**2.2 Floristic composition and structure**

Miombo have an estimated diversity of 8,500 species of higher plants, over 54% of which are endemic and 4% are tree species. Zambia is considered to be the centre of endemism for *Brachystegia* and has the highest diversity of tree species (Rodgers *et al.*, 1996). The diversity of canopy tree species is, however, low and characterized by the overwhelming dominance of trees in the genera *Brachystegia* (*miombo* in Swahili), *Julbernardia* and/or *Isoberlinia*  (Campbell *et al.*, 1996). Other important tree species in miombo include *Pseudolachnostylis maprouneifolia*, *Burkea africana*, *Diplorhynchus condilocarpon* among others. In mature miombo these species comprise an upper canopy layer made of 10-20 m high trees and a scattered layer of sub-canopy trees. The understorey is discontinuous and composed of broadleaved shrubs such as *Eriosema, Sphenostylis, Kotschya, Dolichos* and *Indigofera* and suppressed saplings of canopy trees. A sparse but continuous herbaceous layer of grasses, forbs and sedges composed of *Hyparrhenia, Andropogon, Loudetia, Digitaria* and *Eragrostis* (Campbell *et al*., 1996; Desanker *et al.*, 1997) dominate the ground-layer.

Species composition and structure of miombo vary along the rainfall gradient across the region. In the dry miombo, canopy height is less than 15 m and the vegetation is floristically

Remote Sensing of Biomass in

characteristic Range of values Source

1.5 Mg/ha (3-6 years old coppice) – 144 Mg/ha (Mature wet miombo); 53-55 Mg/ha in dry and sub-humid miombo

0.1-4.0 Mg/ha (2-5%

Table 1. Summary of structural characteristics of miombo woodlands.

**2.3 Ecological role of disturbances in miombo woodlands** 

tissues to fire damage and death (Laws, 1970; Sukumar, 2003).

of the total aboveground biomass)

Woody density 1,500-4,100 stems/ha

Tree density 380-1,400 trees/ha

Structural

Aboveground biomass

Aboveground herbaceous biomass

(Frost, 1996).

the Miombo Woodlands of Southern Africa: Opportunities and Limitations for Research 81

Stand basal area 7-24 m2/ha Banda *et al.,* 2006 ; Backéus *et al*., 2006;

Changes in the landscape of many types of woodland in Africa have been attributed directly to the interactive effect of elephants and fire (Buechner & Dawkins, 1961; Guy, 1981, 1989; Laws, 1970; Mapaure & Campbell, 2002; Ribeiro *et al*., 2008a; Sukumar, 2003; Walpole *et al.*, 2004). In general, the pattern of change is the same: as elephants over-browse the woodlands, laying waste to mature trees, there is an increase in the low woody vegetation and grass cover as well as a dramatic increase in fuel load. This allows fire to become progressively more intense. Fiercer and frequent fires affect both large trees and saplings lowering species diversity. Debarking of large trees by elephants may further expose inner

Nearly 90% of fires in miombo are anthropogenic and associated with several human activities in the woodland including: hunting, honey collection and shifting agriculture (Figure 3). They occur every 1 to 3 years in the dry season from May to October/November with a peak in the late dry season (August-October). They are largely fuelled by grasses and take place in the understorey with flame heights generally low (Gambiza *et al.,* 2005; Trollope *et al.*, 2002). Thus fire intensity and frequency is linked through grass production to the previous season rainfall, the intensity of grazing and the extent of woody plant cover

Fire frequency in miombo is expected to be locally highly variable according to fuel accumulation rates, the proximity to sources of ignition and interannual climatic variations (Chidumayo, 1997; Frost, 1996; Kikula, 1986; Ribeiro 2007; Trapnell, 1959). The impact of fire on plants depends on its intensity, frequency, seasonality and interaction with herbivory (Bond & Van Wilgen, 1996; Frost, 1984; Ribeiro, 2007; Trollope, 1978). The effect of seasonality was studied by Chidumayo (1989), indicating that stem mortality measured over

Campbell *et al.*, 1995; Chidumayo 1997; Grundy 1995; Guy 1981; Ribeiro *et al.*, 2008a; Strang, 1974; Trapnell, 1959.

Banda *et al.*, 2006; Campbell *et al.,* 1995; Chidumayo, 1985; Grundy, 1995; Guy, 1981; Strang, 1974; Trapnell, 1959.

