الفهرس Only 14 pages are availabe for public view
 |  | from | 157 |  |  |
| | | from | 157 | | |
Abstract
The distribution of plant communities and the pattern of species diversity at both the standing vegetation and germinable seed bank levels were studied along an altitudinal gradient (0-2100 m a.s.l.) in the north western sector of the Red Sea. The standing vegetation was studied in 58 stands distributed along the altitudinal gradient, using ten quadrats (10 × 10 m) per stand. Soil samples for seed bank analysis were collected from the same stands used for the determination of the standing vegetation. The germinable seed bank was estimated by the seedling emergence method. The interspecific abundance-occupancy relationships were also determined for species constituting the above-ground vegetation, at both the whole landscape (the entire study area), and individual habitat levels. Moreover, the influence of altitude on plant population demography and dynamics was investigated by evaluating the age-specific survival, mortality and reproduction of Moringa peregrina (Forssk.) Fiori (Moringaceae) along its altitudinal range (550-1000 m a.s.l.) as a case study in the study area.
The application of the Two Way Indicator Species Analysis technique (TWINSPAN) on the standing vegetation and the germinable seed bank of the study area produced different groups corresponding to different altitudinal ranges. The variations in the standing vegetation and seed bank with elevation was also confirmed by the DCA ordination which indicates that the TWINSPAN groups of either the vegetation or seed bank are mainly distributed along an elevation gradient. Edaphic factors such as soil texture, CaCO3, organic carbon and electrical conductivity contribute to the distribution of plant communities of the standing vegetation. The results demonstrate also significant associations between these factors and the floristic composition of the seed bank.
A total of 66 species were recoded in the standing vegetation while some 46 species were present in the germinable seed bank. Of the 66 species recorded in the standing vegetation, only 43 species (65.2%) were present in the seed bank. Three species, Ifloga spicata, Reichardia tingitana and Rumex vesicarius were recorded only in the seed bank and not found in the standing vegetation. All the species that present only in the standing vegetation are perennials, whereas the species which recorded only in the seed bank are annuals. Motyka’s index indicates that the similarity between the seed bank and standing vegetation ranges from 12.5% to 91.7%. The similarity between the standing vegetation and seed bank approaches a U-shaped pattern along the elevation gradient in which the lowest values are mostly at elevations between 400 and 1000 m a.s.l. About 34.8% of the species that constitute the standing vegetation are vulnerable to elimination from the standing vegetation because they are not represented in the seed bank
The diversity of the established vegetation and the germinable seed bank (as measured by species richness and Shannon index) shows a hump-shaped pattern along the altitudinal gradient. This pattern reflects the variations in water supply along the elevation range where maximum diversity occurs generally at elevation between 550 and 800 m a.s.l. which correspond to the mountain bases with the highest water resources allover the study area. Beta diversity decreases with altitude at both the above-ground vegetation and seed bank levels.
The abundance-occupancy relationships are positive and highly significant at both the whole landscape, and individual habitat levels, but stronger at habitat level than at the level of the whole landscape. Niche-breadth was estimated as the number of habitats occupied regionally by a species, and was significantly related to both abundance and occupancy. Niche breadth explains just 10.1% of variation in abundance but some 56.2% of variation in occupancy. Using empirical data, we tested whether abundance-occupancy relationships diverged significantly from a theoretical null model. The relationships diverged significantly from the null model at both whole landscape and habitat levels. Applications of abundance-occupancy relationships for plant conservation show that about 36% of the species is at risk of extinction at least within the study area because they have only one occurrence in the area and their local abundance is very low.
The survivorship curves of M. peregrina populations approach Deevey type III, in which the highest mortalities occur in the early life stages. The populations are dominated by adult individuals and the seedling recruitment is extremely limited. The maintenance of M. peregrina populations is achieved mainly by persistence (in situ maintenance of established individual plants) more than by regeneration (the replacement of individuals by seedlings). The net reproductive rate (R0) ranges from 0.0023 to 0.0040. The intrinsic rate of increase per capita per year (r) varies between -0.121 and -0.081, suggesting that the populations are declining and their survival can not be ensured without conservation management.
Altitude contributes to variations in M. peregrina populations. Along the altitudinal range of M. peregrina, the plant density, seed output, average number of seeds per individual (bx), net reproductive rate (Ro), and intrinsic rate of increase (r) of populations decrease significantly with elevation. This can be related to the fact that the lower elevations along the altitudinal range of M. peregrina correspond to the mountain bases with more water supply. The phenological development of M. peregrina was delayed with increasing altitude. This seems to be a consequence of the lower water supply and lower temperatures at higher altitudes. The reduction in fecundity, net reproductive rate and intrinsic rate of increase with increasing altitude implies that populations at the end of the species elevation range are at high risk of extinction.
Based on the results of the present study, the following recommendations are essential for vegetation conservation and ecosystem management: (1) high priority for conservation of species with both low abundance and low occupancy; (2) plants having economic importance and the species which lack seed bank must receive much attention to save them from the risk of extinction; (3) soil seed bank can be used for restoration of the vegetation of the degraded sites at low and high altitudes where the similarity between the standing vegetation and seed bank is relatively high; (4) in situ conservation of M. peregrina populations, in particular those located at the highest elevations of the species altitudinal range; (5) since the populations of M. peregrina rely mainly on persistence for their survival more than on seedling recruitment, conservation efforts should focus on the protection of established individuals against human disturbances such as cutting and over-grazing; (6) encouraging botanical gardens and research institutes to maintain ex situ populations of M. peregrina; (7) protecting and encouraging traditional utilization of plant resources in ways compatible with their conservation and sustainable use; and (8) since the area is of great importance in terms of biodiversity and cultural heritage, it is advisable to investigate further its case as a candidate biosphere reserve.
The present study demonstrated that: (1) altitude contributes significantly to the distribution of plant communities at both the standing vegetation and germinable seed bank levels; (2) the diversity of the established vegetation and the germinable seed bank shows a hump-shaped pattern along the altitudinal gradient; (3) there are significant associations between edaphic factors and the floristic composition of the above-ground vegetation and seed bank; (4) the similarity between the standing vegetation and seed bank approaches a U-shaped pattern along the elevation gradient; (5) the abundance-occupancy relationship is positive and highly significant at both the whole landscape and individual habitat levels, but stronger at habitat level than overall across the entire regional landscape; (6) habitat niche-breadth explains just10.1% of variation in abundance but some 56.2% of variation in occupancy; (7) along the altitudinal range of M. peregrina, the plant density, seed output, average number of seeds per individual (bx), net reproductive rate (Ro), and intrinsic rate of increase (r) of populations, decrease significantly with elevation.