Hydraulic-trait diversity increases tropical forest resistance to water deficits.

Amazon rainforests host a unique biodiversity, store vast amounts of carbon, and are an essential component of the Earth System. Future water balance changes put the Amazon's carbon storage potential at risk. Evidence from grasslands indicates that diversity can mediate responses to drought; however, it remains unclear how tropical forests will respond. We show that functional diversity increases forest resistance to biomass loss during sudden catastrophic drought and chronic climate change-associated precipitation reductions by up to 25%. Using a model capable of simulating drought responses and functional diversity, we found that distinct strategies emerged along hydraulic and carbon allocation axes of trait variation. Climate change and elevated CO₂ caused the re-assembly of communities towards increased water-triggered phenological strategy dominance, whereas climate change alone negatively influenced biomass stored across all strategies. By removing water-triggered evergreen and deciduous strategies, we show that more biomass is lost in the absence of these strategies and thus clearly illustrate that higher diversity buffers the impacts of water balance changes. Our results demonstrate that a predictive understanding of trait diversity and plant hydraulic traits is essential to understand the complexity of diversity-biomass relations under future climate.