Size Structure and Floristic Diversity of Acacia trees population in Taif Area, Saudi Arabia

Acacia trees are considered keystone species in many desert ecosystems and suffer from different anthropogenic effects. This study estimated the size structure of Acacia trees population in El-Taif area, which indicated that all the populations of Acacia present in El-Taif Area seems to be young as the proportion of small and medium individuals is greater than that of large individuals except the species Acacia albida. Absence of plant species under the canopy of Acacia trees may be due to the severe impact of grazing. In general, distribution of Acacia trees is controlled by physiographic features, and topographical irregularities. Phytosociologically, the area is inhabited by (79) species belonging to (59) genera and related to (26) families. the most characteristic family is Fabaceae (16 species) followed by Asteraceae (15 species). The life-form spectrum in the present study is characteristic of an arid desert region with the dominance of chamaephytes (43% of the recorded species) followed by phanerophytes (31%), therophytes (16%) and hemicryptophytes (10%). The preponderance of annuals and shrubs reflects a typical desert flora, where it is closely related to topography. Phytogeographically, the shrub layer is composed mainly of the Saharo-Arabian with a Sudano-Zambezian focus on distribution. Pure Mediterranean taxa are not represented in the therophyte and chamaephyte layers, whereas they are represented in the biand pluriregional taxa. This may be attributed to the fact that plants of the Saharo-Arabian region are good indicators for desert environmental conditions, while Mediterranean species represent more mesic environments.


Introduction
In arid lands, the population size class structure and dynamics are related to species density as an evidenced to the correlation between the two characteristics over the population growth history [1]. Following the cessation of grazing, increasing population density is accompanied by a high-rate plant growth, where individuals attain a plant size above the average [2]. The role of individuals in the population can be expressed by their size, height, breadth and biomass; as these characteristics usually express their survival and reproductive possibilities. Due to the presence of various growth modifying agents, tree size differences may occur between even aged individuals in the same growth stage. These differences become much more conspicuous if the population has uneven-aged individuals, where the individual's size and biomass change with age [2,3]. The world's fast pace of urbanization has exerted profound pressure on urban trees [4].
The demography of tree species populations, particularly when dealing with field data including density, spatial distribution, size or age classes, seedling establishment, and mortality rates will support species management and conservation efforts [5][6][7]. The change in population tree size is characteristic of the species having high growth rates and high fecundity. Usually, these species possess functional properties which may cause the elimination of neighbors, for example, by occupying wider niche space as a factor, plays a significant role in population structure and dynamics [8][9][10], where some tree species may attain within a short time, an absolute dominance over other phytocoenotic components, by increasing their number and spatial distribution.
Population size may play a significant role in influencing the dynamic of plant populations [11,12]. Size differences may be caused directly or through differences in growth rates due to age differences, genetic variation, heterogeneity of resources, herbivory, and competition [10]. Little studies have been carried out on the size structure of tree populations such as [13][14][15][16].
Published demographic studies on subtropical trees don't enable an evaluation of vegetation climax stage [17]. The populations of Acacia trees, one of the most important tree components of the desert wadis in the central region of Saudi Arabia area subjected to tremendous pressure from human impacts. Apart from human caused changes in the unprotected habitats, other biotic and abiotic factors may also play a role in the decline of the plant population. This study aims to analyze the floristic diversity and the population size structure of Acacia sp within El-Taif area, Saudi Arabia.

Materials and Methods
The study area Taif region lies south-east of Jiddah and the Holy City of Makkah and is situated in the central foothills of the western mountains at an altitude of approximately 2500 m above sea level (21°16`N -40°25`E) ( Figure 1). It is an important place for the people due to its scenic views and fertile valleys which support the growth of many fruits and vegetables. The geological units that outcrop in the Taif area from the

Floristic analysis
The collected plant specimens were identified and named [18][19][20]. Species life forms were determined depending upon the location of the regenerative buds and the shed parts during the unfavorable season [21]. A chronological analysis of the floristic categories species was made to assign the recorded species to World geographical groups [22,23].

