FIGURE 1: The distribution of mopane-dominated woodlands in southern Africa.

Previous studies have demonstrated that rainfall, altitude and soil types influence the distribution of mopane in southern Africa (e.g. Burke 2006; Cole 1986; Mapaure 1994; Voorthuizen 1976; Werger & Coetzee 1978). Mopane occurs in areas receiving low to moderate annual rainfall ranging from 400 mm to 800 mm (Madams 1990; Thompson 1960; Werger & Coetzee 1978). These are normally areas at altitudes ranging from 200 m.a.s.l. to 600 m.a.s.l. (White 1983), with variable soils, but usually fine-grained, having textures ranging from sandy through loamy to clayey. The species is also known to occupy both shallow and deep soils, containing significant amounts of exchangeable sodium (Madams 1990; Thompson 1960; Werger & Coetzee 1978).

Other factors influencing the distribution of mopane include minimum temperature and dry season day length (Stevens et al. 2014). Mopane is commonly distributed in high temperature areas (Table 2) and minimum temperature of < 5 °C limits its distribution (Burke 2006; Cole 1986; Henning 1976; Stevens et al. 2014; Timberlake, Nobanda & Mapaure 1993; Werger & Coetzee 1978; White 1983; Whitecross, Archibald & Witkowski 2012), especially in its southern range (Stevens et al. 2014). However, although mopane is predominantly in frost free areas, the species is capable of withstanding light frost (Thompson 1960) and tall mopane trees of > 4 m in height can survive minimal frost damage (Whitecross et al. 2012).

Mopane is considered an important plant species to people, and wild and domestic animals in its distribution in southern Africa. Rural dwellers use it for firewood (Liengme 1983), construction of traditional structures (Makhado et al. 2009) and, to a lesser extent, for medicinal purposes (Madzibane & Potgieter 1999; Mashabane, Wessels & Potgieter 2001). In some parts of the region, there has been increasing use of mopane in urban areas for firewood as the cost of electricity keeps increasing. Mopane also hosts mopane worms, larvae of the moth Imbrasia belina, which are consumed for their nutritional value (Dreyer & Wehmeyer 1982; Voorthuizen 1976) and traded to generate income (Styles 1996). Dry mopane leaves, twigs and pods provide a valuable source of browse for wild animals such as elephants (Ben-Shahar & MacDonald 2002) and greater kudu (Hooimeijer et al. 2005), especially during the dry season and drought periods (Bonsma 1942; Macala, Sebolai & Majinda 1992; Mosimanyana & Kiflewahid 1988; Timberlake 1995), when the tannins have leached out. In addition, the secretion of Arytaina mopane nymphs, commonly known as lerp, increases the palatability of mopane leaves (Ross 1977; Van Wyk 1972), because the lerps have high sucrose content (Styles 1993). The lerps are highly sought after by baboons, monkeys, birds (Herremans-Tonnoeyr & Herremans 1995) and even humans, especially in the northern part of South Africa (Pettey 1925) and Botswana (Sekhwela 1989), because of their sweetness; they contain about 53% water-soluble sugars (Sekhwela 1989).

Considering the extensive distribution of mopane in the low-lying areas of southern Africa and its importance to human livelihoods, domestic and wild animals throughout its distribution range, it becomes a research challenge when factors influencing its distribution are not easily detectable (Siebert 2012) or not even well understood (Stevens et al. 2014). Various sources have contributed to the understanding of mopane distribution (e.g. Cole 1986; Du Plessis 2001; Henning 1976; Madams 1990; Mapaure 1994; Thompson 1960; Timberlake 1995; Timberlake et al. 1993; Werger & Coetzee 1978; White 1983), but there is still a gap in identifying the underlying factors influencing mopane distribution in southern Africa (Siebert 2012; Stevens et al. 2014). This is creating a subsequent gap in our ability to effectively manage mopane and the wild animals it supports. Current climatic changes provide further complexities for predicting the distribution of mopane. As a result, there is a need to adequately review existing information in an integrated manner. This will allow a better understanding of the factors influencing the distribution of mopane in southern African. This review is also critically important because it gives insight into the potential future distribution scenarios of mopane. An extensive review of a mixture of literature (i.e. journal articles, books, conference proceedings and reports), specifically dealing with adaptation of mopane and factors influencing its distribution in southern Africa was carried out. The aim of this article is therefore to provide a review of the mechanisms that enable mopane to survive disturbances caused by fire, browsing activity by large herbivores and environmental stresses in the savanna ecosystem. The article further tested the effect of environmental factors on the distribution of mopane in the southern Africa’s savanna ecosystem.

