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Species loss in fragmented tropical forest is the greatest single threat to biodiversity. Fragmentation involves changes in the size, shape and spatial distribution of existing forest. Understanding these effects and comparing their impact with the influence of other environmental variables is of utmost importance in prioritising forest blocks for conservation. The spatial geometry of forest blocks in Ghana, West Africa, was examined with respect to tree species number and composition. Tree species number proved to be positively related to forest block area, according to laws of probability and ecological determinism. In addition, tree species composition was dictated by forest block size, with larger reserves recording significantly more rate species than smaller ones. Additional species encountered in reserves of increasing size followed a predictable pattern, which allowed the identification of species vulnerable to habitat loss. An effect of forest block shape on the structure and composition of tree species was visible using coarse spatial resolution satellite sensor imagery. Fragmentation of a previously continuous canopy led to distinct forest edge effects. This was verified by ground data, which showed an increase in the proportions of common pioneer species along reserve peripheries. Significant trends were only detected across small areas: indicating that shape is not an important consideration in the design of larger forest reserves. The effect of forest block proximity on tree species number was examined using regression analysis, but any rôle of proximity was dominated by the influence of area. The influence of climate and fire activity was strongly expressed throughout the examination of spatial geometry in the form of residual variation about the regression line. Results of multivariate analysis indicated that the management of size and disturbance should be considered in preference to shape and proximity if higher levels of diversity and bioquality are to be maintained.