Freson *et al.,*1974 ; Lawore *et al.*, 1994;

2008b; Sitoe *et al.* (*unpubl. Data*)

*Data*); Sitoe *et al.* (*unpubl. Data*)

Chidumayo, 1991; Chidumayo, 1997; Guy, 1981; Malaisse & Strand, 1973; Ribeiro *et al.*,

Frost, 1996; Ribeiro and Matos (*unpubl.* 

poor. *Brachystegia spiciformis*, *B. boehmii* and *Julbernardia globiflora* are the dominant canopy species. The herbaceous layer varies greatly in composition and biomass and contains grasses and suppressed saplings of canopy trees. The wet miombo in turn, presents canopy heights greater than 15 m. The vegetation is floristically rich and includes nearly all of the characteristic miombo species. *B. floribunda*, *B. glaberrina*, *B. longifolia*, *B. wangermeeana* and *Marquesia macroura* are widely distributed. The understorey comprises a mixture of grasses, bracken (*Pteridium aquilinum*) and shrubs.

Biomass distribution is uniform across the ecoregion, with the woody component comprising 95-98% of the aboveground biomass in undisturbed stands (Figure 2); grasses and herbs make up the remainder (Chidumayo, 1997). However, biomass production is a function of rainfall and nutrients. For example, Chidumayo (1997) showed a variation of aboveground biomass of 53 t/ha in western dry miombo to 93 t/ha in wet miombo, in Zambia.

Fig. 2. Beginning of the wet season in mature miombo woodlands in Niassa National Reserve, northern Mozambique. A conserved stand where the homogeneous canopy layer is evident (Photo by: Ribeiro, N.).

The key indicator of the linkage between rainfall and miombo production is the observed structural and compositional variations following the rainfall gradient, from the drier fringes of the miombo to the wetter core area (Desanker *et al*., 1997). The nutrient cycling seems to follow also the rainfall gradient across the region revealing that nutrients limitation is a function of moisture regime. Local variations are expected to be much higher and strongly affected by disturbances, especially fires and herbivory. The main structural characteristics of miombo woodlands are summarized in Table 1.

*Dambos* are distinctive features of the miombo and occupy seasonally waterlogged shallow valley depressions across the prominent catenas in the region (a regular alternation of two or more types of vegetation) (Campbel *et al*., 1996; Scholes, 1997). *Dambos* are small *islands* of hygrophilous treeless grasslands emerged in the miombo landscape, which can make up to 40% of the landscape. They have a particular importance for the ecology of miombo, especially as habitat for animal species including some herbivores.

poor. *Brachystegia spiciformis*, *B. boehmii* and *Julbernardia globiflora* are the dominant canopy species. The herbaceous layer varies greatly in composition and biomass and contains grasses and suppressed saplings of canopy trees. The wet miombo in turn, presents canopy heights greater than 15 m. The vegetation is floristically rich and includes nearly all of the characteristic miombo species. *B. floribunda*, *B. glaberrina*, *B. longifolia*, *B. wangermeeana* and *Marquesia macroura* are widely distributed. The understorey comprises a mixture of grasses,

Biomass distribution is uniform across the ecoregion, with the woody component comprising 95-98% of the aboveground biomass in undisturbed stands (Figure 2); grasses and herbs make up the remainder (Chidumayo, 1997). However, biomass production is a function of rainfall and nutrients. For example, Chidumayo (1997) showed a variation of aboveground biomass of 53 t/ha in western dry miombo to 93 t/ha in wet miombo, in

Fig. 2. Beginning of the wet season in mature miombo woodlands in Niassa National Reserve, northern Mozambique. A conserved stand where the homogeneous canopy layer is

characteristics of miombo woodlands are summarized in Table 1.

especially as habitat for animal species including some herbivores.

The key indicator of the linkage between rainfall and miombo production is the observed structural and compositional variations following the rainfall gradient, from the drier fringes of the miombo to the wetter core area (Desanker *et al*., 1997). The nutrient cycling seems to follow also the rainfall gradient across the region revealing that nutrients limitation is a function of moisture regime. Local variations are expected to be much higher and strongly affected by disturbances, especially fires and herbivory. The main structural

*Dambos* are distinctive features of the miombo and occupy seasonally waterlogged shallow valley depressions across the prominent catenas in the region (a regular alternation of two or more types of vegetation) (Campbel *et al*., 1996; Scholes, 1997). *Dambos* are small *islands* of hygrophilous treeless grasslands emerged in the miombo landscape, which can make up to 40% of the landscape. They have a particular importance for the ecology of miombo,

bracken (*Pteridium aquilinum*) and shrubs.

evident (Photo by: Ribeiro, N.).

Zambia.


Table 1. Summary of structural characteristics of miombo woodlands.
