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Flora
Getting ahead by falling down: the strange case of Choerospondias axillarisby Richard T. CorlettThe Hog Plum, Choerospondias axillaris (Anacardiaceae), is a large, deciduous tree whose natural distribution in Hong Kong has been obscured by widespread planting. It has large (c. 30 mm x 25 mm x 25 mm), yellow, fleshy fruits, each with a single large (20 mm x 15 mm x 15 mm) and very woody stone. Uniquely in the Hong Kong flora, these fruits are dropped to the ground as soon as they are ripe. The fruit flesh is watery and has a high sucrose content, suggesting that it is targeted at mammals, rather than birds (Corlett, 1996; Ko et al., 1998). I have seen macaques eating Choerospondias, but it makes no sense to present fruits to an arboreal primate on the ground. We have also occasionally found the distinctive stones in civet scats, which makes a bit more sense, although civets climb well and obtain most of the other tree fruits they eat direct from the canopy. Neither civets nor macaques, however, explain the occurrence of small groups of cleaned stones deposited in forest patches on Ma On Shan and elsewhere, often with no Choerospondias tree in sight.
The name "Hog Plum" suggests pigs as possible dispersal agents, and this cannot be ruled out, but a more likely explanation is provided by a recent paper in Biotropica (Chen et al., 2001). Jin Chen and his colleagues at the Xishuangbanna Tropical Botanical Garden in Yunnan have shown that, in the Mengla National Nature Reserve, Choerospondias axillaris is dispersed by the Indian Muntjac, Muntiacus muntjak � the same muntjac (barking deer) as we have in Hong Kong. The fruits are apparently swallowed whole and then the seeds are regurgitated undamaged after several hours of rumination. The authors suggest that the relatively high protein and calcium content of the pulp may make these fruits an important part of the muntjac�s herbivorous diet. Chen et al. say that the muntjac is also an important dispersal agent for several other large-seeded woody plants in the same area. The only one of those mentioned that grows in Hong Kong is Phyllanthus emblica (Euphorbiaceae), which has spherical, pale green fruits around 18 mm in diameter, with a single hard stone.
Bibliography Chen, J., Deng, X.B., Bai, Z.L., Yang, Q., Chen, G.Q., Liu, Y. and Liu, Z.Q. (2002). Fruit characteristics and Muntiacus muntjac vaginalis (Muntjac) visits to individual plants of Choerospondias axillaris. Biotropica 33: 718-722. Corlett, R.T. (1996). Characteristics of vertebrate-dispersed fruits in Hong Kong. Journal of Tropical Ecology 12: 819-833. Ko, I.W.P., Corlett, R.T. and Xu, R.J. (1998). Sugar composition of wild fruits in Hong Kong, China. Journal of Tropical Ecology 14: 381-387.
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Spontaneous, ballistic release of pollen into the air, by stamens held under tension until anthesis, occurs in the Urticaceae and in one tribe of the closely related Moraceae, which includes the Hong Kong genera Morus, Broussonetia and Maclura. Although bees occasionally visit the flowers of many of these species, the automatic release of clouds of tiny pollen grains makes most sense as an aid to wind pollination. Both floral morphology and the aerial pollen count also suggest that at least some tropical members of the related family Ulmaceae may be wind-pollinated. This has been proposed for Gironniera spp. in Sarawak (Momose et al., 1998), although the same authors considered that bees pollinated Trema tomentosa. In Hong Kong, Celtis sinensis flowers while leafless and releases pollen when touched, but it is also visited by bees that sometimes contact both anthers and stigmas.
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Coastal habitats, with their strong air movements, relative openness and tendency to single-species dominance, would be expected to favour wind-pollination more than inland areas. In fact, even here, most species have animal vectors, but Cox (1990) considered that the coastal pandan, Pandanus tectorius, was wind-pollinated, with the staminate inflorescences producing copious amounts of loose, dry pollen and the pistillate inflorescences acting as highly efficient pollen receivers. Kato (2000), however, considered that beetles (mostly Nitidulidae) were the main pollinators in the Amami Islands.
The degree of ignorance and uncertainty surrounding this apparently straightforward issue may be surprising, but the relative importance of wind and insects as pollinators is still debated for many well-studied temperate zone taxa. Proving pollination by wind is a lot more difficult than one might think. The traditional test has been to exclude insects with a mesh bag, but a mesh fine enough to keep out thrips and other very small insects will also greatly reduce air movements. A combination of mesh bags and insecticide might be worth trying. There is a future PhD in this for someone!
Bibliography
Corlett, R.T. (2001). Pollination in a degraded tropical Landscape: a Hong Kong case study. Journal of Tropical Ecology 17: 155-161.
Cox, P.A. (1990). Pollination and the evolution of breeding systems in Pandanaceae. Annals of the Missouri Botanical Garden 77: 816-840.
Kato, M. (2000). Anthophilous insect community and plant-pollinator interactions on Amami Islands in the Ryukyu Archipelago, Japan. Contributions from the Biological Laboratory Kyoto University 29: 157-252.
Krishnan, R.M. and Davidar, P. (1993). Nectar secretion and pollinator visitation patterns in Mallotus albus. In Pollination in the tropics (eds. G.K. Veeresh, R.U. Shaankar and K.N. Ganeshaiah), International Union for the Study of Social Insects, Bangalore, pp. 157-158.
Momose, K., Yumoto, T., Nagamitsu, T., Kato, M., Nagamasu, H., Sakai, S., Harrison, R.D., Itioka, T., Hamid, A.A. and Inoue, T. (1998). Pollination biology in a lowland dipterocarp forest in Sarawak, Malaysia. I. Characteristics of the plant-pollinator community in a lowland dipterocarp forest. American Journal of Botany 85: 1477-1501.
Moog, U., Fiala, B., Federle, W. and Maschwitz, U. (2002). Thrips pollination of the dioecious ant plant Macaranga hullettii (Euphorbiaceae) in Southeast Asia. American Journal of Botany 89: 50-59.
Yumoto, T. (1987). Pollination systems in a warm temperate evergreen broad-leaved forest on Yaku Island. Ecological Research 2: 133-145.
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