In addition, mopane is highly browsed by large herbivores such as elephants (Ben-Shahar 1993; Ben-Shahar & MacDonald 2002; Smallie & O’Connor 2000), mainly owing to its high nutritional value (Ben-Shahar & MacDonald 2002; Bonsma 1942; Macala et al. 1992; Mosimanyana & Kiflewahid 1988). Elephants’ preference for mopane makes it susceptible to elephant-induced damage (Lewis 1991). Elephants’ feeding behaviour can transform mopane woodlands to coppiced shrubby stands. Furthermore, elephants also inhibit height recruitment of mopane by repeatedly breaking the branches, ring-barking, heavy browsing and toppling the tree (Lewis 1991; Smallie & O’Connor 2000). As a result, fire and browsing activity has a notable effect on mopane structure, which also has implications on the growth and distribution of mopane in southern Africa’s savannas.

Despite the disturbances caused by fires and browsing activity by large herbivores, mopane is capable of surviving through its coppicing ability and production of chemicals for defence. Various authors have shown that mopane coppices rapidly (Luoga, Witkowski & Balkwill 2004; Mlambo & Mapaure 2006; Mushove 1992; Mushove & Makoni 1993; Tietema 1989) after it has been disturbed by fire and browsing animals. In addition, mopane wood contains crystals of calcium oxalate, which contribute to high wood density (Prior & Cutler 1992) and also enhance resistance of the wood to fire (Centro Informatico Cientifico de Andalucia [CICA] 1996). These crystals effect the burning properties of the wood through producing considerable amounts of carbon dioxide, which retards the fire flame (CICA 1996). During the growing season, mopane also produces a high concentration of secondary metabolites, such as tannins and phenols, in order to deter herbivores from browsing it (Kohi et al. 2010; Wessels, Van der Waal & De Boer 2007), regardless of its high nutritional value. Therefore, the ability of mopane to coppice after disturbances and produce chemical defence enables it to survive disturbances caused by fire and browsing animals.

Another advantage is that the B horizon under mopane sodic soils is relatively impermeable (Dye & Walker 1980), which provides more moisture retention to the A and O Horizons where most of mopane roots are found. The relatively impermeable B horizon further restricts moisture from filtrating down to the C horizon. As indicated by Dye and Walker (1980), these characteristics enable shallow-rooted species such as mopane to have a competitive advantage for moisture uptake over deep-rooted species. It is believed that the shallow rooting system of mopane, complemented by its high root biomass, enables it to quickly absorb and store the available moisture and nutrients near the soil surface, enabling it to survive the ‘harsh’ environmental conditions of southern Africa’s savanna.

The roots play a further critical role in the survival of mopane. Mopane coppices easily (Mushove 1992; Mushove & Makoni 1993; Tietema 1989; Tietema, Kgathi & Merkesdal 1988), mainly because its roots have the ability to produce root suckers, which enables the shoots to grow faster than newly established seedlings (Luoga et al. 2004). As indicated by Mantlana (2002), the ability of mopane roots to coppice confers a degree of resilience to natural and anthropogenic disturbance, which is critical in ensuring its survival.

