UNDERSTOREY VEGETATION AT TWO MUD VOLCANOES IN NORTH-EAST BORNEO

Mud volcanoes constitute one of the natural causes of disturbance in tropical rainforest but the actual effect on the surrounding vegetation is little studied. The objective of this study was to document the effects of mud volcanoes on the species richness, abundance and composition of the surrounding vegetation. The study was conducted at two mud volcanoes in Tabin Wildlife Reserve, Sabah, Malaysia to determine the effect of mud flow disturbance on the distribution of plant species. Both the mud volcanoes have a central area free of vegetation. Four series of plots were established at increasing distance from the periphery of the open area towards N, E, S and W into the surrounding forest. The estimates of canopy openness using the Crown Illumination Ellipses were highest closest to the volcano. At both volcanoes, the abundance decreased, and the species richness and Shannon index of the plots increased with distance from the central area up to 30 m. Species composition was highly correlated with distance from the mud volcano. Plots close to the mud volcanoes had species adapted to open canopies and the adverse soil conditions caused by the mud flows. Some of these species are otherwise mostly known from mangroves, a vegetation type which is at least 10 km away. The study provides baseline data on a rarely studied natural cause of disturbance and thus contributes to a better understanding of the dynamics of tropical forests.