Small understory trees increase growth following sustained drought in the Amazon

Droughts pose a major threat to the Amazon rainforest, yet the mechanisms enabling trees to maintain growth under prolonged drought remain poorly understood, particularly in the understory layer. We leveraged a 22-yr Throughfall Exclusion (TFE) in a 1-ha plot in eastern Amazonia, paired with a Control plot, to test whether small understory trees (1-10 cm diameter) grow faster under long-term drought due to acquisitive resource-use strategies and competition release, given that the TFE plot experienced large-tree mortality and canopy gap formation over time. Despite a 51% reduction in density, small trees grew 2.2 times faster in the TFE than in the Control. At the species scale, growth rates increased with acquisitive traits, such as high foliar nutrient concentrations, greater hydraulic conductivity, and higher leaf-to-wood area ratio, but only in the TFE. These shifts towards acquisitive resource-use strategies were observed within species, indicating plastic responses to drought. At the community scale, growth rates were negatively associated with neighbour density in the TFE, suggesting that competition release facilitates growth under drought. Our findings reveal that plastic and competitive processes stabilise the growth of surviving small understory trees after drought-induced self-thinning, highlighting key mechanisms that can enhance forest resilience to future climate extremes.