Journal of Tropical Ecology (1999) 15:437–449. With 4 figures Copyright  1999 Cambridge University Press The mechanism of general flowering in Dipterocarpaceae in the Malay Peninsula MASATOSHI YASUDA* 1 , JUN MATSUMOTO**, NORIYUKI OSADA†, SAN’EI ICHIKAWA††, NAOKI KACHI§, MAKOTO TANI§§, TOSHINORI OKUDA‡, AKIO FURUKAWA‡‡, ABDUL RAHIM NIK and N. MANOKARAN‡‡‡ * Wildlife Ecology Laboratory, Forestry and Forest Products Research Institute, PO Box 16, Tsukuba Norin, Ibaraki 305-8687 Japan (myasuda@ffpri.affrc.go.jp) ** Faculty of Science, University Tokyo, Japan † Faculty of Agriculture, Kyoto University, Kyoto, Japan †† Japan Science and Technology Corporation, Tokyo, Japan § Faculty of Science, Tokyo Metropolitan University, Tokyo, Japan § Forestry and Forest Products Research Institute, Ibaraki, Japan ‡ National Institute for Environmental Studies, Ibaraki, Japan ‡‡ Faculty of Science, Nara Women University, Nara, Japan ‡‡‡ Forest Research Institute Malaysia, Kuala Lumpur, Malaysia (Accepted 7th February 1999) ABSTRACT. The mechanism of general flowering in Dipterocarpaceae in the Malay Peninsula is revealed through field survey and meteorological data analyses. The regions of general flowering coincide with those which experienced a low night-time temperature (LNT) c. 2 mo before flowering. This supports the hypo- thesis that low air temperature induces the development of floral buds of diptero- carps. LNT was found to be caused by radiative cooling during dry spells in winter when the northern subtropical ridge (STR) occasionally migrates southwards with a dry air mass into the equatorial region. LNT events usually occur in La Nin ˜a episodes, not in El Nin ˜o episodes as believed previously. This is because the south- ward migration of the STR is associated with the intensification of local meridional Hadley Circulation in the western Pacific, which is strengthened in a La Nin ˜a episode. Results suggest that El Nin ˜o-like climate change in increased atmospheric carbon dioxide concentrations may be critical for the tropical rain forest biome in south-east Asia. KEY WORDS: Dipterocarpaceae, El Nin ˜o Southern Oscillation, general flowering, La Nin ˜a-STR hypothesis, low night-time temperature, Malaysia, reproductive ecology 1 To whom all correspondence should be addressed. 437