Topsoil samples were collected from two 1 m2 quadrats in six mature forest sites, each representative of a Ghanaian forest type, and regularly watered to promote seed germination. Each sample was subdivided; half the subsamples were exposed to full sunshine, while the other half were shaded until germination ceased and then transferred to full sun. A total of 2028 seedlings belonging to 90 species (of which only 10 species occur in mature forest) were recorded in the sunlit boxes. Only 120 seedlings, of 25 species, germinated in the shade, but a further 41 species were recorded when the shaded boxes were subsequently exposed to sunshine. We conclude that most seedlings germinated from seeds whose dormancy is broken by exposure. WHEN MATURE TROPICAL FOREST is cut and cleared, it is quickly replaced by fast-growing herbs, climbers, and woody plants of species quite different from those present in the original forest. Various opinions have been advanced as to how this happens. One possible explanation is that the seeds from which these colonizing pioneers grow lie dormant in the soil from the time of one gap phase to the next. If this were true, seed longevities of up to several hundred years would be implied. Alternatively, dispersal agents might be responsible for a more-or-less continuous rain onto the forest floor of seed from pioneer species growing in gaps or secondary forest elsewhere. Two possible fates may be envisaged for seeds of pioneer species dispersed onto the floor of intact forest: they might germinate, but the resulting seedlings quickly succumb, or they might lie dormant until the forest is disturbed and the canopy removed. Four main aspects of the problem thus require investigation. Firstly, the size and composition of the pioneer seed stock in soils under mature forest; secondly the distribution and intensity of pioneer seed rain; thirdly the length of viability of pioneer seed; and finally the environmental requirements for germination. Several workers have investigated soil seed stocks by removing samples of topsoil from apparently undisturbed forest, watering the samples under exposed conditions, and assessing the seedlings which come up in them. Symington (1933), one of the first to undertake such experiments, reported that Malaysian forest soils contain abundant dormant pioneer seed, but Whitmore (1978) points out that Symington failed to protect his samples from possible contamination by seed which could have reached them from nearby secondary forest species during the course of the experiment. Other workers have avoided this problem: Liew (1973) and Guevara and GomezPompa (1972) protected their seed boxes from contamination by placing them in greenhouses, while Keay (1960) noted that the species of seedlings recorded in his boxes were not present in the area where the experiments were performed. With respect to the characteristics of the seed rain, Richards (1952: 383) has pointed out that most pioneer species fruit more or less continuously, and that their propagules are provided with efficient dispersal mechanisms. Gomez-Pompa et al. (1976) recently discovered, in an investigation in Mexico, that three of the four commonest seed species to be found in the guts of birds were pioneer species recovered from the soil in nearby primary forest. Although a heavy, widespread rain of poineer seed thus seems probable, direct measurements have not been published. Neither are data available on longevity of dormant seed. So far as the requirements for germination are concerned, all the authors mentioned above concur that seed dormancy is widespread among pioneer species. Ng (1978) states, however, that 'dormancy is not particularly characteristic of pioneer species in the humid tropics,' but by dormancy he implies only mechanical dormancy or the need for after-ripening. Most experiments have, in fact, failed to distinguish between the various possible causes of dormancy. V6azques-Yanes (1976) concludes from an extensive series of experiments, both in the laboratory and the field, that a light requirement is universal for the lPresent address: Department of Botany, The University, Aberdeen AB9 2UD, Scotland, U.K. 256 BIOTROPICA 12(4): 256-263 1980 This content downloaded from 157.55.39.157 on Fri, 08 Jul 2016 05:20:45 UTC All use subject to http://about.jstor.org/terms germination of pioneer species in the Mexican tropical forest where he worked, but he does not seem to have considered temperature as a possible factor.