Bare Dunes Research Articles

LONG-DISTANCE WANDERING AND MATING BY THE DANCING WHITE LADY SPIDER (LEUCORCHESTRIS ARENICOLA) (ARANEAE, SPARASSIDAE) ACROSS NAMIB DUNES

Adult males of the Dancing White Lady Spider (Leucorchestris arenicola, Araneae, Sparassidae) occurring in the dunes of the Namib Desert, Namibia, frequently wander far out of their 3m radius territories on dark nights. They move across bare dune slopes in search of mating opportunities and subsequently return to their burrows. In the current study, I describe the long-distance movements and navigational ability of males and examine how their wandering behavior relates to mating and interactions with other males. In 16 observed complete excursions, male spiders walked 51 m (median, range 16–91 m) from their burrow along a path of 134 m (42–314 m). The return path was shorter than the outgoing path, had less than ⅛ as many turns, and rarely retraced the outgoing path. Typically, the return path across open terrain had a straight section (median 33 m, range 10–89 m) which was directed towards the home burrow with a maximum angle of deviation of 5°. Males crossed 0–5 territories of adult males and as many female territories, mating in about half of the encounters with females. Males avoided each other and signaled with intense sand drumming. Adult males differ in size and there are indications that they compete with each other for mates by long-distance movements, drum-signaling each other, and interfering with mating. During three years of observations of a L. arenicola population, 8% of the largest males did 51% of the mating. Spiders of both sexes were promiscuous, and individuals mated with each other on several occasions. The current study prompts future investigations concerning male orientation and its neurophysiological basis, their ability to locate females, as well as the inter- and intrasexual relationships of L. arenicola.

Aeolian processes across transverse dunes. I: Modelling the air flow

This paper discusses a two-dimensional second-order closure model simulating air flow and turbulence across transverse dunes. Input parameters are upwind wind speed, topography of the dune ridge and surface roughness distribution over the ridge. The most important output is the distribution of the friction velocity over the surface. This model is dynamically linked to a model that calculates sand transport rates and the resulting changes in elevation. The sand transport model is discussed in a separate paper. The simulated wind speeds resemble patterns observed during field experiments. Despite the increased wind speed over the crest, the friction velocity at the crest of a bare dune is reduced compared to the upstream value, because of the effect of stream line curvature on turbulence. These curvature effects explain why desert dunes can grow in height. In order to obtain realistic predictions of friction velocity it was essential to include equations for the turbulent variables in the model. In these equations streamline curvature is an important parameter. The main flaw of the model is that it cannot deal with flow separation and the resulting recirculation vortex. As a result, the increase of the wind speed and friction velocity after a steep dune or a slipface will be too close to the dune foot. In the sand transport model this was overcome by defining a separation zone. Copyright © 1999 John Wiley & Sons, Ltd.

Aeolian processes across transverse dunes. I: Modelling the air flow

This paper discusses a two-dimensional second-order closure model simulating air flow and turbulence across transverse dunes. Input parameters are upwind wind speed, topography of the dune ridge and surface roughness distribution over the ridge. The most important output is the distribution of the friction velocity over the surface. This model is dynamically linked to a model that calculates sand transport rates and the resulting changes in elevation. The sand transport model is discussed in a separate paper. The simulated wind speeds resemble patterns observed during field experiments. Despite the increased wind speed over the crest, the friction velocity at the crest of a bare dune is reduced compared to the upstream value, because of the effect of stream line curvature on turbulence. These curvature effects explain why desert dunes can grow in height. In order to obtain realistic predictions of friction velocity it was essential to include equations for the turbulent variables in the model. In these equations streamline curvature is an important parameter. The main flaw of the model is that it cannot deal with flow separation and the resulting recirculation vortex. As a result, the increase of the wind speed and friction velocity after a steep dune or a slipface will be too close to the dune foot. In the sand transport model this was overcome by defining a separation zone. Copyright © 1999 John Wiley & Sons, Ltd.

Soil moisture and dry bulk density patterns in bare dune sands

Recent research has shown that irregular wetting and preferential flow may occur in heterogeneous field soils. Bare dune sands, which at first sight show the greatest homogeneity, were sampled volumetrically at nine locations in temperate and arid areas to determine the spatial distribution of soil moisture content, θ, and dry bulk density. Soil moisture patterns were irregular, even in extremely dry situations. As κ - θ functions for dune sands are very steep in the dry θ range, small differences in water content result in huge differences in hydraulic conductivity, κ. These differences, induced by soil moisture variability, may result in anisotropy of the system, and hysteresis tends to magnify this phenomenon. So, if soil moisture differences exist, they will be consolidated and even amplified during new rain events. At one site on a recently deposited dune sand, infiltrating water was able to transform a one-layer soil system into a two-layer soil system, inducing preferential flow at the layer interface. At another site, preferential flow paths appeared at the soil surface as ‘sand columns’ as a result of the displacement of the dry sand between the flow paths, caused by wind erosion. In all the dunes studied, dry bulk density patterns were irregular and tended to be slightly vertically orientated. Spatial distributions of volumetric and gravimetric soil water content showed great similarity.

Mesquite (Prosopis glandulosa Torr.) dunes and interdunes in southern New Mexico: a study of soil properties and soil water relations

Soil properties and soil water relations were compared between soils of mesquite (Prosopis glandulosa Torr.) dunes and soils of adjacent interdune areas in southern New Mexico. Particle size, bulk density, organic matter content, electrical conductivity, pH, and temperature of dunal and interdunal soils were measured. Except for particle size and temperature, dunal and interdunal soils did not differ greatly. Soils of dunes had more sand and less silt and clay than did soils of interdunes. Soil temperatures were higher in interdunes than in vegetated dunes. Soil temperatures were similar in dunes with plant cover removed and in interdunes. Infiltration and runoff, hydraulic conductivity, and water retention measurements showed that although dune soils had greater infiltration and more rapid hydraulic conductivity, interdune soils retained more water above −1·5 MPa matric potential. Evaporative losses, as measured with atmometers 3 cm from the soil surfaces, were greater from interdunes than from vegetated dunes. Through two growing seasons, measured total soil water content at a depth of 30·5 cm was always greater for interdunes than for either vegetated or bare dunes. The dune soils stored water less efficiently than interdune soils and reached −1·5 MPa matric potential with much greater frequency.

Dinosaur- and mammal-bearing aeolian and associated deposits of the Upper Cretaceous in the Gobi Desert (Mongolia)

The Barun Goyot Formation (? Campanian-Maastrichtian) consists of continental red-bed type sandstones. The formation was interpreted in terms of intertonguing and/or alternating dune deposits and sediments of intermittent lakes and streams. The mega cross-stratified sandstone units which show considerable lateral continuity and little variation of foresets dip were described as buried transverse dunes. They are rather poor in animal fossils. Most dinosaurs and mammals occur in water-deposited interdune sediments. This is due to ecological differences between bare dunes and the interdune depression which had a vegetation cover.