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Energetics of Pollination

Annals of the Missouri Botanical Garden (Impact Factor: 0.54). 11/2003; 6:139-170. DOI: 10.1146/annurev.es.06.110175.001035
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    ABSTRACT: 1. The effect of male tree density on pollen flow and fruit set in females was examined for three species of dioecious tree (Neolitsea dealbata, Litsea leefeana and Diospyros pentamera) in complex notophyll rain forest in tropical, north-eastern Australia. 2. The spatial distribution for all reproductive individuals combined was significantly clumped in Diospyros but not in either Litsea or Neolitsea. Separate male and female groups were significantly clumped in all species. More than 80% of nearest male neighbours occurred within 10 m of Neolitsea females and within 20 m of Litsea and Diopsyros females. The ratio of males to females was 1.6:1 (Neolitsea), 1.5:1 (Diopsyros) and 1.3:1 (Litsea). 3. No species produced fruit asexually when flowers were isolated in mesh bags. All species populations flowered synchronously but Diopsyros flowered for a longer period (15 weeks), than either Neolitsea (3--4 weeks) or Litsea (4--5 weeks). All species had small, unspecialized flowers and were pollinated by small, generalist insects. Male trees produced more flowers per reproductive shoot than females and flower production was positively correlated with tree size in males but not in females. Fruit set was not correlated with tree size in female trees. 4. The ratio of fruit set to flowers in females was negatively related to two measurements of male to female distance (distance to the nearest male tree (all species) and mean distance to the nearest 10 (Neolitsea and Diospyros) or three (Litsea) males, termed `local male distance'). The rate of reduction in fruit set with increasing nearest male distance was the same for all species, although Diospyros produced a higher proportion of fruit relative to flowers. The ranges of fruit set for Neolitsea, Litsea and Diospyros were 1-30%, 2-20% and 20-40%, respectively. Fruit set in Neolitsea females was similar in two successive seasons, indicating a consistent effect of local male distance on fruit set. 5. The amount of pollen trapped at female trees over the whole flowering season was unrelated to fruit set or to local male distance in all species. Amounts of pollen trapped at females after the first 6 (Neolitsea) and 15 (Diospyros) days of flowering were negatively correlated with local male distance. In Neolitsea females, the amount of pollen trapped at females after 6 days of flowering was positively related to fruit set, indicating that effective fertilizations occur early in the flowering period. 6. Although the mechanism for density-dependent pollen movement is the same for all three species, the longer flowering period and more gradual production of flowers found in Diospyros allowed more time for pollen to move through the population. It is concluded that rain-forest trees which have obligate outcrossing breeding systems, brief, synchronous flowering periods and are pollinated by a non-specialized fauna risk reduced rates of fruit set when relatively isolated from conspecifics.
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