Size structure and dynamics of some woody perennials along elevation gradient in Wadi Gimal, Red Sea coast of Egypt

The population structure of 10 common woody perennials was investigated in terms of size distribution, height, diameter and density in Wadi Gimal along the Red Sea coast of Egypt. It was attempted to assess the effect of elevation on the size, distribution and density of the studied species. These species are: five trees (Acacia tortilis subsp. raddiana, Acacia tortilis subsp. tortilis, Balanites aegyptiaca, Tamarix aphylla, and Tamarix nilotica), two shrubs (Leptadenia pyrotechnica and Nitraria retusa) and three shrublets (Pulicaria undulata, Zilla spinosa, and Zygophyllum coccineum). The size estimations were then used to classify population into six size classes: 20-80 cm for shrublets, 100-500 cm for shrubs, and 2-10 m for trees. The absolute and relative frequency of individuals and mean height, diameter and height to diameter ratio per individual in each size class were determined. Density of occurrence of most species, except B. aegyptiaca, decreased as elevation increased. The height-to-diameter ratio was less than unity for most of the recorded species except T. nilotica. Several forms - including, positively and negatively skewed, inverse J-shaped, bell shaped and more or less J-shaped distributions - were recognized along the different elevations. The size structure of some species was positively related with soil variables, such as T. nilotica with sulphate, while some others were negatively significant related to the substrate characteristics, such as Z. spinosa with salinity.