Agradecimentos: We are grateful to the LBA-INPA office in Santarém for management of the LBA flux tower site K67 in the Tapajos National Forest, specially Louro Lima, to Sky Dominguez for logistical support; and to the parataxonomists Ednaldo Augusto Pinheiro Nascimento and Miguel Pastana do Nascimento. We also thank Dr. Marcos Longo for providing the height-diameter model. We thanks to Laboratory at LBA-ECO flux tower, Laboratory of the "Biofísica da Região Amazônica e Modelagem Ambiental"—BRAMA, and in the "Laboratório de Tecnologia da Madeira"—LTM, both in University of Western Pará (UFOPA—"Universidade Federal do Oeste do Pará), for let us use their logistics to process our samples. This study was financed by the U.S. National Science Foundation (NSF) award DEB-1754803 and DEB-1949894. S.C.S. was supported by NSF award DEB-1754357, synergistic DEB-1950080, and the USDA NIFA. Luciana F. Alves was supported by NSF award DEB 1753973, Deliane Penha was supported by Serrapilheira Institute award 1709-18983. RSO and GST acknowledges CNPq for a productivity scholarship Ver menos

Abstract: How tropical plants cope with water availability has important implications for forest resilience, as severe drought events are expected to  increase with climate change. Tree size has emerged as a major axis of drought vulnerability. To understand how Amazon tree species are distributed along size-linked gradients of water and light availability, we tested the niche acclimation hypothesis that there is a developmental gradient in ontogenetic shift in embolism resistance and tree water-use efficiency among tree species that occurs along the understory-overstory gradient. We evaluated ontogenetic differences in the intrinsic water-use efficiency (iWUE) and xylem hydraulic traits of abundant species in a seasonal tropical forest in Brazil. We found that saplings of dominant overstory species start with a high degree of embolism resistance to survive in a dense understory environment where competition for water and light among smaller trees can be intense during the prolonged dry season. Vulnerability to embolism consistently changed with ontogeny and varied with tree species' stature (maximum height): mature individuals of larger species displayed increased vulnerability, whereas smaller species displayed unchanging or even increased resistance at the mature stage. The ability to change drought-resistance strategies (vulnerability to embolism) through ontogeny was positively correlated with ontogenetic increase in iWUE. Ecologically, overstory trees appear to shift from being hydraulically drought resilient to persisting under dry soil surface layer conditions to being more likely physiological drought avoiders as adults when their roots reach wetter and deeper soil layers Ver menos