The Amazon is known to be one of the most important carbon reservoirs on the planet, absorbing and storing tons of CO2 annually. The problem is that over the course of this century, a large decrease in rainfall is expected in some parts of this region - what will the consequences be? A new study recently published in Nature Ecology and Evolution, led by the University of Edinburgh and in which CREAF participates, suggests that the smallest trees will be able to adapt, but the largest will disappear. They have discovered this thanks to an in-situ experiment in the Amazon rainforest of Brazil where they simulated a prolonged drought for more than two decades. According to the team, this adjustment has consequences, since, with the death of the largest specimens, an amount of wood, leaves and roots equivalent to 40% of the biomass of these forests is degraded and, therefore, the stock of atmospheric carbon stored in their structure is released into the atmosphere. In addition, some of the biodiversity living in these giants, such as fungi, plants and animals such as rodents, is also lost.
In the first 15 years of the experiment we observed that most of the larger trees died, and this caused a loss of more than a third of the total biomass. However, after this initial period, the forest was reconfigured and the survivors showed the same water stress as the trees in areas not affected by the drought.
“In the first 15 years of the experiment we observed that most of the larger trees died, and this caused a loss of more than a third of the total biomass. However, after this initial period, the forest was reconfigured and the survivors showed the same water stress as the trees in areas not affected by the drought”, explains Pablo Sánchez Martínez, first author of the study, a researcher at the University of Edinburgh who did his PhD at CREAF.
According to the study, one of the reasons why the smaller specimens survive is that, as the larger ones die, there is less competition for water and more available light.
It could be that, being taller, they are more exposed to heat, wind and water evaporation stress, in contrast to the shorter ones that are protected by shade.
Another possibility is that the smaller ones, which are usually the youngest, are more plastic and capable of developing an internal memory that allows them to recover and adapt their structure to drier environments, “although this theory is very incipient and we are currently investigating it”, explains Maurizio Mencuccini, ICREA researcher at CREAF and one of the authors of the study. As for why the larger specimens die, “it could be that, being taller, they are more exposed to heat, wind and water evaporation stress, in contrast to the shorter ones that are protected by shade”, adds Mencuccini.
The experiment has been carried out for more than 20 years in plots in the Caxiuanã National Forest, located in Brazil, where hundreds of trees per plot have been included, corresponding to more than 150 species representative of the Amazon forest. In one of the plots they recreated a prolonged drought, where they reduced the amount of water to 50% - the drought that is predicted for the end of this century - thanks to plastic covers that prevented water from reaching the soil, and compared it with another plot made up of the same species where no action was taken. But the team also warns that the drought conditions will be worse than they have been able to recreate in the study, because they will be accompanied by more heat, changes in air humidity and the combined effects of storms or fires, “so further and longer-term research will have to be done”, adds Sanchez Martinez.
Panels installed in the forest to simulate drought. Image: Pablo Sánchez Martínez.
What is lost when a giant tree dies
According to the team, the ancient trees of the Amazon forest are the most important. Apart from their function as a carbon sink, they are also responsible for creating the so-called flying rivers, as they form the rain clouds that are transported to other parts of the world through the process of evapotranspiration. Thus, if this cycle is altered, the rainfall pattern on a continental scale would also be at risk, especially in regions of Bolivia, Argentina, Paraguay and Brazil. .
But there is more. Centennial trees such as the Brazil nut, mahogany or jatoba wood not only stand out for their size and longevity - they can live more than 500 years - but also for being true vertical ecosystems that are home to a great deal of biodiversity. Among their canopies and trunks live birds such as toucans and macaws, arboreal mammals, reptiles, insects, frogs and plants such as orchids and bromeliads. “These giants create unique microhabitats, so their disappearance also represents the loss of the world they shelter within their branches”, warns Maurizio Mencuccini.
The study has been led by the University of Edinburgh, with the participation of the University of Exeter and Cardiff University in the UK; CREAF and ICREA in Spain; the Australian National University and the University of Adelaide in Australia; and several Brazilian institutions such as the Federal University of Pará (UFPA), the Museu Paraense Emílio Goeldi and the Universidade Federal Rural da Amazônia (UFRA).
Reference article: Sanchez-Martinez, P., Martius, L. R., Bittencourt, P., Silva, M., Binks, O., Coughlin, I., Negrão-Rodrigues, V., Silva Jr, J. A., Da Costa, A. C. L., Selman, R., Rifai, S., Rowland, L., Mencuccini, M. & Meir, P. (2025). Amazon rainforest adjusts to long-term experimental drought. Nature Ecology & Evolution. https://doi.org/10.1038/s41559-025-02702-x
