Tree water potentials supporting an explanation for the occurrence of Vachellia erioloba in the Namib Desert (Namibia)

The objective of this study is to develop a testable explanation of the occurrence and distribution of V. erioloba by site-vegetation relations, in terms of soil water availability in the rhizosphere, under extreme conditions of two example sites in the Namib Desert.

The Namib Desert is defined as a hyper-arid area, where rainfall is so low that water controls most biological processes (Seely 1987). This water-controlled ecosystem with infrequent, discrete and largely unpredictable water inputs (Berry 2000) and far higher rates of potential evaporation than precipitation (Mendelsohn et al. 2002) allows only a few plant and animal species inhabitation. In this context, availability of soil water is considered as being a limiting factor for plants (Hobbs 1984; Mitlöhner 1998; Serrano et al. 1998; Reynolds et al. 2004; Otieno et al. 2005a; Nobel 2017). Access to available soil water and the ability to endure water stress varies from species to species and can determine growth success if not solely enabling any growth or survival. Here, water stress is determined in terms of the plant water relation parameter ‘water potential’ (compare Vertovic et al. 2001 and Nobel 2017). In this understanding, the term ‘site-vegetation relation’ reflects the interaction between specific sites and specific plant species. It encompasses manifold factors and functions like soil conditions, climatic parameters, as well as genetic pre-conditions which, on the other hand, determine certain characteristics of plant-reaction and adaptation. The more difficult the site conditions are for plant growth, the higher is the level of species adaptation requirement and the fewer the number of species that are able to adapt and solely survive under these conditions (NATH 2000). It is remarkable to discover the comparably rich growth of higher plants or even trees at certain sites within the hyper-arid Namib Desert.

Tree species investigated are V. erioloba E. Mey (Mimosoideae), the ‘Camelthorn Tree’ (basionym: Acacia erioloba E. Mey), Faidherbia albida A. CHEV. (Mimosoideae, ‘Ana-tree’), Euclea pseudebenus E. MEY ex A. DC. (Ebenaceae, ‘African Ebony’), and Tamarix usneoides E. MEY ex BUNGE (Tamaricaceae, ‘Wild Tamarisk’).

Throughout V. erioloba’s extensive range within central and western Southern Africa, it varies from a small, spiny shrub, of barely two metres in height, to a tree of up to 16 m tall. Occurring in dry woodlands in stony or sandy arid areas, this is one of the major tree species of the desert regions of Southern Africa (Palgrave 2002). The outstanding adaptation of V. erioloba to desert sites is reflected in its root-system. In addition to surface roots growing in a radius of up to 20 m around the tree, a strong tap root reaches deep into the ground (Palgrave 2002; Burke 2006).

Faidherbia albida is a large tree reaching up to 30 m in height with wide-spreading branches and a rounded crown. It is a lower elevation species which is widely distributed throughout Africa and usually found on alluvial floodplains, along dry watercourses, in riverine fringe forests and around pans and swamps (Palgrave 2002).

Euclea pseudebenus forms a shrub to medium-sized slender tree of 3 to 10 m in height, with slender drooping branches. It occurs in stony and sandy desert areas where few tree species can survive, but usually in depressions and along watercourses (Palgrave 2002).

Tamarix usneoides is an evergreen, gregarious, many-branched shrub or small tree of up to 5 m in height, occurring in desert areas and their fringes, along shore lines and brackish rivers, and frequently in dry riverbeds.

First, following limitations of the study must be considered: Conclusions on soil water availability are technically limited to locations where plants can be measured. Thus an absence of plants can not be explained by this approach and such sites can not be regarded as less suitable. Further, other potential determinants concerning the occurrence of trees are not assessed. Also possible differences of atmospheric site conditions are not investigated. In consequence, the results can only offer one explanation for the occurrence and distribution of trees: access to soil water in the rhizosphere.

At Namibrand, the plant-internal water potentials at midday remain fairly even (Fig. 3). The changing volume of the dune, recorded by topography, is the only gradient noted and it seems not to impact these values: They seem to describe the actual species’ limit of ‘suction force’ within its environment at the given time and under high atmospheric water demand. The recorded plant water potentials at pre-dawn, on the other hand, do show decreasing values (stronger plant potentials) with the dune’s ascending topography. In the assumption that the soil substrate (the coarse dune sand) is fairly homogeneous within the whole transect, these results allow the suggestion that the pre-dawn plant water potentials are related to the position of the tree in relation to the volume of the dune ridge. Pre-dawn water potentials were plotted versus elevation to support this hypothesis in Fig. 4.