As a result of the osmotic adjustment, mopane has the ability to germinate and establish root growth at lower water potentials than otherwise would be possible. The seeds of mopane can germinate and withstand water stress from -0.2 MPa to -0.51 MPa without wilting (Choinski & Tuohy 1991; Henning 1976; Johnson et al. 1996). Although the predawn xylem pressure potential analysis for mopane suggests a high water stress in the dry season (February et al. 2007), the species is able to survive water-stress conditions because of its ability to use water efficiently (Mantlana 2002), which is probably the result of its osmotic adjustment. By using a combination of physical and physiological adaptations, mainly involving roots and leaves, mopane is able to tolerate hot, dry conditions mainly found in low-lying areas of southern Africa.

All these adaptation mechanisms enable mopane to use the available limited moisture and nutrients efficiently in order to survive semi-arid to arid conditions of southern Africa. This article has further reviewed and tested the effect of environmental factors on the distribution of mopane in southern Africa. The variables used include rainfall, temperature, altitude, and soil types.

The data used were derived from various sources (Table 1 and Table 2). The areas covered by mopane in southern Africa (Table 1) were plotted against minimum, average and maximum rainfall, temperature and altitude using a polynomial regression analysis (Figures 2–4). The rationale for using polynomial regression was that all functions (linear and non-linear) were showing weak relationships and this was worse when fitting a linear function. However, a polynomial function gave a better fit compared to linear function, which is the reason it was used here.

FIGURE 2: Effect of rainfall on the distribution of mopane in southern Africa.

FIGURE 3: Effect of temperature on the distribution of mopane in southern Africa.

FIGURE 4: Effect of altitude on the distribution of mopane in southern Africa.

The review showed that low rainfall positively correlates with the distribution of mopane in southern Africa, but the relationship becomes weak when the annual rainfall exceeds 600 mm per annum (Figure 2). This corroborates well with the findings by Porter (1968) and Henning (1976), who both indicated that an increase in rainfall to > 800 mm per annum becomes the limiting factor to mopane distribution. As also indicated by Thompson (1960), the limitation of mopane from higher rainfall zones is probably the result of competition with other species, which are more suited to those wetter conditions, low temperature, acidic soil conditions and high frequency of disturbances such as fires.

However, our analysis indicated that the relationship between rainfall and mopane distribution was higher at minimum or low annual rainfall (R² = 0.38), slightly declined at average rainfall (R² = 0.32) and then significantly declined at maximum or higher rainfall (R² = 0.28). Although this relationship is positive, it clearly gives a less than 40% confidence (Figure 2), which concurs with Stevens et al. (2014) that rainfall alone cannot be considered as the major factor determining the distribution of mopane. It is further indicated that the probability of mopane presence drops to < 50% when precipitation exceeds 380 mm in the wettest quarter (Stevens et al. 2014), which confirms that the species favours low rainfall areas (Figure 2). The possibilities of rainfall decline as a result of climate change means that it will further favour the distribution of mopane in areas such as Zambia, which is currently considered a high rainfall area.

The review revealed that the distribution of mopane is best associated with an average daily maximum temperature of 30 °C (R² = 0.42) (Figure 3), but that relationship declines when the mean daily maximum temperature drops (Figure 3). Although the relationship between mean daily temperature and mopane distribution is positive, it clearly gives a less than 43.0% confidence (Figure 3). Stevens et al. (2014) also found that minimum temperature in the coldest month was the strongest determinant for mopane distribution, accounting for 42.2% of the modelled distribution. However, these results do not give a degree of confidence of at least > 50.0% to better explain an important factor associated with the distribution of mopane. This therefore means that temperature alone cannot be considered as the most important factor determining the distribution of mopane in southern Africa. However, it is important to take into account that the probability of mopane presence drops below 50.0% at minimum temperatures less than 5 °C in the coldest month of July. Therefore, minimum temperature is predicted to limit the distribution of mopane from entering the cold interior of the southernmost boundary of southern Africa (Stevens et al. 2014). The limitation of mopane at low temperature zones is because of occasional events of frost (Stevens et al. 2014; Whitecross et al. 2012), which mainly destroys trees and shrubs less than 4 m in height (Whitecross et al. 2012). However, an increase in temperature will further facilitate the distribution of mopane in areas currently considered as cold, especially in areas west and slightly south of its current distribution range (Stevens et al. 2014).

