Vegetation History And Logging Disturbance: Effects On Rain Forest In The Lope Reserve, Gabon (With Special Emphasis On Elephants and Apes)

Abstract

An investigation of the effects of commercial mechanised selective logging on rain forest vegetation and mammals, was undertaken in the Lope Reserve, central Gabon, between January 1989 to July 1991. Vegetation in Lope is mostly semi-evergreen lowland tropical rain forest, but there are some localised patches of savanna., which are thought to be natural in origin, but which are maintained today by regular f i r e s started by humans. Study sites were established in areas of forest logged 20-25, 10-15, and 3-5 years previously.. a fourth was logged during the study, and a f i f t h remained unexploited. None of the study sites had been subject to hunting in the recent past. A line-transect five kilometres in length was cut across the drainage in each site. Forest composition and structure was assessed along each transect, by identifying and measuring trees and lianes in botanical plots, counting stem density of herbaceous vegetation i n the families Marantaceae and Zingiberaceae, and by measuring canopy cover at three heights. At total of 4885 trees and lianes of 327 species occurred in five 2.5 ha samples of plants greater than 10 cm dbh, whilst 1832 individuals of 137 species were found i n five 25ha samples of trees greater than 70cm dbh. There were marked differences i n structure and species composition both between and within sites. A model was developed to attempt to explain this variation, based upon the theory that much of Lope had been covered by savanna vegetation during a previous cool., dry climatic phase, and that forest structure and composition reflected recolonisation of the savanna by forest. Physical features such as swamps, rocky outcrops and altitude were also considered. Two types of multivariate analysis were applied to botanical data and supported the model. The effects of logging on forest vegetation were assessed by returning after logging to botanical plots established before exploitation. Damage levels were low, compared to other parts of the World, resulting in about a 10% reduction in canopy cover. Patterns of f r u i t production were studied by counting fallen ripe and unripe f r u i t on transects. Fruits encountered were classified on the basis of their morphology and dispersal mechanism. There was a period of low f r u i t production during the major dry season, when frugivores are likely t o suffer dietary stress. A number of plant species which did produce f r u i t at this time were identified as species which might represent 'keystone' resources. Over 70% of f r u i t species were animal-dispersed, demonstrating the the important role animals play in the ecology of tropical rain forests. Forest elephant (Loxodonta africana cyclotis) diet, ecological role and group structure were analysed. The bulk of the diet consisted of the bark and leaves of trees, and some monocotyledons in the families Marantaceae and Zingiberaceae, but fruit was also important. Large-scale seasonal movements in response to f r u i t availability were detected. Elephants were important seed dispersers f o r many plant species, and were responsible for less than 1% of natural tree mortality. The social structure of these forest elephants differed from that of populations that have been studied i n east and southern Africa. Average group size was 2.8, and no groups of more than 10 individuals were encountered. Densi ties of primates, ungulates and squirrels were assessed using standard line-transect censuses. Resolution was poor, but statistical differences were detected between sites for some species. Chimpanzee, (Pan t. troglodytes) densities declined i n logged forest, but no other species could be shown to decline after logging. Some other differences between sites were related to vegetation composition. Biomass was high, estimated a t up to 4692.6 kg km-2, but was dominated by elephants, which made up 25-82%. Conservation implications of this study are discussed, and recommendations made.