Species distribution is strongly driven by local and global gradients in water availability but the underlying mechanisms are not clear. Vulnerability to xylem embolism (P 50 ) is a key trait that indicates how species cope with drought and might explain plant distribution patterns across environmental gradients. Here we address its role on species sorting along a hydrotopographical gradient in a central Amazonian rainforest and examine its variance at the community scale.We measured P 50 for 28 tree species, soil properties and estimated the hydrological niche of each species using an indicator of distance to the water table (HAND).We found a large hydraulic diversity, covering as much as 44% of the global angiosperm variation in P 50 . We show that P 50 : contributes to species segregation across a hydrotopographic gradient in the Amazon, and thus to species coexistence; is the result of repeated evolutionary adaptation within closely related taxa; is associated with species tolerance to Ppoor soils, suggesting the evolution of a stress-tolerance syndrome to nutrients and drought; and is higher for trees in the valleys than uplands.The large observed hydraulic diversity and its association with topography has important implications for modelling and predicting forest and species resilience to climate change.

Studies report the forest expansion process toward open savanna areas; however, most of them were conducted by means of satellite images and aerial photographs. This study has investigated the forest expansion process through community dynamics over 15 years of permanent plots monitoring. The study was conducted at a forest continuum of three distinct phytophysiognomies (riparian forest, dry forest, and woodland savanna), sampling the trees with diameter at breast height C4.77 cm, distributed into 211 plots (10 m 9 10 m). Density and basal area were compared using a paired t test, and Shannon-Wiener diversity was determined through Hutcheson's t test. The number of dead and recruit trees, and basal area increment and decrement were compared among the phytophysiognomies using variance analysis, followed by Tukey's test. Dynamics rates were calculated for the interval of 15 years, as well as Jaccard and Morisita-Horn's similarity indices among phytophysiognomies. In woodland savanna, density and basal area increased and richness and diversity decreased, whereas in dry forest density decreased and richness and diversity increased. No changes in floristic parameters were observed for riparian forest. The similarity among phytophysiognomies increased over time due to advance of typical dry forests species toward woodland savanna and riparian forest; and local decrease in richness of typical savanna species in woodland savanna (decline of 13 species) and riparian species in the riparian forest (three species). From floristic analysis, our results support the dry forests expansion process toward woodland savanna and riparian forest, which seems to be strongly related to the control of fire and decrease in river flow, respectively.