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

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

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 (Frost, 1996).

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

Remote Sensing of Biomass in

area.

**woodlands** 

estimation.

section.

collection and transmission.

in the Niassa National Reserve of northern Mozambique.

the Miombo Woodlands of Southern Africa: Opportunities and Limitations for Research 83

out as the main cause of decline. The authors noticed that 82% reduction in the elephant population by culling between 1979 and 1982, result in a noticeable vegetation recovery. However, field observations indicate that some areas were not reverting to miombo, but to *Combretaceae*-dominated thickets. This behavior was also observed by Ribeiro et al. (2008a)

Dublin *et al.* (1990) studied the effects of elephants and fires in the Serengeti-Mara woodlands in Kenya and found that the combined effect of fire and elephants caused consistently decrease in the recruitment of tree species. Recently Walpole *et al.* (2004) assessed the status of the Mara woodlands (adjacent to the Serengeti National Park). They report that species diversity is relatively low, possibly because of decline in the density and extent of woodland and thickets originated by increasing elephant population within the

Remote Sensing refers to the acquisition and recording of information about objects on Earth without any physical contact between the sensor and the subject of analysis. It provides spatial coverage by measurement of reflected and emitted electromagnetic radiation, across a wide range of wavebands. Remotely sensed data provide in many ways an enhanced and very feasible alternative to manual observation with a very short time delay between data

Since the first launch of an earth observatory satellite, in July 1972 by the US called Earth Resources Technology Satellite (ERTS, later renamed Landsat), remote sensing has been increasingly used to acquire information about environmental processes and over the last three decades this use has had a substantial increase with the development of modern remote sensing technology as radar sensors (ERS, RADARSAT, etc), LiDAR systems and the new generation of optical sensors (IKONOS, SPOT-5, GeoEye, etc.). Thus, remote sensing

The capabilities of remote sensing techniques have been lately expanded to serve monitoring efforts as well as to provide valuable information on degradation in specific areas. Remote sensing, due to its potential to estimate biophysical parameters with detention of temporal and spatial variability, becomes a powerful technique. These, in combination with field sampling methods may provide detailed estimations of forestry parameters. In forest ecosystems, estimation of biophysical parameters has been made by a succession of methods. However, the extension of these estimations in space and time has obvious limitations especially in highly variable ecosystems such as miombo. Moreover, the network of field plots across the miombo ecoregion is not sufficient enough to cover it, which imposes further limitations in the use of remote sensing for biomass

In this section we highlight the uses of remote sensing techniques to derive biomass information in miombo. Emphasis is given to optical remote sensing techniques because the majority of measurements have been concentrated on these techniques. Radar and microwave remote sensing in miombo have, in the last decade, become a useful source of information for biomass estimation, but has been barely applied in the region. Therefore, radar and microwave remote sensing are also briefly discussed in this

**3. Remote sensing techniques for biomass estimation in the miombo** 

techniques are a time-and-cost-efficient method of observing forest ecosystems.

a two-year period in wet miombo was only 4-3% when both woodland and coppice plots were burned in early dry season but 18% and 40% respectively when burned in late dry season.

Fig. 3. Agriculture and associated fires in the Gorongosa Mountain, Central Mozambique (Photos by Ribeiro, N.)

The tolerance or susceptibility of miombo plants to frequent late dry season fire is a function of their growth form, developmental stage, physiological conditions and phenological state at the time of burning. For instance, grasses and many non-woody herbs tolerate intense, late dry season fires better than most woody plants. During this time of the year most tree species produce new leaves, which make them less tolerant to intense fires. Frequent late dry season fires also destroy young trees and shrubs, or their aboveground parts, preventing the development of taller, more fire resistant size classes (Brookman-Amissah *et al*., 1980; Hopkins, 1965).

Scientific accounts of elephant damage to vegetation in African savannas and woodlands emerged during the 1960s. Buechner & Dawkin (1961) analyzed the vegetation changes in the Murchinson Falls National Park, Uganda between 1932 and 1956 and found that tree populations had halved during that period. Thomson (1975) report that nearly 67% of the 500 original mature trees of *B. boehmii* in Chizarira National Park, Zimbabwe died, and another 20% were damaged, transforming relatively dense woodlands into more open wooded grasslands.

