GSA QUATERNARY GEOLOGY AND GEOMORPHOLOGY DIVISION FAROUK EL-BAZ AWARD FOR DESERT RESEARCH: ECO-GEOMORPHIC PROCESSES ACTING ON DESERT SAND DUNES

About 20% of the world's arid zones are covered by aeolian sand. Half of these areas are partially covered by vegetation. We will question the geomorphic causes and reasons for the appearance and default of plants on sand dunes in deserts worldwide.

Practically, dune sand is inert soil considered to be devoid of any positive ecological characteristics. Sand is distinguished by low field capacity, low value for soil moisture availability for plants, and a high rate of permeability and leaching, resulting in the washing away of nutrient elements necessary for plant growth. However, these negative characteristics are an advantage in sandy deserts resulting in richer biomass compared to other soil types such as loess. Stabilized dunes in deserts are covered by filamentous cyanobacteria that form crust on the dunes' surface to further fix the dunes.

Erosion of sand, more than lack of moisture, is the major limiting factor for vegetation growth on dunes. Hence, there are two conditions for vegetation growth on dunes in deserts: 1. Average annual rainfall above 80 – 100 mm. 2. Low rate of wind erosion.

The rate of erosion of sand on dunes depends on the mechanism of mobility. For barchan and transverse dunes the windward slope is under the erosion of sand. The lee slope is under the deposition of sand. Accordingly, on the crest there is only sand transport with no erosion or deposition of sand. Therefore, vegetation may exist on the crest of barchan and transverse dunes. It may also be found on the slip-face, when the rate of deposition is low. On linear dunes that elongate, the crest is the dune's most active part. Vegetation cannot cling there or on the slopes, which are undergoing erosion. Vegetation is therefore found at the dune's lower slope plinth and in the interdune area.