Amazônia doesn’t forget: Tropical trees with drought memory resist El Niño

Amazônia is the largest tropical rainforest in the world storing between 150-200 petagrams of carbon in its vegetation and soils, and it contributes significantly to the stability of the Earth’s climate system. However, there is alarming evidence of a continuous decline of Amazônia’s capacity to absorb net CO₂. Abnormal carbon losses and an overall weakening of Amazônia’s carbon sink have been related to land use and climate change, raising concerns about its long-term stability and the potential culmination of a related tipping point. The 2023/24 El Niño marked the worst drought in the Amazon on record, with large impacts on forest functioning. This suggests that Amazonian rainforest trees were exposed to immense hydraulic stress during that period. Insights into how the Amazon rainforest responds to recurrent droughts can be gained from results from a long-term throughfall exclusion experiment (TFE) based in the eastern Amazon in Caxiuanã, simulating the recurrence of an El Niño type drought for 23 consecutive years. However, how do trees that have been subjected to recurrent droughts respond to future extreme events? Critically, we need to understand whether recurrent drought exposure affects the long-term future drought responses of Amazonian trees: could Amazonian trees have drought memory?

We tested this idea during the historic 2023/24 El Niño drought, by quantifying resistance, resilience, plant water-use regulation and growth in Amazonian rainforest trees. We compared tree performance between trees that had experienced natural rainfall over the past 23 years (control trees, CTs), and those that had survived repeated drought (drought-exposed trees, DETs) by being subjected to 23 years of artificial El Niño-type soil moisture reduction within the droughted forest plot (TFE plot). At the onset of the El Niño drought, both the TFE and Control plots displayed similar values of available soil water per biomass, indicating that all trees in each plot had a similar water availability. Astonishingly, the DETs were significantly more resistant than CTs during the 2023/24 El Niño and were more resilient post-drought as far as hydraulic transport is concerned. In addition, DETs were much less conservative with regard to water loss during the dry season. However, at the peak of the drought, control trees had used significantly more soil water despite strong water use regulation. While CTs displayed apparent negative stem growth due to the loss of water stored in their trunks, some DETs displayed positive stem growth, indicating that these trees not only survived, but physiologically functioned and grew during the extreme drought of 2023/24, without depleting available soil water. These findings suggest that Amazonian trees that have been subjected to recurrent drought events may build a form of drought memory that enables functional responses to future drought that exceed those from short-term phenotypic plasticity. Such plant memory may underpin phenotypic acclimation to new environmental conditions, ensuring survival and competitive advantage even in a rapidly changing climate.