Short-term drought causes synchronous leaf shedding and flushing in a lowland mixed dipterocarp forest, Sarawak, Malaysia

Tropical rain forests are evergreen and experience a climate suitable for plant growth year round (Whitmore 1998). However, most tropical rain-forest trees display periodic shoot growth (Borchert 1991) and show synchronous leaf flushing at the community level (Itioka & Yamauti in press, Medway 1972, Ng 1981). Synchronous leaf flushing may have a great impact on animal population such as herbivores, because young leaves are suitable food resources for many herbivores (Aide 1988, 1992; Coley 1983, Itioka & Yamauti 2004, Lowman 1985). As to the triggering mechanism of leaf flushing, Larcher (2003) suggested that even tropical plants respond to small changes in climate. But there are still no clear answers about environmental triggers that induce leaf flushing in the tropical rain forest (Medway 1972, Ng 1981). In tropical dry forests, which have a severe dry season lasting several months, it is known that flushing of many evergreen and leaf-exchanging species is immediately preceded by, and presumably triggered by, leaf shedding after severe drought at the end of dry season and/or occasional abnormal drought even during the rainy season (Borchert et al. 2002, Williams et al. 1997). In the tropical rain forest of South-East Asia, after the unusually severe drought associated with the 199 7/98 El Niflo event (Kinnaird & O'Brien 1998, Nakagawa et al. 2000, Potts 2003, Williamson & Ickes 2002), an extreme burst of leaf shedding and subsequent leaf flush was observed in many species (Harrison 2001, Itioka & Yamauti in press, Nomura et al. 2003). Therefore, we can hypothesize that severe droughts induce leaf shedding and flushing in the tropical rain forest. However, there have been few studies on the environmental triggers of leaf flushing in tropical rain-forest trees under normal conditions of high rainfall. The aim of this paper was to identify the threshold of drought-induced synchronous leaf shedding and flushing at the community level in a tropical rain forest in SouthEast Asia. We paid particularly close attention to the relation between the pattern of leaf shedding and flushing and rainfall fluctuations. Furthermore, we investigated the relationship between vegetative phenology and cambium growth. Our study was conducted in the Canopy Biology Plot (CBP, 200 x 400 m) at Lambir Hills National Park, Sarawak, Malaysia (4020' N, 113 50' E, 150-250 m asl; Inoue et al. 1995, Kato et al. 1995). The vegetation of the study site is typical lowland mixed dipterocarp forest (Ashton & Hall 1992), which is characterized by an extremely high tree species diversity; within the park 1174 tree species were identified in a 52-ha plot (Condit et al. 2000, Lee et al. 2002). The area has a perhumid tropical climate with a weak seasonal change in rainfall and temperature (Kato et al. 1995). Annual precipitation 1Corresponding author. Address: Hokkaido University Forests, FSC Kita-9, Nishi-9, Kita-ku, Sapporo 060-0809, Japan. Email: ichie@fsc.hokudai.ac.jp