Dynamics of Forest and Thickest Vegetation on the Accra plains, Ghana
Date
1979-06
Authors
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Publisher
University of 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.
Description
Thesis(Phd)- University of Ghana