.) Endl. and its associated species and the relationships of this pattern to environmental variables. Species abundance was estimated in 32 stands spanning most of the apparent physiographic and edaphic variations in a 0.8-million-hectare area in the northeastern sector of the western desert of Egypt. Cluster analysis of the vegetation led to the recognition of nine clusters often comparable to communities previously defined in the study area. These clusters are distinguished along environmental gradients of decreasing calcium carbonate and increasing salinity and exchangeable cations and soluble anions. Sand, silt, and clay soil fractions exert some influence on vegetational changes but at comparatively low levels of statistical significance. On the axes of the "Wisconsin School" ordination and to a lesser extent on the axes of the principal component analysis, we distinguished four sets of stands: a central set of the T. hirsuta cluster, with environmental conditions intermediate in all respects, and three peripheral sets reflecting variations in soil characteristics. The first peripheral set includes the grouping of Euphorbia paralias L. and Echinops spinosissimus Turra, characterizing the calcareous dune soils. The second set, comprising the groupings of Stipagrostis siliata L., Anabasis articulata (Forssk.) Moq., Artemisia monosperma Del., and Artemisia judaica L., reflects the nature of a vegetation associated with high environmental aridity and in most cases with silicious sandy soils. The third set, including the groupings of Asphodelus microcarpus Salzm. & Viv. and Limoniastrum monopetalum (L.) Boiss.-Plantago albicans L., characterizes the vegetation of the littoral plains and depressions which have the least soil moisture stress and in certain cases high salt content.