DYNAMICS OF FOREST AND THICKET VEGETATION ON THE ACCRA PLAINS, GHANA..

ABSTRACT Aspects of seasonal dynamics, population dynamics, and successional dynamics were investigated in three kinds of woody vegetation which occur on the Accra Plains: dry tropical forest, large thickets, and isolated clumps of thicket. Flowering, fruiting, foliage behaviour, and girth changes show a strong seasonal pattern in the study area; moisture deficits limit phenological activity within the community, although patterns vary among species. Synchrony was high within species in reproductive phenology and girth changes, and was high within the community in foliage behaviour. Synchronous flushing was shown to be a significant mechanism of herbivore escape in species lacking alternative defences such as hairy leaves or chemical deterrents. The success rate of fruit set during the study period was low; this probably resulted from both pollination failure and moisture stress. Girth increments for the year were negative, due to the unusually low rainfall. Patterns of seed dispersal and vegetative regeneration tend to maintain the floristic identity of thicket and forest respectively; little mixing between habitats occurs in the seed rain, and differential mortality of seedlings further constrains the adult species composition in the two habitats. Root suckering is particularly important in thicket clumps, due to the mosaic nature of the habitat. Regeneration is adequate in both thicket and forest to maintain the vegetation in a steady-state, although during the study period, seedling mortality exceeded recruitment. Species of closed canopy dry forest showed good stocking in all size classes; some gap-exploiting species were deficient in the small size classes or showed a highly irregular size class distribution. In most species, survivorship rates were constant from one size class to the next through the first stage of the tree's growth (up to 201 of its maximum size), improving in the later stage; in two understorey species, survivorship rates were constant throughout the lifespan of the tree. Successional patterns within the forest are dominated by patchy disturbances (caused by tree falls); gaps are filled by any of a number of rather rare, long-lived emergent species, and old gaps show higher species diversity than is found in other areas. In the absence of disturbance, the composition converges on a low-diversity forest dominated by Diospyros abyssinica, Drypetes parvifolia, and Drypetes floribunda. Most species in the forest show clumped dispersion, resulting from patterns of seed dispersal and vegetative regeneration. Forest of the Accra Plains appears to be stable under present climatic conditions. Reduced rainfall probably has the effect of reducing the cover of woody vegetation, and the damaging effect of fire and wind may be augmented under particularly dry conditions. Changes in vegetation resulting from drought, fire, and wind are neither rapid nor pronounced. Cutting of wood, however, quickly brings about apparently irreversible changes in the species composition, physiognomy, and stature of the vegetation: a dense, low thicket replaces the forest, and this thicket is, itself, stable under the influence of further cutting. Thicket clumps, which differ floristically from both forest and large thickets, exist as isolated relics of more continuous thicket vegetation where the moisture level is insufficient to support larger patches of woody vegetation.