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.

热带植物如何应对水资源供应对森林的恢复能力具有重要影响，因为严重干旱事件预计会随着气候变化而增加。树木大小已成为干旱脆弱性的一个重要轴。为了了解亚马逊树种如何沿着与大小相关的水和光可用性梯度分布，我们测试了生态位适应假设，即沿树种发生的栓塞抗性和树木水分利用效率的个体发育转变存在发育梯度。林下-林间梯度。我们评估了巴西季节性热带森林中丰富物种的内在水分利用效率（iWUE）和木质部水力特征的个体发育差异。我们发现，优势树种的树苗一开始就具有高度的抗栓塞能力，可以在茂密的林下环境中生存，在漫长的旱季，小树之间对水和光的竞争可能会很激烈。对栓塞的脆弱性随着个体发育而不断变化，并随着树种的高度（最大高度）而变化：较大物种的成熟个体表现出增加的脆弱性，而较小的物种在成熟阶段表现出不变甚至增加的抵抗力。通过个体发育改变抗旱策略（栓塞的脆弱性）的能力与 iWUE 的个体发育增加呈正相关。从生态上讲，上层树木似乎从水力干旱恢复能力转变为在干燥的土壤表层条件下持续存在，当它们的根到达更潮湿和更深的土壤层时，成年后更有可能成为生理上的干旱避免者。