Guy (1981) studying miombo changes in the Sengwa Wildlife Research Area, Zimbabwe found that the biomass was reduced by 54% between 1972 and 1976. The decline was associated with the decreasing number of the dominant tree species, *B. boehmii*, a species markedly selected by elephants. In 1989 the area was dominated by *J. globiflora*, *P. maprouneifolia* and *Monotes glaber* but *B. boehmii* was rare (Guy, 1989). Comparing the woodlands inside the research area with those outside (where the elephants population and fires were excluded) the author found less tree density (267 stems/ha over 334 stems/ha), lower stem area (3.56 m2/ha compared to 9.52 m2/ha) and lower biomass (8.5 t/ha compared to 26.2 t/ha). Recently Mapaure & Campbell (2002) working in the same area report an overall rate of decrease in woody cover of 0.75% per year. Elephants (significantly high negative correlation coefficient, r=-0.90, p<0.05), and fire (r=-0.35, p=0.61) were pointed

a two-year period in wet miombo was only 4-3% when both woodland and coppice plots were burned in early dry season but 18% and 40% respectively when burned in late dry

Fig. 3. Agriculture and associated fires in the Gorongosa Mountain, Central Mozambique

The tolerance or susceptibility of miombo plants to frequent late dry season fire is a function of their growth form, developmental stage, physiological conditions and phenological state at the time of burning. For instance, grasses and many non-woody herbs tolerate intense, late dry season fires better than most woody plants. During this time of the year most tree species produce new leaves, which make them less tolerant to intense fires. Frequent late dry season fires also destroy young trees and shrubs, or their aboveground parts, preventing the development of taller, more fire resistant size classes (Brookman-Amissah *et* 

Scientific accounts of elephant damage to vegetation in African savannas and woodlands emerged during the 1960s. Buechner & Dawkin (1961) analyzed the vegetation changes in the Murchinson Falls National Park, Uganda between 1932 and 1956 and found that tree populations had halved during that period. Thomson (1975) report that nearly 67% of the 500 original mature trees of *B. boehmii* in Chizarira National Park, Zimbabwe died, and another 20% were damaged, transforming relatively dense woodlands into more open

Guy (1981) studying miombo changes in the Sengwa Wildlife Research Area, Zimbabwe found that the biomass was reduced by 54% between 1972 and 1976. The decline was associated with the decreasing number of the dominant tree species, *B. boehmii*, a species markedly selected by elephants. In 1989 the area was dominated by *J. globiflora*, *P. maprouneifolia* and *Monotes glaber* but *B. boehmii* was rare (Guy, 1989). Comparing the woodlands inside the research area with those outside (where the elephants population and fires were excluded) the author found less tree density (267 stems/ha over 334 stems/ha), lower stem area (3.56 m2/ha compared to 9.52 m2/ha) and lower biomass (8.5 t/ha compared to 26.2 t/ha). Recently Mapaure & Campbell (2002) working in the same area report an overall rate of decrease in woody cover of 0.75% per year. Elephants (significantly high negative correlation coefficient, r=-0.90, p<0.05), and fire (r=-0.35, p=0.61) were pointed

season.

(Photos by Ribeiro, N.)

*al*., 1980; Hopkins, 1965).

wooded grasslands.

out as the main cause of decline. The authors noticed that 82% reduction in the elephant population by culling between 1979 and 1982, result in a noticeable vegetation recovery. However, field observations indicate that some areas were not reverting to miombo, but to *Combretaceae*-dominated thickets. This behavior was also observed by Ribeiro et al. (2008a) in the Niassa National Reserve of northern Mozambique.

Dublin *et al.* (1990) studied the effects of elephants and fires in the Serengeti-Mara woodlands in Kenya and found that the combined effect of fire and elephants caused consistently decrease in the recruitment of tree species. Recently Walpole *et al.* (2004) assessed the status of the Mara woodlands (adjacent to the Serengeti National Park). They report that species diversity is relatively low, possibly because of decline in the density and extent of woodland and thickets originated by increasing elephant population within the area.
