Study sites
Two research locations with contrasting site characteristics were selected in the hyper-arid Namib Desert of Western Namibia: The location ‘Namibrand’ within the ‘Sand Sea’ of the South Namib, and the location ‘Homeb’ at the Kuiseb River, the border between the ‘Sand Sea’ and the gravel plains of the Namib Desert (Barnard 1998). Figure 1 indicates the location in Namibia.
The Namibrand location (25°13′S–16°02′E) is situated ca. 122 km east of the Atlantic Ocean in the South Namib at an elevation of ca. 1070 m a.s.l. The site is characterised by an almost 200 m high dune ridge, which is situated on top of permanent sand plains originating from Tertiary deposits (Seely 1987). Annual precipitation is around 40 mm (usually occurring between November and February), annual potential evaporation is around 3400 mm (Mendelsohn et al. 2002). The climatic conditions are described as hyper-arid (Seely 1987) with a mean minimum temperature of 4 °C to 6 °C and a mean maximum temperature of 34 °C to 36 °C (Mendelsohn et al. 2002). Two to three nights with frost can occur annually. A few days of fog may prevail annually, predominantly originating from the cool Benguela current at the coast (Shanyengana et al. 2002; Eckhardt et al. 2012). The sand dunes move about one metre per year in the prevailing south-westerly wind direction (NATH 2000). The continuously homogenous, coarse dune sand of the Namib is a combination of silica-oxide and mica/feldspar in a relation of 9:1 from eroded quartz (NATH 2000). Human impacts on the vegetation are not noted. The woody vegetation of Namibrand is limited to a dispersed stock of V. erioloba. However, even long tap roots of V. erioloba are not expected to be long enough to reach a ground-water Table. A nearest borehole, located ca. 4 km S of the Namibrand research location, reveals its ground-water table to be at ca. 1010 m a.s.l., which is 50 to 160 m below the dune’s surface. Figure 2 illustrates the location after rare precipitation.
In addition to V. erioloba, the endemic, perennial Stipagrostis sabulicola (Poaceae, ‘Namib Dune Bushman Grass’), and Cladoraphis spinosa (Poaceae, ‘Spiny Love Grass’) grass species occur. Both species show an intensive system of surface roots (Ganssen 1968). According to NATH (2000), the extensive shallow root system of S. sabulicola may extend up to 20 m2 laterally close to the sandy soil surface. Powrie (2017) describes the perennial grass S. sabulicola as occurring sporadically on large dunes which contain substantial water storage. Roth-Nebelsick et al. (2012), on the other hand, describe S. sabulicola’s outstanding self-irrigating ability: The endemic grass species seems to rely on its ‘highly efficient natural fog-collecting system’ by specific leaf surface structures. C. spinosa is portrayed by Dallwitz et al. (1999) as a species of open habitats on desert dunes and in sandy beds of dry watercourses.
A transect of 3200 m by 60 m was recorded in a straight line of 154° from north to cover, as best possible, the gradient of the dune’s profile. The starting point was located at 1086 m a.s.l. At the end, the dune surface touches the Tertiary sand layer at 1062 m a.s.l. The transect crosses two ridges (highest elevation: 1108 m a.s.l.). All trees (total number of 77) on 19.2 ha, with a total basal area of 0.32 m2∙ha−1 were assessed. The mean diameter at breast height is 31.2 cm (±14.6 cm SD). Following the age determination approach for V. erioloba by Steenkamp et al. (2008), the median age is presumed at 79 years.
The Homeb location (23°38′S–15°10′E) is situated ca. 68 km east of the Atlantic Ocean in the Namib Desert, at 440 m a.s.l. along the ephemeral Kuiseb River. The Kuiseb is the largest ephemeral river in Namibia and carries surface water almost every year for several days. At the Gobabeb Research and Training Centre, ca. 25 km upstream of the research site, Henschel (pers. com.) recorded between 1963 to 2000 an average of 17.6 days of annual flooding. Homeb receives a mean annual precipitation of about 20 mm (Mendelsohn 2002). The mean minimum and maximum temperatures vary between 8 °C to 10 °C and 30 °C to 32 °C respectively, the annual potential evaporation is ca. 3200 mm.
Homeb itself represents an open dale within the gorge of the Kuiseb river. Because the Kuiseb has regular surface flows and a perennial aquifer, it supports a closed forest stand of almost 5 ha of V. erioloba, F. albida, E. pseudebenus and T. usneoides. The latter dominates the stand on the southern part of the riverbed and forms an almost impenetrable bush (diameter at breast height: 3 to 7 cm, height: 2 to 3 m).
A section crossing the riverbed of 250 m by 10 m (0.25 ha) was demarcated as research site. Within that section elevation does not vary. Only the riverbed itself forms a depression of ca. 2 m in depth. All 51 trees with a minimum of 10 cm DBH were recorded in terms of position, height, diameter, and crown dimension. The water potentials of twelve individuals with sufficient sample material (leaves) at both pre-dawn and midday were recorded.
Plant water potential
Plant water potential was measured using a pressure chamber (Scholander et al. 1965), which is a method widely used (Sellin 1996; Mitlöhner 1998; Rodriguez et al. 2003; Gebrehiwot et al. 2005; Gebrekirstos et al. 2006). Extensive reviews on pressure chamber measurements are available, e.g. in Tyree and Hammel 1972; Ritchie and Hinckley 1975; Boyer 1995 and Richter 1997. In line with Horton et al. (2001a, 2001b), Mitlöhner (1995, 1998), Teskey and Hinckley (1986), and Hennessey and Dougherty (1984), the pre-dawn water potential is understood to reflect the soil water potential of the rhizosphere, which affects the assessed trees. The water potential assessed at midday describes the strongest negative water potential applied by the sample species to access soil water at the given measuring time.
The field measurements were carried out in the late dry season in September, just before the fall of leaves. Within designated transects (3200 m × 60 m at ‘Namibrand’ and 250 m × 10 m at ‘Homeb’), all trees providing sufficient sample material were measured by two samples at pre-dawn (4:00 to 6:00), the time of presumed highest water potential, and by two samples at midday (12:30 to 14:30) when the water potential is expected to be at its daily minimum. The samples were taken from the same part of a branch and the same insertion height and exposure: The height of two to three metres at the western exposure of trees was chosen and only fully developed leaves of about the same age status were selected. The pressure chamber measurements were carried out in the field, at each tree as the leaf was picked.
Soil samples
Soil samples were taken at three different places within the transect of Namibrand (after the first, second and third quarter of the transect), while one was taken in the Tertiary sand layer. At Homeb, two samples were taken at river centre, two samples 50 m to the north and another two samples 50 m to the south. All samples have been taken at 30 cm depth. The general soil classification was based on literature, soil samples taken were analysed by a standard soil analysis (Soil Analysis by the Soil Laboratory, Ministry of Agriculture, Water and Rural Development, Windhoek).
Statistical analysis
The statistic evaluation of recorded data was carried out using QED and SPSS: After verifying the data’s normal distribution and the homogeneity of variance, the means were evaluated with a one-way ANOVA test. Additionally, the Tukey test (Tukey’s Honestly Significant Difference, LSD test) was carried out to obtain more sensitive statements for distinguishing significantly mean differences (vide Fowler et al. 1998). According to these procedures, all results presented are statistically significant (at P < 0.05), unless indicated otherwise.