The epiphytic orchid Pomatocalpa spicatum was studied with regard to substrate diversity, demography, and fruit set in two floristically and structurally different areas of lower tropical rain forest in southeastern Thailand. At both sites, the orchid was found to grow directly on the bark of the phorophytes, from near ground level to 2 to 3 m in height. The relative frequencies of P. spicatum on the four types of substrate (viz. tree trunk, branch, twig, and climber) differed markedly, however, between the two populations. We conclude that an apparently specific substrate requirement of this species at any one locality may reflect local forest structure rather than specific requirements on behalf of the orchid. Despite the large structural and floristic differences between the study sites, both the demographic patterns and the patterns of flowering and fruit set were remarkably similar. In comparison to published observations of other epiphytic orchids, including P. naevatum, small/young individuals were under-represented in both populations—possibly an effect of recent climatic fluctuations on a regional scale. The low relative fruit set in the populations (5.2% and 7.5%, respectively) suggests an allogamous breeding system. The Lorenz curves prepared for the two populations are very similar and document a disproportionately high female genetic contribution by relatively few individuals.
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