Regeneration and management of Brachystegia spiciformis Benth. and Julbernardia globiflora (Benth.) Troupin in Miombo woodland, Zimbabwe
Miombo is dry deciduous woodland dominated by leguminous tree species, covering a significant area of Africa south of the equator, including large parts of Angola, Malawi, Mozambique, Tanzania, Zaire, Zambia and Zimbabwe. This woodland type reaches its driest, most species-poor limit in Zimbabwe. Brachystegia spiciformis and Julbernardia globiflora are the two main dominants of the drier form of miombo. Miombo woodland represents a rich and diverse resource base for small-scale farmers throughout the region, the importance of which has been ignored in the past by managers and policy-makers. The development of sustainable management strategies for this resource has therefore been hampered by a lack of knowledge. For this reason, the overall objective of this research was to define the basis for sustainable management of these woodlands, firstly by developing techniques for measuring biomass and monitoring woody growth; secondly, by examining the effect of site conditions on productivity; and thirdly, by investigating biological and social aspects of the management of these woodlands. This research has found that good estimations of standing wood biomass of these two species can be derived from diameter and stem length measurements, thus providing woodland managers with a means of assessing the standing stock. The finding that B. spiciformis forms annual rings can be used in ageing the trees, projecting future yields more accurately, as well as monitoring the effects of management on growth. In the investigation of the effects of climatic and edaphic conditions on tree growth, soil depth was found to have the greatest influence, followed by mean annual rainfall and clay content. Dominant height was found to be the best morphological variable to estimate site potential, in the absence of detailed studies of soil characteristics. The experimental findings from silvicultural trials have important implications for management. Regrowth from coppice stools was vigorous even in dry years, and greatest from medium to large stools. Productivity in general was extremely variable, both of coppiced and uncoppiced trees, due to site effects. The most productive sites are also potentially the best for agriculture and are therefore unlikely to be left exclusively for woodland management. The silvicultural techniques of coppice reduction and water harvesting were not found to enhance coppice regrowth. Browsing by livestock was found to severely reduce coppice regrowth, particularly of J. globiflora, at least in the initial stages, so that protection from browsing in the first year or two after felling is recommended if maximum regrowth is desired. An assessment of the use and management of the indigenous wood resources in a resettlement area in central Zimbabwe suggests that the present harvesting of wood products is unsustainable, due largely to the lack of any woodland management policy for these areas. Local villagers feel powerless to exclude outsiders from their resource, and thus the incentives to manage it sustainably are low. Major changes in government legislation are necessary to alter this situation. In the interim, resource-sharing schemes are suggested as a method of improving the sustainability of use. Some of the results reported here have a direct application in miombo management; these are summarised at the end of this work in the form of guidelines for management. Others identify the need for further work to expand these initial findings. One of the outcomes of this research has been the reminder of how little is known of the appropriate management of this important vegetation type.