Plasticity in leaf-to-sapwood area ratio enables saplings to increase growth under long-term drought in Amazon

Frequent and severe drought events in the Amazon threaten the carbon sink capacity of the world’s largest tropical rainforest. However, the extent to which trees can buffer the impacts of drought by adjusting their physiology to sustain growth remains uncertain. This is particularly relevant for the understory strata, which comprise trees that will mature over decades to centuries, shaping the future forest structure and function. Here, we leveraged 22 years of experimental throughfall exclusion in a 1-ha plot in Eastern Amazon, paired with a similarly sized control plot, to investigate how prolonged drought affects the growth of understory saplings. Given that drought has reduced the above-ground biomass by half and increased canopy openness, we hypothesized that (1) saplings grow faster under long-term drought conditions, (2) traits which correlate with sapling growth rate differ under drought and control conditions, and (3) trait plasticity under drought conditions increased sapling growth during drought. Our findings suggest that two decades of imposed drought increased sapling growth rates relative to saplings in control conditions. Despite sapling density being 51% lower in the droughted plot, droughted saplings grew on average three times faster than their control counterparts. In the droughted plot, growth rates increased with leaf-to-sapwood area ratio, leaf nitrogen content, stem conductivity, and leaf minimum conductance. Whereas growth rates increased with embolism vulnerability in the control plot. Within species, plasticity in the leaf-to-sapwood ratio emerged as the single driver of faster growth rates observed in droughted individuals relative to control individuals. In conclusion, we found that prolonged drought reduces understory sapling abundance, alleviating competition, and enabling the surviving individuals to maximize photosynthetic capacity and growth. This implies that drought reshapes the forest into a novel, low-density, fast-growing state which understory trees respond to by increasing their total leaf area.