Population size structure
Size distribution analysis of Acacia population using canopy cover, tree height and Diameter (Figure 3) shows presence of juveniles in some of Acacia species such as Acacia gerrardii var. gerrardii, Acacia tortilis, Acacia ehernbergiana, Acacia nubica, Acacia origena, Acacia hamulosa, Acacia asak and Acacia gerrardii subsp. negevensis, on the other hand it was absent in Acacia etabica, Acacia raddiana and Acacia albida. clear reduction in numbers of small and large categories and increasing in numbers of medium categories in Acacia johnwoodii and Acacia etabica. This pattern of distribution indicates the absence of recruitment to supplement the smallest size categories. In addition, human disturbance in the study area is responsible for high mortality, which is clear from the decline numbers of small and large trees. The relationships between the individual heights and diameters of Acacia trees population are simple linear (Figure 4). The result shows that the relation between height and diameter of trees was positive in all Acacia species except Acacia gerrardi (ger) and Acacia tortilis was negative. On the other hand, ( Table 2) shows that the mean height to diameter ratio for both Acacia species was the highest in Acacia origena (60.0) and the lowest in Acacia hamulosa (1.6). Regarding the variation in the canopy size was the highest in Acacia albida (204.4) and the lowest in Acacia origena (13.6). Also, the height has the highest value (17.8 m) in Acacia albida and lowest (3.31 and 3.32 m) Acacia nubica and Acacia hamulosa, respectively. The diameter has the highest value (2.07 m) in Acacia hamulosa and lowest (0.1 m) in Acacia origena.

Discussion
The area is inhabited by (79) species belonging to (59) genera and related to (26) families. the most characteristic families are Fabaceae and Asteraceae (15 species, each). The life-form spectrum in the present study is characteristic of an arid desert region with the dominance of chamaephytes (43% of the recorded species) followed by phanerophytes (31%), therophytes (16%) and hemicryptophytes (10%). The preponderance of annuals and shrubs reflects a typical desert flora, where it is closely related to topography [26,27]. On the other hand, they may be a response to the hot, dry climate and human and animal interferences. A comparison of the life-form spectra of the northern part of the Eastern Desert of Egypt [28] and those in the Tihama coastal plains of the Jizan region in south-western Saudi Arabia [29] showed the same results.
Phytogeographically, the shrub layer is composed mainly of the Saharo-Arabian with a Sudano-Zambezian focus on distribution. Pure Mediterranean taxa are not represented in the therophyte and chamaephyte layers, whereas they are represented in the bi-and pluriregional taxa. This may be attributed to the fact that plants of the Saharo-Arabian region are good indicators for desert environmental conditions, while Mediterranean species represent more mesic environments.
The results of the size distribution study indicated that all population of Acacia present in El-Taif Area seems to be young as the proportion of small and medium individuals is greater than that of large individuals except the species Acacia albida.
Acacia trees are considered keystone species in many desert ecosystems [30,31], due to the many provided services and goods, Thus, the conservation of Acacia trees is important particularly in arid and hyper arid deserts for regulating microclimate, improving conditions for survival of associated plant and animal species [32], providing direct benefits for local inhabitants, and for starting sustainable development [33]. Due to aridity and anthropogenic disturbances, floristic diversity of the study area is characterized by a paucity of trees and annuals. The canopy of Acacia shows negative influence on the understory vegetation. Most of the associated species were recorded between canopies.
The absence of plant species under the canopy of Acacia trees may be due to the severe impact of grazing. In no disturbed areas, [13] reported that A. tortilis has a positive influence on the understory herbaceous vegetation. In general, distribution of Acacia trees is controlled by physiographic features, and topographical irregularities, which all act through modifying the amount of soil water availability [34].
The height/diameter ratio gives an idea about the growth habit of the plant. In the present study, this ratio is less than unity for Acacia species which means that the individual diameter exceeds, the relation between height and diameter of trees was positive in all Acacia species except Acacia gerrardi and Acacia tortilis was negative tend to the twospecies adapted to escarp from drought seasons. This may be a strategy of the desert plants to provide safe sites for their self-regeneration, as the horizontal expansion usually provides shade, which leads to decrease the severe heating effect and increase the soil moisture [35,36].
In conclusion, the total size structure of Acacia populations in El-Taif area is characterized by the preponderance of the young individuals comparing with the old ones, that we found continuously regeneration of most of Acacia population except one species named: Acacia albida which suffer from unregeneration due to some anthropogenic effects in the region.