The term phenology is derived from the Greek word phaino meaning to show or to appear. Hence, phenology is defined as the study of the seasonal timing of life cycle events. For plants the seasonal timing of such events can be critical to survival and reproduction. In agriculture the most common failure of introduced crops is the inability to adjust to the seasons imposed by the new, environment (68). In the past few years, interest in the ecology and evolution of timing of life cycle events has grown. Here we review the literature on phenological patterns of germination, flowering, and fruiting (including dispersal).

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Phenological Patterns of Terrestrial Plants

Patterns of tree abundance and dispersion in a tropical deciduous (dry) forest are summarized. The generalization that tropical trees have spaced adults did not hold. All species were either clumped or randomly dispersed, with rare species more clumped than common species. Breeding system was unrelated to species abundance or dispersion, but clumping was related to mode of seed dispersal. Juvenile densities decreased approximately exponentially away from adults. Rare species gave evidence of poor reproductive performance compared with their performance when common in nearby forests. Patterns of relative species abundance in the dry forest are compared with patterns in other forests, and are explained by a simple stochastic model based on random-walk immigration and extinction set in motion by periodic community disturbance.

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Tree Dispersion, Abundance, and Diversity in a Tropical Dry Forest

Plant-Pollinator Interactions in Tropical Rain Forests

There is a voluminous literature on pollination and dispersal, very little of which deals with the consequences of reproductive failure and its most extreme consequence: extinction. The risk of plant extinctions can be assessed by considering the probability of dispersal or pollinator failure, reproductive dependence on the mutualism and demographic dependence on seeds. Traits for ranking species rapidly according to these three criteria are indicated. Analysis of case studies suggests that plants often compensate for high risk in one of the three categories by low risk in another. For example, self-incompatible plants with rare specialist pollinators often propagate vegetatively. Some systems, including elements of the Cape flora and lowland tropical rain forest, lack compensatory traits and the risk of plant extinction from failed mutualism is high. 'What escapes the eye, however, is a much more insidious kind of extincnction: the extinction of ecological interactions' Janzen (1974).

Do mutualisms matter? Assessing the impact of pollinator and disperser disruption on plant extinction

The concept of sexual selection (intrasexual competition for mates and mate preference) is used in revising the classical explanation of dioecy in plants. Male and female functions of hermaphroditic flowers can be subject to different sexual selection pressures, which may conceivably lead to the separation of male and female structures. This suggestion does not exclude genetic advantages that may accrue from outcrossing; the two aspects of selection probably operated together in the evolution of dioecy. The hypothesis regarding the importance of sexual selection could be tested by monitoring variation in individual fitness via pollen donation and via ovule maturation. The evolution of pollinia and other large pollen dispersal units is also viewed in terms of sexual selection. Availability of a reliable pollen vector is a prerequisite to the evolution of pollen packages. From the point of view of a flower functioning as a male, large PDU's ensure fertilization of more ovules and possible preemption of stig...

Sexual Selection in Plants

THE exceptional species diversity of tropical rain forests is well known; of these the lowland forests of the Malay Peninsula are among the richest1. This, and the size of the trees, pose obvious difficulties for the maintenance of pan-mixis, and tempt speculation on the processes by which such diversity has evolved and may now be favoured. Fedorov2 suggested that natural selection is low and self-pollination prevalent, favouring random genetic drift. Prevalence of random drift, or apomixis—agamospermy—would contradict Ashton's3 view that, given a stable environment, evolution proceeds through ecotypic differentiation while diversity accrues through ever increasing niche specialisation; for this view to be correct, maintenance of genetic variability within breeding groups would be essential. Solid evidence has been meagre but there is evidence4 that genetic polymorphism, and hence heterozygosity, is common in the emergent tree Shorea leprosula Miq., and Xerospermum intermedium Radlk., an undeirstorey species. The pattern of spatial variation is consistent with the view that these species are outbreeders in which predominantly short-range pollen and fruit dispersal are accompanied by short-range heterogeneity in gene frequencies. However, both are common trees with minimal spatial isolation. We now report the occurrence of apomixis in some other rain forest trees.

K. Jong

Papers

Apomixis may be widespread among trees of the climax rain forest

Reproductive patterns of selected understorey trees in the Malaysian rain forest: the sexual species

Cytoembryology of some Malaysian dipterocarps, with some evidence of apomixis