Abstract: Diversity is positively correlated with water availability at global, continental and regional scales. With the objective of better understanding the mechanisms that drive these relationships. we investigated the degree to which variation in water availability affects the performance (recruitment, growth a rid survival) of juvenile trees. Precipitation was supplemented throughout two dry seasons in a seasonal moist forest in south-eastern Peru. Supplementing precipitation by 160 mm mo(-1), we increased soil moisture by 17%. To generate seedling communities or known species composition, we sowed 3840 seeds of 12 species. We monitored the fates of the 554 seedlings recruited from the sown seeds. as well as 1856 older non-sown seedlings (10 cm <= height < 50 cm), and 2353 saplings (> 1 m tall). Watering significantly enhanced young seedling growth and survival, increasing stern density and diversity. Watering diminished the recruitment of species associated with upland forests, but increased the survival of both upland- and lowland-associated species. Though supplemental watering increased the growth of older seedlings. their density and diversity were unaffected. Sapling performance was insensitive to watering. We infer that variation in dry-season water availability may affect seedling community structure by differentially affecting recruitment and increasing overall survival. These results suggest that differential seedling recruitment and survival may contribute to the observed relationships between water availability, habitat associations and patterns of tree species richness.

Abstract: Although the potential contribution to tropical tree species coexistence of niche differentiation along light gradients has received much attention, the degree to which species perform differentially along soil resource gradients remains unclear. To examine differential growth response to soil resources, we grew seedlings of nine tropical tree species at 6.0% of full sun for 12 mo in a factorial design of two soil types (clay and white sand), two phosphate fertilization treatments (control and addition of 100 mg P kg(-1)) and two watering treatments (field capacity and water limitation to one-third field capacity). Species differed markedly in biomass growth rate, but this hierarchy was almost completely conserved across all eight treatments. All species grew more slowly in sand than clay soils. and no species grew faster with phosphate additions. Only Eperua grandiflora and E. falcata showed significant growth increases in the absence of water limitation. Faster-growing species were characterized by high specific leaf area, high leaf allocation and high net assimilation rate but not lower root allocation. Slower-growing species exhibited greater plasticity in net assimilation rate. suggesting that tolerance of edaphic stress in these species is related more to stomatal control than to whole-plant carbon allocation. Although relative growth rate for biomass was correlated with both its physiological and morphological components. interspecific differences were best explained by differences in net assimilation rate across six of the eight treatments. A suite of traits including high assimilation and high specific leaf area maintains rapid growth rate of faster-growing species across a wide gradient of soil resources, but the lack of plasticity they exhibit may compromise their survival in the poorest soil environments.

Abstract: We tested the response of seedlings of Diconyina guianensis, a major timber tree species of French Guiana, to mycorrhizal symbiosis and water limitation in a semi-con trolled experiment under natural light conditions. Under well-watered conditions, mycorrhizal colonization resulted in an increase of net photosynthesis, growth and phosphorus uptake. When submitted to water stress, no growth reduction of mycorrhizal seedlings was observed. Mycorrhizal seedlings were more sensitive to drought than non-mycorrhizal ones in terms of carbon assimilation, but not with regard to stomatal closure. In contrast to previous studies on temperate tree seedlings, this result precludes a mycorrhizal effect on the hydraulic properties of this species. Furthermore, our results suggest that below a specific threshold of soil moisture, carbon assimilation of D. guianensis seedlings was decreased by the mycorrhizal symbiosis. This is probably related to the competition between the plant and its host fungus for carbon allocation under low light intensity, even though it did not seem to have a significant effect on mortality in our experiment.

Abstract: Whereas broad-scale Amazonian forest types have been shown to influence the structure of the communities of medium-to large-bodied vertebrates, their natural heterogeneity at smaller scale or within the terra firme forests remains poorly described and understood. Diversity indices of such communities and the relative abundance of the 21 most commonly observed species were compared from standardized line-transect data across 25 study sites distributed in undisturbed forests in French Guiana. We first assessed the relevance of a forest typology based on geomorphological landscapes to explain the observed heterogeneity. As previously found for tree beta-diversity patterns, this new typology proved to be a non-negligible factor underlying the beta diversity of the communities of medium-to large bodied vertebrates in French Guianan terra firme forests. Although the species studied are almost ubiquitous across the region, they exhibited habitat preferences through significant variation in abundance and in their association index with the different landscape types. As terra firme forests represent more than 90% of the Amazon basin, characterizing their heterogeneity-including faunal communities-is a major challenge in neotropical forest ecology. © 2015 Cambridge University Press.