The relationship between altitude and mopane distribution is positive; although, that relationship gives a < 45% confidence (Figure 4). This also means that altitude alone cannot be considered as the important factor determining the distribution of mopane in southern Africa. However, the distribution of mopane correlates well at low altitude (R² = 0.44), but that relationship declines at higher altitudes (R² = 0.28) (Figure 4). This finding corroborates the findings from various authors who also indicated that the distribution of mopane is associated with low-lying, flat and undulating areas (e.g. Werger & Coetzee 1978; Cole 1986; Mapaure 1994; Siebert 2012). This implies, as also indicated by Henning (1976), that limited distribution of mopane at higher altitudes might be the result of combined influences of increased precipitation, lower temperatures, acidic soils and disturbances such as fires.

The soils in areas where mopane occurs tend to have high exchangeable sodium content (Dye & Walker 1980; Werger & Coetzee 1978), which inevitably results in reduced permeability and increased susceptibility to soil erosion (Scholes 1997). Mopane mainly survives on alkaline soils (Werger & Coetzee 1978) and is less common on acidic soils (Henning 1976). White (1983) further indicated that mopane does not occur on true saline soils in which water-soluble salts exceed 0.2% – 0.3%. As a result, mopane is thus considered as an indicator species of alkaline soil (Werger & Coetzee 1978).

Soil having low nitrogen (< 0.2% at 0 cm – 10 cm), phosphorus (< 1.5 ppm), low moisture (15.0%) and exchangeable magnesium favours the growth and performance of mopane, but an increase in soil sodium and potassium levels results in a decline in the growth yield, which is probably because of increased soil osmotic suction, whilst increasing magnesium seems to improve soil moisture uptake (Henning 1976). Therefore, mopane exhibits a shrub structure on shallow sodium-rich soils or clay soils derived from basalt (Mapaure 1994; Mlambo 2006). These are areas with limited soil depth and are normally occupied by ‘bonsai’ shrubby mopane which grow up to 1.5 m in height. The ‘cathedral’ mopane grow quite tall on deep nutrient-rich alluvial soils (Mapaure 1994; Timberlake 1995): up to 6 m in height on heavy impervious soils and up to 25 m in areas having sandy-loamy and alkaline soils (Werger & Coetzee 1978). It is also important to note that the distribution of mopane is limited in the acrisolic soils, possibly because acrisols derived from acid igneous and metamorphic rocks, limit the growth of mopane, but support the growth of other species such as Acacia, Boscia, Grewia, Combretum and Terminalia (Madams 1990).

FIGURE 5: Ordination of the drivers of mopane distribution in different countries in southern Africa.

It is further concluded that the physical, chemical and physiological responses of mopane enable it to survive various disturbances and ‘harsh’ environmental conditions in southern Africa’s savanna ecosystem. This means that a better understanding of the adaptation mechanisms and distribution of mopane is critical and can be used to explain the distribution and survival of the species in these ‘harsh’ conditions in southern Africa. This understanding can also be used to further identify the ecology of the many mammalian and invertebrate herbivores that are found within the mopane ecosystem. Such information is essential for holistic management of mopane woodland and shrublands in southern Africa.

However, because of the complexity associated with identifying factors which associate best with the distribution of mopane in southern Africa, we recommend that such complexity be addressed through the development of an integrated model. Such a model needs to include climatic factors (e.g. rainfall and temperature), topographical factors (e.g. altitude and slope), edaphic factors (e.g. soil types and soil nutrients) and disturbances (e.g. fires, herbivory and competition). Once developed, such a model can significantly improve the precision of predicting the distribution of not just mopane, but also other vegetation formations and associated wild animals in the savannas.

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