Reproductive biology and ecology of some tropical pioneer trees
Some members of two tropical genera of the Euphorbiaceae, Macaranga and Mallotus were studied to determine how and why they are successful pioneers. Aspects of reproductive biology were studied in a total of eight species of Macaranga and two of Mallotus, although detailed observations were limited to a single species of each genus. The majority of work was carried out. in the Pasoh Forest Reserve and Ulu Gombak Forest Reserve in W. Malaysia. 2. All species of Macaranga and Mallotus studied produced flowers at least once each year. Both Mallotus macrostachyus and Mallotus paniculatus bloomed regularly each-July, but there was a wide variation amongst Macaranga species since M.heyneii flowered continuously while the majority of other species flowered at about eight month intervals. Observations on M.hypoleuca demonstrated an alternation between the vegetative and reproductive phases in mature trees. 3. Most Macaranga species produced large numbers of flowers with male trees carrying about 35x more than females. Flowering populations alsio usually contained more male trees. Macaranga flowers are generally small, green and inconspicuous, arising from the leaf axils. Mallotus however produces inflorescences terminally, and although the flowers are small, they are more noticable by virtue of their bright yellow colour. 4. M.hypoleuca produces seed through cross-pollination and most probably also by agamospermy, thereby maintaining genetic variability while allowing the replication of well adapted genotypes through facultative apomixis. Mallotus macrostachyus sets seed only following cross-pollination, Therefore these two pioneers, occurring in the same habitat, exhibit two different kinds of breeding system. 5. Male trees of both genera were visited by generalised pollinators e.g. Trigonids, but these were not found on female flowers, Chironomids did visit flowers of both sexes and may have transferred pollen by ''mistake". Thrips were present in both male and female flowers and therefore may be potential pollinators. 6. High fruit production was a feature of most species, and Macaranga fruits especially attracted many birds and squirrels. Bulbuls especially carried seed away from the tree thereby probably effecting successful dissemination. Many seeds fell directly beneath the tree, and most appeared to germinate immediately in this position. 7. A small proportion of Macaranga seeds appeared capable of remaining viable for up to one year, but most fresh seeds seem to geminate very soon after fruit fall. 8. Although no significant seed bank was found in forest soil, indirect evidence such as the rapid mass germination of Macaranga seeds after forest clearance, followed by no further germination, indicated that it was highly likely that dormant seed was present in the soil. 9. There was a very low Macaranga seed rain inside the forest, and this . source seemed unlikely to account for the mass seed germination following forest clearance or formation of large gaps. 10.Macaranga seeds germinate successfully both in disturbed and undisturbed soil, indicating the operation of a number of stimuli for germination, light and temperature variations probably being most important. 11. The germination of Macaranga seed under greenhouse conditions was enhanced by red light and inhibited by green light. Germination experiments in the forest showed that there was significant sprouting Macaranga seeds in gaps of all sizes, while seeds placed beneath the primary forest canopy failed to germinate. 12. Despite the presence of a dynamic gap phase in Pasoh F.R., Macaranga species are not well represented except in large natural gaps. 13. Macaranga species are the dominant pioneer species in most cleared areas inside the forest, and in most man-made clearances. As forest disturbance continues in Malaysia, Macaranga species are increasingly becoming a major feature of regrowth vegetation. 14. Various topics for further study have been recommended, Macaranga and Mallotus species being ideal subjects, since they occur widely in S. E. Asia.