Miombo is the vernacular (plural) name for Brachystegia boehmii and similar trees found across Southern Africa (Coates Palgrave et al. 2002). However the name miombo is widely used to describe the savanna woodlands of Southern Africa that are dominated by trees of the subfamily Caesalpinioideae of the Leguminosae, mainly of the genera Brachystegia, Julbernardia and Isoberlinia. The area dominated by these trees is coincident with White’s (1983) Zambezian Phytochorion, the largest of his regional centres of endemism in Africa (Chidumayo 1997). Miombo woodlands covers about 2.7 million km2 in Southern Africa (Frost 1996).
As well as a diverse tree community, miombo is rich in other plants (Coates Palgrave et al. 2007). Common grasses include the genera Hyparrhenia, Andropogon, Loudetia and Digitaria (Frost 1996). For a given rainfall, miombo has only around 20% of the grazer biomass expected on more fertile savanna (2.2 Mg dry matter ha-2) and the grazing community is dominated by elephant and buffalo, with some large antelope (including Lichtenstein’s hartebeest and Sable) (Frost 1996). The nutrient-poor soils in miombo mean that the grass has very low nutrient content and this, combined with the presence of the Tstse fly in much of the region, means that miombo is not widely used for grazing cattle.
Miombo occurs on well drained soils that are derived from the African and post-African planation surfaces that form the Central African plateau (Cole 1986). They are nutrient poor and acidic (Frost 1996), with very low organic content. The dominant trees utilise ectomycorrhizae to obtain phosphorus (Hogberg 1986; Hogberg et al. 1986). Several non-dominant species have nitrogen-fixing nodules.
White’s (1983) map of African vegetation. Miombo woodlands are shown in dark green.
In the context of African savannas, miombo can be classed as a moist/distrophic savanna (van Wilgen 1997). As a moist savanna, tree cover is more limited by disturbance than by climate (Sankaran et al. 2005). In general, Southern African savanna structure is less determined by nutrients (Scholes 1990) than by climate and fire (Bond et al. 2003; Bond et al. 2005), and this appears true of miombo. Rooting structure of the dominant trees typically consist of a both a deep taproot and large lateral roots (Fig. 4 and (Timberlake et al. 1993), allowing deep water access and efficient nutrient uptake.
The miombo region (along with all of Southern Africa) burns regularly, with a mean fire return interval of 2-4 years (Scholes et al. 1996; Barbosa et al. 1999; Mouillot et al. 2005). However satellite estimates of burned area are probably underestimates, as detection of small patchy burns is difficult (Laris 2005; Sa et al. 2007). The fires are primarily fuelled by the senesced grass layer which is flammable throughout the dry season. Nearly all fires are ignited by humans, and this has been so since at least the Iron Age (Chidumayo 1997; Eriksen 2007). Fire is used for many purposes, including hunting, clearing land, producing a new flush of grass for grazers, as well as to clear paths around houses to reduce wildlife hazards (Eriksen 2007; Stronach 2009).
Miombo woodland in Nhambita, Mozambique
The dominant trees are described as fire-tender (Trapnell 1959), although this is disputed (Chidumayo 1988). What is clear however is that fire is a major determinant of miombo woody biomass. Several long term fire experiments in Zimbabwe (Furley et al. 2008) and Zambia (Trapnell 1959) have show that fire exclusion leads to the formation of closed canopies and a succession towards forest. Annual fires have been shown to destroy all woody vegetation.
Miombo trees are variously described as semi deciduous, semi evergreen, drought deciduous or simply deciduous. The seasonal cycle of leaf display is broadly in sync with the rainfall seasonality (Chidumayo 2001), but pre-rain leaf flush is often reported (Frost 1996). In many areas, the new leaves of miombo trees are coloured red, brown and yellow, due to the presence of anthocyanins (Tuohy et al. 1990; Choinski et al. 1993; Frost 1996). Evergreen trees are found in the wetter miombo areas, and along river lines (Chidumayo 1997). Other phenological strategies are also observed among co-occurring species, including the reverse phenology of Faidherbia albida (Barnes et al. 2003) which drops its leaves during the wet months and displays them through the dry season, making it useful in agroforestry systems.
Miombo is often divided into wet and dry miombo based on the 1000 mm isohyet. In dry miombo aboveground woody biomass averages around 55 t dry matter ha-1, whilst in wet miombo 90 t dry matter ha-1 is typical (Frost 1996). There is a significant correlation between rainfall and woody biomass. These biomass values are slightly lower than dry forests under similar conditions in other continents (Frost 1996). Root biomass can comprise between 32% of total woody biomass (in Zambia, Chidumayo 1997) and 20% (in Tanzania, Malimbwi et al. 1994). However there are very few studies of root biomass, partly because taproots can exceed 5 m in depth (Timberlake et al. 1993). Grass biomass decreases with increasing tree biomass, but in a non-linear and complex way (Robertson 1984; Frost 1996).
The root system of a typical miombo tree, Brachystegia spiciformis Benth. from a site in Chati, Zimbabwe on deep sandy soils. The tree was 26 m high. Source: (Timberlake et al. 1993).
|The effect of fire in miombo at Marondera, Zimbabwe. The photo on the left shows a plot that was burned annually for 50 years. The plot on the right has been protected from fire for the same period. See Furley et al (2008) for further details.|
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