Tropical trees can be hermaphroditic, when both sexes are in the same flower: examples include leguminous species and citrus; monoecious, when both sexes are in the same plant but in different flowers, i.e., there are separate male and female flowers on the same individual: examples include pines, palms, and oaks; or dioecious, when sexes are on different individuals, i.e., male flowers occur on different trees to female flowers: examples include species belonging to the families Araucariaceae, Myristicaceae, Piperaceae, and Euphorbiaceae, among others. About 60-65% of trees in lowland rainforests have been found to be hermaphroditic; 11-14% monoecious, and 23-26% dioecious (Bawa 1992).
The mode of reproduction has important implications for management and conservation. Research has shown that most tropical woody plants are strongly outcrossed (Bawa 1992). This means that pollen from the flower of one tree fertilizes the flowers of another tree of the same species. In most tropical forests there is a large distance among individuals of the same species. As a result of timber extraction, distance among individuals of the same species may become even larger, and it may be harder for the pollinators to transfer the pollen to another tree of the same species. This poses a limit to the number of individuals that can be extracted from the forest, and also on the minimum size needed for a forest reserve, so as to ensure successful reproduction of the species.
Research on genetic characteristics of tropical trees has shown that there is considerable genetic variation within populations (Bawa 1992). Genetic heterogeneity can be an advantage for withstanding stress and for adapting to new environmental conditions, such as those encountered by trees when they are planted in a plantation. Cross pollination increases genetic heterogeneity. Distance between two individuals of the same species is the main factor affecting cross-pollination. As cross-pollination has great survival value, dioe-cism is an advantage because it impedes self-pollination. The frequency of dioecism needs to be taken into account when planning the minimum size of area for preservation, since dioecious plants require larger areas than monoecious or hermaphroditic trees. Dioecious trees generally require larger areas, because the chances that the nearest neighbor of the same species is of the opposite sex is 50%, while in the case of hermaphroditic or monoecious trees, the chances that the nearest neighbor will have a flower or flower parts of the opposite sex is 100%. In addition, in the case of monoecious or hermaphroditic trees, if other individuals of the same species are not close enough, the trees may still be able to reproduce by self pollination, although this might result in a decrease in genetic heterogeneity for the next generation. In the case of the dioecious trees, they must necessarily have another individual of the opposite sex close enough to reproduce.
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