According to Gebrehiwot et al. (2005) and Gebrekirstos et al. (2006), the difference between the soil-induced pre-dawn value and the climate-induced midday value of the plant water potential reflects the range of plant water potential to overcome the soil water potential. Thus it is inferred that the results of the pre-dawn water potential (in relation to the changing topography) and the midday water potential reveal consequences for the plant-internal needs in balancing its internal water budget. At point A in Fig. 3, the noticeable difference between the pre-dawn values and the midday values indicates an actual situation without severe water stress: For the re-saturation at pre-dawn, the trees only need to apply about a third of the ‘suction force’ of midday. On the other hand, at point B, the situation indicates higher water stress: Some trees have to apply almost double the force than at point A to balance their internal water condition at pre-dawn. Since soil texture conditions do not change within the sand dune, it is assumed that the changing topography causes these indications - as supported by Fig. 4. The area within section II appears to be a more favourable location for V. erioloba in consideration of soil water availability (Fig. 4). The diurnal range indicates lowest values at around 1081 to 1097 m a.s.l. Only at a distance of about 2100 to 2600 m from the transect starting point, the pre-dawn water potential is revealed as increasing in relation to declining elevation (at the right side next to B in Fig. 3).

In contrast, however, at point C (Fig. 3), the pre-dawn water potential decreases again. At an elevation of ca. 20 m below point A and about 30 m lower than point B, the impact of the consolidated sand layer seems to come into account (point C). This observation is supported by Fig. 4: at elevation levels lower than 1079 m the pre-dawn water potentials decrease. Here the water potential is likely affected by the different soil conditions of the ground layer. The consolidated sand layer below the dunes contains higher proportions of silt and clay than the dune sand. An impact of salinity might come into account as well: The electric conductivity was tested as three times higher in that layer than within the dunes.

A gap between point A and B is characterised by the absence of sample trees and describes a valley between two major dune ridges. On this plain S. sabulicola densely covers the sand.

Nevertheless, the recorded range between the pre-dawn plant water potentials and the midday plant water potentials at Namibrand is still surprisingly low (indicated in Fig. 4). Stronger values had been expected for a site in the Namib Desert, which receives such little precipitation (compare to e.g. Mitlöhner 1998; Gebrehiwot et al. 2005; Gebrekirstos et al. 2006). Thus unexpected soil water access is reflected in relation to the dune’s volume, but the source is unknown. Access to a deep ground-water table (known by a borehole ca. 4 km south) cannot be considered. Fog precipitation may also not come into account in sufficient extent for the growth of trees in that distance to the sea. Although a few days of fog occur annually and these may support growth of S. sabulicola (Roth-Nebelsick 2012), Schachtschneider and February (2010) state that also riparian trees in hyper-arid environments of the Namib Desert are rather relying on a ‘seasonally fluctuating combination of ground water, shallow soil water and deep soil water’ than on fog precipitation. Thus it might be presumed that the dunes store noteworthy amounts of soil water from precipitation. It may be caused by a water-storing effect of the coarse sand, which allows deep percolation and minimizes runoff and evaporation losses, and thus can be expected to provide storage of plant available water (compare Ganssen 1968; Weidelt 1989; Minnich 1995; Bodnar and Hulshof 2006). According to the author’s own observations, even after a little rainfall of less than 10 mm, the surface dries fast (illustrated in Fig. 2), while water is stored within a few centimetres below the surface for many days and even weeks. The coarse pore system hardly allows any capillary water ascent (Mensching 1990). According to Weidelt (1989), such ‘basic soil moisture’ of sites without any noteworthy capillary ascent, can support a limited stock of perennial grass and potentially even trees, even in extremely arid areas over years. However, since such ‘profile water’ would not explain the observed, elevation-related changes of midday water potential unless situated in deeper layers of the dunes, it could alternatively be assumed that the observations are linked to another underground aquifer in the dune, possibly accumulated from episodic events and a while ago. Such episodic events were observed by Namibrand Nature Reserve authorities e.g. in 2001 and 2011 (seasonal precipitation of 130 and 420 mm, respectively, pers. com.), but may have supported water availability for V. erioloba also a decade ago – considering the age of present individuals. Following these observations, a reasonable water supply may be assumed to be available for deep-rooted trees.

