Tree growth is influenced by many factors, including genetics, climate and soils, as well as levels of disturbance of fires, diseases, slash-&-burn agriculture and charcoal production. Among the most studied and utilized species in Southern Woodlands are Brachystegia spiciformis, Pterocarpus angolensis, Julbernadia paniculata, Isober — linia angolensis, Brachystegia floribunda and Sclerocarya birrea (Shackleton 2002; Grundy 2006; Syampungani et al. 2010; Helm 2011). The majority of tree species studied have diameter increments ranging from 0.03 to 2.6 cm per annum (Helm 2011; Syampungani et al. 2010; Timberlake et al. 2010). Brachystegia spiciformis in western Zambia grew by 0.24-0.33 cm diameter per annum while in Zimbabwe, the species was reported to grow by 0.03-0.27 cm per annum (Grundy 2006; Trouet et al. 2006). A study in Zambia (Syampungani et al. 2010) revealed that there is significant difference in annual ring width between species (Table 4.3); but the increment in annual ring width of similar species did not differ significantly between disturbance factors.

Woody biomass is said to increase with increase in mean annual rainfall across the Southern African region (Frost 1996). Aboveground biomass in old — growth, uneven aged stands has been reported to be 55 Mg ha_1 in dry miombo woodland of Zambia and Zimbabwe on average (Chidumayo 1991; Guys 1981) while the average biomass in wet miombo woodland has been observed to be about 90 Mg ha_1 (Table 4.2). Additionally, lower values have been observed in young re-growth stands of miombo woodlands (1.5 Mg ha_1) in a 3-6 year old coppice stands. Similar variations have also been observed in Mopane woodland namely; 1.1 Mg ha_1 in south eastern Zimbabwe to 79 Mg ha_1 in northern Botswana (Tietema 1993). Shackleton and Scholes (2011) also observed an increase in basal area and biomass from an arid locality to mesic one. They attributed this to differences in stocking between the mesic locality, and the arid and semi-arid localities. This also suggests that moisture availability has an influence on biomass accumulation across ecosystems.

Root biomass studies have been limited in Southern African woodlands. How­ever, what has been documented clearly is that the Zambezian woodland species have horizontally and vertically extensive root systems. Maximum recorded lateral distance ranges from 15 to 27 m in dominant miombo species namely Julbernadia paniculata and Brachystegia spiciformis (Strang 1965; Savory 1963). In dry

Table 4.3 Growth rates of key Miornbo species under different disturbances

miombo woodland of Central Zambia, an average of 32.7 Mg ha_1 as root biomass was observed by Chidumayo (1993). In the Transvaal region of South Africa, Roux et al. (1994) recorded total root biomass of 29.79 t Mg ha_1 he in dense Mopane woodland.