Lee Kong Chian Natural History Museum

Dipterocarpaceae

Photo credit: Edward Lim
Kingdom:Plantae
Phylum/Division:Magnoliophyta
Class:Eudicots
Order:Malvales
Family:Dipterocarpaceae

Description

The family derived its name from the genus Dipterocarpus. Dipterocarpaceae is largely found in Asian forests, particularly in the Sundaland – a region that includes the Malay Peninsula, Singapore, Borneo, Java, Sumatra and their surrounding islands. In these areas, the dipterocarps form the architectural framework of the forests, both by their tall statures as well as by their abundance. As such, depending on the altitude, forests of these regions are commonly referred to as “lowland, hill or upperhill dipterocarp forests”.  

Within a forest, populations of individual species have preferences for specific soil fertility and altitudinal ranges. They are found in loose clusters – breeding mature trees surrounded by dense juveniles. This could in part be due to the trees’ obligate relationship with ectotrophic mycorrhizal (fungus whose mycelium stay outside of the trees’ roots), which might be spread through the soil. Pollination by weak flying insects and poor fruit dispersal could also have been a result of, or help to maintain this clumped distribution (see below).
 
Dipterocarps exhibit mass flowering at two to six years time interval, although sporadic bloomings by individual trees are not uncommon. Masting events have been found to correlate with the El Nino years, during which minimum temperature is depressed over many nights, which is thought to trigger flowering. Dipterocarp flowers are bisexual, in parts of five and typically small in size, with the genus Dipterocarpus as an exception. Species are highly self-incompatible. It has been found that flowering in closely related species tend to be staggered with some amount of overlapping, which helps to maintain outbreeding. The flowers of these trees open at night and drop by midday of the following day. Blooms can last between two to three and a half weeks.
 
There is still much that we do not know about the pollination and dispersal of these trees. Indepth research conducted on a closely related group of the genus Shorea found that at the height of the bloom, the cream-white flowers produce a pervasive sickly and sweet scent that attract thrips. These insects, which are poor fliers, enter the corolla and very probably fall to the ground with the shedded corolla in the morning.  In the evening, the thrips, with attached pollens, then make another weak spiral flight towards the canopy, possibly carried by the air currents to a neighbouring tree. Thus it was suggested that a combination of wind and insect pollination could be possible for these Shorea. Aside from thrips, moths have also been posited as pollinators, due to the nocturnal blooms. Sweat bees and honey bees have also been observed to visit the flowers.
 
Despite the massive flowering event, a high percentage of the fruits die prematurely. Fruits are one-seeded nuts that are covered by a woody fruit wall, with wings that are developed from the sepals. Although many of the fruits are winged, they appear to be poorly dispersed by wind, and gyrate slowly to a distance not far from the mother tree. However, occasional strong monsoonal winds could disperse the seeds further away. Secondary dispersal by ground mammals is also a possibility. 
 
Weevils are major pests which feed on the seeds while the fruits are on the trees. After dispersal, seeds can be predated by other insects, birds or mammals. In years of sporadic fruiting by few individuals only (non-masting years), researcher observed lower seed production due to lower pollination success, greater failure of fruit sets and almost complete seed destruction by animals. Thus masting events could have evolved to maximize pollination success and to satiate seed predators, eventually allowing more seedlings to survive.  Dipterocarp seeds lack dormancy. The seedlings are shade tolerant and are able to persist in the understorey for many years. 
 
Aside from its ecological importance, the family is also highly valued for its resinous hardwood. The high market value of these timbers is a driving force for the high rate of deforestation in countries like Indonesia. 
 
A big family with 20 genera and 500 species, locally in Singapore we have 6 genera and 30 species found in the wild. The six genera are Dipterocarpus, Shorea, Vatica, Hopea, Anisoptera and the less known Cotylelobium. 
 
References
 
Ashton P.S. 1988. Dipterocarp Biology as a Window to the Understanding of Tropical Forest Structure. Ann. Rev. Ecol. Syst. 19:347 – 370 
 
Chan 1980a. Reproductive Biology of some Malaysian Dipterocarps I. Flowering Biology. Malaysian Forester 43: 132 – 143.
 
Chan 1980b. Reproductive Biology of some Malaysian Dipterocarps II. Fruiting Biology and Seedling Studies.  Malaysian Forester 43: 438 – 451.
 
LaFrankie, J.V. 2010. Trees of Tropical Asia: An Illustrated Guide to Diversity. Black Tree Publications, Inc. Phillppines. pp 435.
 
Symington, C.F., 2004. Foresters' Manual of Dipterocarps. Revised by Ashton, P.S. & Appanah, S., Edited by Barlow, H.S. Caxton Press Ltd, Kuala Lumpur, Malaysia.  


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