Vegetation Distribution on Mega Sand Dunes
in Nafud Ath Thuwayrat, Saudi Arabia
Abdulatif H. Al-Nafie
Department of Geography, Faculty of Social Sciences,
Imam Muhammad Ibn Saud Islamic University,
P.O. Box 5760, Riyadh, 11432, Saudi Arabia
Abstract
This paper examines the vegetation distribution on mega sand dunes in Nafud Ath Thuwayrat. Observations of the sand dune environment show that both sand dune and interdune areas have a variety of vegetation types that appear to be highly associated with the topography of the sand dune and the sand movement. The distribution patterns recorded reflect the availability of moisture and nutrition, along with the effects of sand movements and wind patterns. The presence of vegetation cover limits sand mobility by modifying wind flow, sheltering and protecting the land surface from strong winds, binding sand with roots, and intercepting sand particles, fines, clay and silt, which gather around their base. Key words: Mega sand dunes, Ath Thuwayrat, Saudi Arabia, Vegetation distribution
Introduction The presence and absence of vegetation cover on sand dunes can be attributed mainly to climate, the degree of sand mobility, and the destruction of plants by human activities.
Climate might affect the presence of vegetation cover, regardless of other factors. In areas with very little or no rainfall, vegetation cover might fail to appear, whatever the dune type or shape (Tsoar and Moller, 1986).Yet, the average amount of rainfall in the study area appears to be adequate and within the average of the range required to support vegetation cover (more than 50 mm) when other environmental requirements are favourable.
The density and nature of vegetation cover along the dune elevation gradient are determined by the degree of sand mobility and stability, which, in turn, are influenced by wind velocities and dunes (Ash and Wasson, 1983, Warren, 1988). Dune shape itself affects the rate of sand erosion and sand shifting. Areas that experience high wind velocity and, therefore, a high rate of erosion, show little or no vegetation at all for the following two reasons:
1- There is a continuous loss of sand moisture as a result of sand texture, which results in high percolation and low moisture tension. Active dune sand has the lowest moisture content at field capacity (Tsoar, 1990). Continuous sand movement also exposes the deeper moist layers to drying by wind and solar radiation, leading to further moisture loss.
2- Sand movement also results in the uncovering of roots of established plants, which kills them and prevents new plant germination.
It is known that all the major sand dune types react differently to sand erosion. Accordingly, the presence and density of vegetation might differ significantly from one type of dune to another.
Seif dunes are formed in areas that are dominated by bi-directional wind regimes, so that the slipface and windward of the dunes alternate seasonally or even diurnally under the influence of wind direction. The crests and near-crest areas are the seif dunes’ most active and driest parts, which mean that vegetation can not germinate or become established on these areas. In contrast, the plinths of the seif dunes are usually stable (Thomas and Tsoar, 1990). As a result, from the dunes’ base, plant coverage decreases upslope.
Transverse dunes are developed in areas that are dominated by unidirectional wind regimes, which results in continuously mobile sand that affords little or no chance for the development of plant cover. However, when winds are moderate and erosion is minimal, vegetation might be found in some parts of the dunes, including the crests and slip-faces.
The linear or longitudinal dunes that extend in a direction parallel to the direction of the prevailing wind are more stable and favour plant growth. The flanks of the ridges of these dunes are normally covered with vegetation though the long ridge tops might be bare. It is possible, however, that the whole dune might become partially vegetated and stabilised.
Although the climate in the study area is arid, about 90% of Nafud Ath Thuwayrat is predominantly composed of stable and semi-stable dune systems, which are capable of supporting sparse vegetation cover (Al-Nafie, 1995).
Active sands are limited to small patches on crests of the inactive and fixed mega dunes, so that large scale movement of the sand is very slow relative to other dune areas and types.
Human activities and interference constitute additional factors that play a major role in the absence of vegetation cover from some parts of Nafud Ath Thuwayrat. Land use history shows that, through many centuries, the cutting and uprooting of shrubs, as well as overgrazing, have led to the absence of vegetation cover over wide expanses of the study area where the vegetation cover used to be rich. The severe destruction of vegetation cover on sand dune slopes causes sand movement that leads to further destruction of vegetation. Clear examples of such destruction can be seen along the completely bare slopes of mega dunes that surround small human settlements scattered in the middle and northern of parts of Nafud Ath Thuwayrat.
Conclusion
It can be concluded that about 90% of Nafud Ath Thuwayrat is composed predominantly of stable and semi-stable mega dunes capable of supporting sparse vegetation cover. Vegetation is absent from the active parts of the mega sand dune as a result of sand mobility and the high rate of erosion, both of which affect the availability of moisture and destroys any plant species that could be established in these areas. As the rate of sand mobility increases, the chances of plant establishment and survival decline. Active sand areas are limited mainly to the crests and upper parts of inactive dunes. There appears to be little difference in vegetation cover between the leeward and the windward slopes of the dunes. However, the windward slopes usually have more vegetation cover, since they are much gentler than the leeward slopes, although, more systematic studies are needed in order to more precisely explain this difference.
Stipagrostis drarii is the most common plant present near mega sand dune crests in the leeward and the windward areas. Sometimes it is the only species found there. Calligonum comosum and Artemisia monosperma might be present in these areas but in a lower numbers. The intervening and interdunal sand-filled areas and gaps between the mega dunes are occupied by Scrophularia hypericifolia, Haloxylon salicornicum, and Rhanterium epapposum communities. In the stable or semi-stabilized lower and middle parts of the dunes, plant species appear to be less widely spaced and more abundant. Human activities also play a major role in the absence of vegetation from areas that can be or once were vegetated. Areas around settlements, bedouin camps, and along car routes and tracks are lacking in vegetation as a result of the destruction of plant species by biotic factors such as intense and prolonged overgrazing and fuel gathering.
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