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, has an area of 544 km2 and a wide range of rain forest formations on several soil types and at altitudes of 5G2376 m. It has an annual rainfall of about 5000 mm. (2) Sites of 1 ha were established in each of four contrasting types of primary lowland rain forest: alluvial forest (AF); dipterocarp forest (DF); heath forest (HF) and forest over limestone (LF). All trees (210 cm dbh) were measured for dbh and height, except for the LF where height was calculated using a regression equation based on height and diameter measurements of a sample of thirty-four trees. Estimates of numbers or biomass or both of small trees (< 10 cm dbh), lianes, ground herbs and ferns, and epiphytes (including ferns) < 3 m from the ground were also made. Epiphytes > 3 m from the ground and bryophytes were not ennumerated. The total above-ground forest biomass (t ha-' dry weight) was calculated as: AF, 250; DF, 650; HF, 470; LF, 380. (3) Leaf and branch material were collected from each tree and, as far as possible, identified to species. There were at least 223 species ha-' in the AF; 214 in the DF; 123 in the HF; and seventy-three in the LF. (4) Soil analyses were carried out using standard methods on twenty-five samples from each site. The A F soil was heterogenous, with gley soils of high base status in the lower, occasionally flooded, part and podzols and peats in the higher part. The D F soil was acid and very low in calcium. It was lower in total exchangeable bases than the very acid podzolic soils of the HF. The LF soils were shallow, highly organic, of high base status, and neutral to mildly acid pH. (5) It was demonstrated that the species-rich D F occurred on very poor soils but there was no simple relationship between soil nutrient element concentrations and biomass or species richness. Many factors are probably involved in controlling these attributes. (6) The causes of the distinctive sclerophyllous leaves of the heath forest are discussed. It is suggested that extreme soil acidity (in the absence of a buffering effect of Alt++) in the organic soils limits nitrogen mineralization and that low levels of biologically active nitrogen favour sclerophylly. Those features of heath forest which reduce transpiration may be important in reducing the mass flow of soil toxins to the root surfaces.

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Summary 1The aim of this study was to document patterns in tree reproductive phenology in a rain forest of central Borneo and examine relationships between phenology and climatic patterns. 2 A 10-year data set (1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000) of monthly observations of flowering and fruit production of 171 trees (including 39 members of the Dipterocarpaceae) at Barito Ulu, Central Kalimantan, Indonesia, showed that most trees (73%) underwent reproductive activity on a supra-annual timescale. 3 There were three general flowering (GF) events, in 1991, 1994 and 1997, which were preceded by major drought periods (30-day sliding total rainfall of less than 100 mm for more than 10 days) in which at least 40% of dipterocarps and at least 18% of all other trees underwent synchronized reproductive activity; there was also a minor event in 1990. Around 1.3% of trees flowered and 3.8% produced fruit in months outside of these four events. 4 At the community level, the strongest negative correlation was found between the percentage of flowering individuals and total rainfall in the preceding 150 days. 5 Within three genera of dipterocarps examined in more detail ( Dipterocarpus , Shorea and Vatica ) there were clear and consistent patterns of sequential flowering with certain species flowering early in the GF events and others towards the end of these events. 6 Our results confirm the importance of large-scale climatic fluctuations (El Niño-Southern Oscillation) on plant reproductive phenology in South-east Asian tropical forests and indicate that drought may be a more important cue than low night-time temperatures.

Mahogany (Swietenia macrophylla, Meliaceae) is the most valuable and intensively exploited Neotropical tree. No information is available regarding the genetic structure of mahogany in South America, yet the region harbours most of the unlogged populations of this prized hardwood. Here we report on the genetic diversity within and the differentiation among seven natural populations separated by up to 2100 km along the southern arc of the Brazilian Amazon basin. We analysed the variation at eight microsatellite loci for 194 adult individuals. All loci were highly variable, with the number of alleles per locus ranging from 13 to 27 (mean = 18.4). High levels of genetic diversity were found for all populations at the eight loci (mean ff E = 0.781, range 0.754-0.812). We found moderate but statistically significant genetic differentiation among populations considering both estimators of F ST and R ST , 9 = 0.097 and p = 0.147, respectively. Estimates of 0 and p were significantly greater than zero for all pairwise population comparisons. Pairwise p-values were positively and significantly correlated with geographical distance under the isolation-by-distance model. Furthermore, four of the populations exhibited a significant inbreeding coefficient. The finding of local differentiation among Amazonian mahogany populations underscores the need for in situ conservation of multiple populations of S. macrophylla across its distribution in the Brazilian Amazon. In addition, the occurrence of microgeographical genetic differentiation at a local scale indicates the importance of maintaining populations in their diverse habitats, especially in areas with mosaics of topography and soil.Keywords: Amazon, conservation genetics, genetic structure, mahogany, microsatellites, Swietenia macrophylla, tropical tree tropical forest ecosystems (Bawa 1994;Young et al. 1996). The genetic threat to tropical trees results from the loss The destruction of tropical forests world-wide has inof genetic diversity associated with the extinction of local creased dramatically in recent decades (Whitmore 1997; populations, reduced population sizes, and the disruption Bawa & Seidler 1998), posing a significant threat to the of mutualisms with pollinators and seed-dispersing animals maintenance of biodiversity and biological processes in (Bawa 1994;Hall et al. 1996; Nasonrt al. 1997; Aldrich etal. 1998;Dick 2001). Moreover, selective logging may promote Correspondence: Maristerra R. Lemes. Fax: + 55 (92) 6433285; dysgenic selection as a result of the continuous exploita-

The spatial distributions of species of tree -> 10 cm gbh were examined in two 4 ha plots and related to the local variation in topography and soil chemistry. The plots were similar in their species composition, particularly in terms of the densities of small trees, and they showed very similar edaphic characteristics. Size class distributions varied little within and between plots. Ordination of 0.25 ha subplots highlighted parallel gradients in the vegetation of both plots when the densities of trees -> 10 cm gbh were considered. Focusing on understorey trees in the 10-<50 cm gbh class at the 0.04 ha subplot scale showed a similar vegetation gradient in both plots closely associated with change from lower slope to ridge. No relationship with soil chemistry was found. On the ridges a special group of understorey species formed clumps and these species contributed importantly to the ordinations. Borneo has a regional history of occasionally severe droughts. It is suggested here that the observed patterns in the understorey are due to differential responses to low soil water supply, the ridges probably tending to dryness more than the lower slopes. Within the large and diverse family Euphorbiaceae, which dominates the understorey at Danum, there may be ecophysiological groupings of species. The long-term effects of disturbance interacting with local edaphic factors on forest structure and composition are discussed.

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