Forest Ecology and Forest Management Group, Wageningen University, 6708 PB Wageningen, THE NETHERLANDS.
Forest-Woodland-Savanna (FWS) mosaics are complex, highly varied and dynamic landscapes that cover extensive areas of the tropical world. Until recently, these mosaics were, however, considered poor in terms of biodiversity. In addition, most specialists viewed them as either mismanaged forest areas or an intermediate stage in a gradual forest degradation towards savanna. Consequently, only few scientific studies have been done on FWS mosaics and little attention has been paid to their conservation.
In Uganda, several FWS mosaics have been identified as areas of high biodiversity but no specific plans for their conservation have been made. Little, however, is known about the species variation within these mosaics and how they have changed over time. The aim of this study was therefore to assess the spatial and temporal variation in the FWS vegetation and to identify the environmental factors that maintain that variation.
The northern part of Budongo Forest Reserve, located in northwestern Uganda, was selected as the main study site because the forest has high species diversity, has low variation in some environmental variables e.g. soil types and has a fairly well known management history. For a specific focus on the effect of fire on woody plants, an area in the southern part of Murchison Falls National Park, located immediately north of Budongo forest, was selected.
Five vegetation cover classes (VCC) and a burnt area cover class were identified in the Budongo study area and were best classified using Spectral Angle Mapper (SAM) combined with an Expert System (overall accuracy was 94.6%). The VCC were well distinguishable in terms of species composition and vegetation structure. Basal area, in particular, differed strongly between them. Also, for each vegetation cover class (except open woodland) indicator species were identified: Uvariopsis congensis, Funtumia elastica, all Celtis species, Caloncoba schweinfurthii, Holoptelea grandis, Diospyros abyssinica and Tapura fischeri for the forest; Terminalia velutina and Albizia grandbracteata for the closed woodland; Grewia mollis and Combretum molle for the very open woodland; and Lonchocarpus laxiflorus, Grewia bicolor, Combretum binderanum and Combretum guenzii for the wooded grassland.
Many tree species, however, occurred in more than one VCC, and the vegetation, in fact, showed a gradient in species composition. To conserve these wide-ranging species, the variety in VCC needs to be conserved. Other species e.g. Uvariopsis congensis showed a very narrow range of distribution. For such species, the specific environment in which they preferentially occur needs to be conserved.
Along a succession gradient, adults and juveniles of most species were found at the same location. Some of the species, however, e.g. Maesopsis eminii had their adult trees at a different position along the succession gradient than their saplings and seedlings. To ensure continuity of such species, the succession gradient locations covering the plants’ growth stages need to be conserved. For the species whose adults and juveniles occur together, the environment in the specific location along the succession gradient needs to be maintained. Uvariopsis congensis, for example, occurred only at one extreme end of the succession gradient, while Grewia mollis occurred at the opposite extreme.
The extent of woodland-savanna vegetation, examined for the period between 1985 and 2002, had increased in 15.1% of the area and decreased in another 14.3%. Whereas most increase occurred between the two forest blocks (Main Budongo and Kaniyo-Pabidi forests) and to the west of Kaniyo-Pabidi forest, most of the decrease was found in the woodland-savanna areas northwest of Main Budongo forest and in the far north of the forest reserve. Most of the areas with vegetation increase were areas jointly managed by the Forest Department (FD) and the Uganda Wildlife Authority (UWA). The areas of vegetation decrease were either managed by the UWA alone, who practice annual burning, or were FD areas where the local people had easy access. The local people also practice annual burning.
Among the environmental variables measured, fire was identified as the major factor influencing woody plant variation over the area. Species composition and indicator species varied between areas with a different fire regime (No fire, Old fire and Recent fire).
Although fire disturbance is essential for the maintenance of some VCC, its excessive use may be detrimental to the existence of the species there in. Three different VCC subjected to a similar fire regime for over 46 years showed a convergence in woody plant composition in a similar direction. Should the fire regime be maintained for a longer period, these VCC will eventually converge. To maintain the existent variation in VCC, fire regimes need to be varied.
The observed variation in vegetation, and species, along the FWS gradient and their interrelationship indicate that each part of the FWS mosaic is essential for the maintenance of the overall diversity within the mosaic. To conserve the variation in composition within the FWS mosaic of Budongo Forest Reserve, the existing variation in vegetation and their spatial and temporal interrelationships need to be conserved. In addition, the variation in environmental factors needs to be maintained. Purposeful fire management is an essential element of this variation. The conservation plans should also provide for maintenance of the varied food resource required by the resident fauna, especially chimpanzees.