The observations at Homeb reveal the underground water supply of the Kuiseb, considering that the pre-dawn water potential reflects the availability of soil water independently from the species which has been assessed: The trees located close to the riverbed seem to enjoy sufficient water supply. The maximum values of the plant water potential at midday close to the riverbed (e.g. sample trees 4, 5 and 9) are even higher (indicating a lower water potential) than the relaxation values reflecting soil water availability (pre-dawn plant water potential) of sample trees are ca. 75 m further away (e.g. sample trees numbers 1, 2 and 11). Figure 6 illustrates different values of water availability according to the sample trees’ position in relation to the riverbed: The abundant ground-water supply, illustrated by all species as the pre-dawn plant water potential, reflects a site-specific value. As long as the assessed tree species root within the same depth and site, the different species reflect the same parameter of soil water condition. Summarising these findings, the availability of ground water is implied by the pre-dawn water potentials of the sample trees: The closer the sample trees are located to the riverbed, the greater the availability of plant-available soil water (Fig. 6). Salinity might be expected to increase with distance away from the riverbed; although the soil samples did not provide clear results for this assumption (Table 1). However, the soil sample depth is questionable and thus limits the information value. Horton et al. (2001b) describe similar experiences with native riparian tree species in Arizona, where the pre-dawn water potential described the availability or the reduction of available soil water according to river-flow regulation. Comparable results are available from Otieno et al. (2005b), who recorded leaf water potentials to assess root access to soil water at varying soil depths and how this affects the expression of morphological and physiological traits developed during drought in the savannah region of Kenya.

The comparable re-saturation values of V. erioloba (the species’ pre-dawn water potentials) observed at both sites support the assumption made for Namibrand - assumed existence of certain soil water access within the dunes. Only the values of the water potential at midday reflect the different site conditions. At Namibrand, V. erioloba applies a much stronger water potential than at Homeb. Furthermore, the fact that the diurnal range at Homeb is quite low (compared to that at Namibrand, vide Table 3) seems to reflect a more sufficient ground water supply at Homeb - the species does not need to apply a stronger water potential to cope with midday water-deficiency.

Whereas site conditions are reflected by the water potential at pre-dawn, the adaptation of the different species is illustrated by the plant water potential at midday (compare Table 2 and Fig. 6). Comparing E. pseudebenus (sample trees numbers 3 and 8) with neighbouring V. erioloba, it is observable that E. pseudebenus shows a more ‘excessive character’ regarding its water potential. By withstanding a similar or even higher pre-dawn water potential than the neighbouring trees, it applies a remarkably lower midday water potential. Gebrehiwot et al. (2005) reveal comparable characteristics of various species by using the same methods in the N-Ethiopian drylands. This difference in the species’ water-usage-character could support an explanation for the different distribution and dominance of the tree species (Mitlöhner 1998). While V. erioloba is widely distributed in the Namib Desert, the species E. pseudebenus, illustrating a ‘more excessive character’, is usually found along dry watercourses and in riverine fringe forests (vide Van Wyk and Van Wyk 1997; Palgrave 2002).

A rather unexpected occurrence of V. erioloba on sand dunes of Namibrand is related to soil water availability in the rhizosphere. Water availability, determined as minimum factor to tree growth or even survival, was assessed by the plant-internal parameter ‘water potential’, and related to the distribution of trees on both extreme sites within the Namib Desert. A species’ internal ability of adaptation towards the sites’ soil water budget was reflected by the diurnal range of water potentials. The comparison between both sites, a bed of the ephemeral river Kuiseb and the sand dunes on Namibrand, indicated a comparable access to soil water in the rhizosphere for V. erioloba.

The relation between a diurnal range of water potentials and the dune’s volume at Namibrand suggests the hypothesis of water storage low down in the dune: Since the dunes are comprised of loose and coarse sand into which precipitation percolates easily, and from which little soil water is lost by evaporation, noticeable precipitation water might have been stored. This could create a situation where water is more difficult to be accessed by trees situated on the dune’s top, but available for trees growing in the mid portion of the dune – as reflected by Fig. 4: That is where V. erioloba experiences lesser midday water potentials. For trees about 15–20 m higher, on the dune top, the water sources seem to be too far down to be reached with V. erioloba’s root system, extensive as it may be. Supporting that hypothesis, the proximity of the Tertiary layer seems to be less favourable in terms of water supply. Also that layer might provide further conditions more unsuitable for V. erioloba, possibly a combination of consolidation and higher EC.

A limitation in this concern is the fact that the origin of that soil water can not be determined. It is assumed, that ‘episodic events’ with unusual precipitation (as recorded in 2001 and 2011) contributed to water storage in the dunes. Considering the median age of the sampled trees, 79 years, such ‘episodic events’ might have occurred with some frequency. Unfortunately, a respective documentation is not available for this part of the Namib Desert.

However, the results encourage further investigations: While water availability was demonstrated, the origin of the source was not. Using stable isotopes of water (²H, ¹⁸O) for determining the sources of water transpired could allow deeper insight, as proposed by Brunel, Walker and Kennett-Smith (1995), or Verweij et al. (2011). More investigations are suggested concerning the soil water abundance in relation to rooting design and position of V. erioloba on the dunes. A distribution and age of V. erioloba within a wider proximity for a potential relation between age clusters could further support conclusions on presumed episodic precipitation events that facilitate the propagation of V. erioloba within the dunes.