Summary 1.Relationships between tropical rain forest biomass and environmental factors have been determined at regional scales, e.g. the Amazon Basin, but the reasons for the high variability in forest biomass at local scales are poorly understood. Interactions between topography, soil properties, tree growth and mortality rates, and treefalls are a likely reason for this variability. 2. We used repeated measurements of permanent plots in lowland rain forest in French Guiana to evaluate these relationships. The plots sampled topographic gradients from hilltops to slopes to bottomlands, with accompanying variation in soil waterlogging along these gradients. Biomass was calculated for >175 tree species in the plots, along with biomass productivity and recruitment rates. Mortality was determined as standing dead and treefalls. 3. Treefall rates were twice as high in bottomlands as on hilltops, and tree recruitment rates, radial growth rates and the abundance of light-demanding tree species were also higher. 4. In the bottomlands, the mean wood density was 10% lower than on hilltops, the basal area 29% lower and the height:diameter ratio of trees was lower, collectively resulting in a total woody biomass that was 43% lower in bottomlands than on hilltops. 5. Biomass productivity was 9% lower in bottomlands than on hilltops, even though soil Olsen P concentrations were higher in bottomlands. 6. Synthesis. Along a topographic gradient from hilltops to bottomlands there were higher rates of treefall, which decreased the stand basal area and favoured lower allocation to height growth and recruitment of light-demanding species with low wood density. The resultant large variation in tree biomass along the gradient shows the importance of determining site characteristics and including these characteristics when scaling up biomass estimates from stand to local or regional scales.

We investigated the relationship between habitat association and physiological performance in four congeneric species pairs exhibiting contrasting distributions between seasonally flooded and terra firme habitats in lowland tropical rain forests of French Guiana, including Virola and Iryanthera (Myristicaceae), Symphonia (Clusiaceae), and Eperua (Caesalpiniaceae). We analyzed 10-year data sets of mapped and measured saplings (stems >150 cm in height and <10 cm diameter at breast height [dbh]) and trees (stems > or =10 cm dbh) across 37.5 ha of permanent plots covering a 300-ha zone, within which seasonally flooded areas (where the water table never descends below 1 m) have been mapped. Additionally, we tested the response of growth, survival, and leaf functional traits of these species to drought and flood stress in a controlled experiment. We tested for habitat preference using a modification of the torus translation method. Strong contrasting associations of the species pairs of Iryanthera, Virola, and Symphonia were observed at the sapling stage, and these associations strengthened for the tree stage. Neither species of Eperua was significantly associated with flooded habitats at the sapling stage, but E. falcata was significantly and positively associated with flooded forests at the tree stage, and trees of E. grandiflora were found almost exclusively in nonflooded habitats. Differential performance provided limited explanatory support for the observed habitat associations, with only congeners of Iryanthera exhibiting divergent sapling survival and tree growth. Seedlings of species associated with flooded forest tended to have higher photosynthetic capacity than their congeners at field capacity. In addition, they tended to have the largest reductions in leaf gas exchange and growth rate in response to experimental drought stress and the least reductions in response to experimental inundation. The corroboration of habitat association with differences in functional traits and, to a lesser extent, measures of performance provides an explanation for the regional coexistence of these species pairs. We suggest that specialization to seasonally flooded habitats may explain patterns of adaptive radiation in many tropical tree genera and thereby provide a substantial contribution to regional tree diversity.

We observed strong positive relationships between soil properties and forest dynamics of growth and mortality across twelve primary lowland tropical forests in a phosphorus-poor region of the Guiana Shield. Average tree growth (diameter at breast height) increased from 0.81 to 2.1 mm yr −1 along a soil texture gradient from 0 to 67% clay, and increasing metal-oxide content. Soil organic carbon stocks in the top 30 cm ranged from 30 to 118 tons C ha −1 , phosphorus content ranged from 7 to 600 mg kg −1 soil, and the relative abundance of arbuscular mycorrhizal fungi ranged from 0 to 50%, all positively correlating with soil clay, and iron and aluminum oxide and hydroxide content. in contrast, already low extractable phosphorus (Bray P) content decreased from 4.4 to <0.02 mg kg −1 in soil with increasing clay content. A greater prevalence of arbuscular mycorrhizal fungi in more clayey forests that had higher tree growth and mortality, but not biomass, indicates that despite the greater investment in nutrient uptake required, soils with higher clay content may actually serve to sustain high tree growth in tropical forests by avoiding phosphorus losses from the ecosystem. our study demonstrates how variation in soil properties that retain carbon and nutrients can help to explain variation in tropical forest growth and mortality, but not biomass, by requiring niche specialization and contributing to biogeochemical diversification across this region.