Seed-bank dynamics of C. controversa, a common tree in Japanese temperate forests, were investigated at Ogawa Forest Reserve by direct measurement of seed input, soil seed density, and seedling output (germination of soil seeds), together with two field experiments on postdispersal seed mortality. Soil seed density varied from 4 to 233 m−2 in the site near conspecific adults (<5 m horizontally from crown edge), and 0 to 18 m−2 in far sites (≥5 m) during the 1988–92 period. Seed input occurred only in 1988 and 1990: 190–740 m−2 at the sites near conspecific adults, and 4–20 m−2 at the far sites. Seedling output was observed every year and accounted for 0–15% of the loss from the seed bank, occasionally 30–60%. Seeds buried at 5-cm depth suffered moderate mortality (c. 0.2 year−1) by fungal pathogens. In contrast, seeds experimentally placed on the ground suffered higher mortality (0.5–1 year−1) through predation by ground mammals, probably wood mice. In the experiment, seeds whose mesocarp was removed survived much better. On the other hand, distance itself did not appear to affect post-dispersal mortality of seeds. Seeds in canopy gaps also tended to suffer higher mortality. Current spatial pattern and mode of seed dispersal should decrease the potential ability of the seed bank of this species to colonize safe sites. Rare and unpredictable events might increase the contribution of the seed-bank strategy to regeneration in this forest.

The physical and chemical seed characteristics of 11 dipterocarp species (four Dipterocarpus, two Dryobalanops and five Shorea species) were quantified to examine variations among species. We measured dry weight, pericarp thickness and concentrations of nitrogen, total phenol, condensed tannin, lignin, lipid, starch and total sugar. Although there was low intraspecific variation among parent trees, each species had unique characteristics. We found significant correlations between chemical defensive compounds (i.e. lignin and total phenol) and nitrogen concentration. However, these relationships were no longer significant when an outlier was removed. Fruiting frequency was not significantly correlated with any chemical compound. Seeds of Shorea species had a higher concentration of lipid than seeds of Dipterocarpus. The implications of variation in the physical and chemical characteristics of dipterocarp seeds are discussed.

Some tree species exhibit large year-to-year variation in seed production, a phenomenon known as masting (Kelly 1994, Kelly & Sork 2002). Even in tropical rain forests, in which the climate is suitable for plant growth all year round with little seasonal variation (Whitmore 1998), there are many reports of masting (Appanah 1993, Hart 1995, Newbery et al. 1998, Newstrom et al. 1994, Wheelwright 1986). In particular, Dipterocarpaceae, the dominant family in lowland mixed dipterocarp forests in South-East Asia, undergo mast fruiting following mass-flowering with strong interspecific synchronization in aseasonal western Malesia (Appanah 1985, 1993; Ashton 1989, Ashton et al. 1988, Curran et al. 1999, Janzen 1974, Medway 1972, Sakai et al. 1999, Whitmore 1998, Wood 1956). In mixed-dipterocarp forests, dipterocarp species contribute more than 70% of the canopy biomass (Bruenig 1996, Curran & Leighton 2000). Masting of dipterocarp species is therefore likely to have a major impact on animal populations, and also on the nutrient cycle in such forest ecosystems by causing fluctuations in the availability of resources (Sakai 2002).

Spatial variation in canopy structure induces highly variable patterns of light availability for juveniles and understorey trees (Nicotra et al. 1999, Oberbauer et al. 1988, Tang et al. 1999). In the heterogeneous light environment, plasticity in resource use is essential for survival of juveniles and hence maintenance of populations (Valladares et al. 2000). One of the most striking characteristics of plants in their response to a given resource availability is their capacity to change morphological traits (Küppers 1994). Previous studies have attempted to understand the significance of morphological adaptations in their contribution to competitive ability, focusing on crown form (Kohyama 1987, Sterck et al. 2003), branching pattern (King 1998), shoot architecture (Canham 1988) and leaf shape (King & Maindonald 1999, Takenaka 1994, Yamada et al. 2000). Several studies have investigated the changes in morphology of saplings or seedlings growing under different light availability across canopy gaps (Ackerly & Bazzaz 1995, King 1994, Valladares & Pearcy 1998). However, the phenotypic plasticity in morphology of saplings adapting to the fluctuation of light availability has been investigated less (Blundell & Peart 2001, Poorter & Werger 1999).

Using 14C dating, the life span and growth rate of Borneo ironwood (Eusideroxylon zwageri, Lauraceae), which is a canopy tree species with extremely durable and decay-resistant wood distributed in tropical rain forests of South-East Asia, were studied. Timber segments collected from 15 logged stumps in Kubah National Park, Sarawak, Malaysia, were analysed by accelerator mass spectrometry and the obtained data were calibrated to determine the age of individuals. The 14C dating turned out to be an effective method for estimating ages of long-lived trees, such as E. zwageri, in the aseasonal tropics because the estimated error was small compared with estimated age. We found that E. zwageri can live more than 1000 y and that the growth rate of this species was very slow, with a mean radial growth rate of 0·058 cm y-1. The life span was much greater and the growth rate was much slower than those observed or estimated for trees of Dipterocarpaceae, the dominant family in this tropical forest. The long life span of this species may be caused by wood durability with a high specific gravity and abundant defensive compounds. Given equal carbon allocation, the high density and carbon-based defensive compounds may result in a reduced growth rate.

The impact of the unusually severe drought associated with the 1997–1998 El Niño on tropical forest dynamics in Sarawak, Malaysia was examined. Mortality during the non-drought period (1993–1997) in a core plot (1.38 ha) was 0.89 % y−1, while that during the drought period (1997–1998) in the same plot and a peripheral plot was 6.37 and 4.35 % y−1, respectively. The basal area lost in the drought interval was 3.4 times that of the annual incremental basal area in 1993–1997. Drought mortality was higher for the smaller trees, though it was less size dependent than the non-drought mortality. Dipterocarpaceae, which is the dominant family in the study plot, had a mortality 12–30 times higher in the drought than the non-drought period. There were no significant differences in mortality among the topographic types. From the results of a log-linear model (multi-factored contingency table), the death of trees was correlated with size class, indicating a change in the size-class structure of the forest. Thus, both the species composition and structure are totally affected by such an episodic drought even in a per-humid tropical forest.