When we look at the towering trees of old-growth forest patches in the Amazon, we might think these ancient beings have reached their maximum size and width.<\/p>\n
It turns out they have not, a new study suggests. It shows that even the largest and oldest Amazonian trees still capture carbon dioxide (CO2)\u2014and keep getting bigger, albeit at a slow pace.<\/p>\n
Led by Adriane Esquivel-Muelbert<\/a>, an ecologist at the University of Cambridge, the researchers analyzed 3 decades of tree measurements from 188 primary forest plots spread across nine Amazonian countries. Each plot was around 1 hectare, about the size of a city block, and was measured by teams using tapes and notebooks, often under harsh conditions.<\/p>\n The plots were selected from the Amazon Forest Inventory Network (RAINFOR<\/a>), which has become one of the most important monitoring efforts in tropical ecology, according to Esquivel-Muelbert. The monitoring period varied between 1971 and 2015.<\/p>\n \u201cWe already knew the Amazon works as a carbon sink,\u201d she said. \u201cBut we wanted to understand what\u2019s happening inside the forest\u2014what kinds of trees are changing, and how.\u201d<\/p>\n The study, published in<\/a> Nature Plants<\/a>, found that the average tree size had increased by 3.3% per decade over the past 30 years. Large-canopy trees\u2014those with trunks wider than 40 centimeters\u2014grew even faster in diameter. Smaller trees shaded by larger ones also grew, while the size of medium-sized trees remained relatively stable.<\/p>\n The consistency across the Amazon basin suggests the increasing amount of CO2 in the atmosphere is the ingredient fattening up the trees. \u201cCarbon is an extra resource,\u201d Esquivel-Muelbert explained. \u201cWith the same amount of light, a plant can photosynthesize more efficiently when there\u2019s more CO2 available.\u201d<\/p>\n In other words, as humans release more carbon into the atmosphere, Amazonian trees seem to be using some of it to grow. The researchers interpreted the pattern as a mix of two effects: a winner-takes-all response, in which the tallest trees gain even more advantage, and a carbon-limited benefit response, in which smaller, shaded trees find it easier to survive in low light. Both effects can occur at the same time, leading to more biomass for both groups at the extremes of the size scale.<\/p>\n The study also found no sign that large trees are dying faster, contradicting earlier hypotheses that canopy giants would be the first casualties of heat and drought. The resilience of these ancient trees\u2014some of them centuries old\u2014is important because they sequester a disproportionate share of the forest\u2019s carbon.<\/p>\n \u201cThe largest 1% of trees account for about half of all the carbon stored and absorbed by the forest,\u201d Esquivel-Muelbert said. Losing them would mean losing much of the Amazon\u2019s buffering power against climate change.<\/p>\n Not Exactly Good News<\/p>\n \u201cIt doesn\u2019t mean carbon dioxide is good for the forest. What we\u2019re seeing is resilience, not relief.\u201d<\/p>\n The findings might sound like good news, but \u201cit doesn\u2019t mean carbon dioxide is good for the forest,\u201d Esquivel-Muelbert said. \u201cWhat we\u2019re seeing is resilience, not relief.\u201d<\/p>\n Carbon dioxide might be fattening up old trees, but its consequences for the global climate totally offset what might look like an advantage or a good thing at first sight, she emphasized.<\/p>\n To Tom\u00e1s Domingues<\/a>, a forest ecologist at the Universidade de S\u00e3o Paulo in Ribeir\u00e3o Preto, the new results offer valuable real-world confirmation of what experimental models have long proposed. \u201cThe study shows that the community as a whole is gaining biomass, presumably due to higher CO2,\u201d he said. \u201cThat aligns perfectly with what we\u2019re testing at AmazonFACE<\/a>.\u201d<\/p>\n AmazonFACE<\/a>\u2014a large-scale open-air experiment near Manaus in the Brazilian state of Amazonas\u2014exposes forest patches to elevated concentrations of atmospheric carbon to simulate future conditions. One of its main goals is to see how long the carbon fertilization effect can last before the forest runs into another limitation: the lack of nutrients such as phosphorus, calcium, magnesium, and potassium.<\/p>\n \u201cThe CO2 effect has a short life,\u201d Domingues explained. \u201cTrees can only turn extra carbon into growth if they have enough nutrients. In the Amazon, everybody\u2014trees, microbes, fungi, insects\u2014is competing for the same scarce resources.\u201d If nutrients become limited, he added, growth could plateau or even reverse, regardless of the CO2 supply.<\/p>\n Still Holding On<\/p>\n The new findings highlight how complex the Amazon\u2019s responses to human-driven change can be. While extra carbon has acted as a growth stimulus so far, climate stressors, especially heat, drought, and windstorms, are also intensifying.<\/p>\n Previous studies suggested that the Amazon\u2019s overall carbon storage capacity is starting to weaken<\/a>. Changes in species composition, repeated droughts, and the spread of degradation along the southern and eastern edges of the basin are already weakening parts of the system. \u201cThe forest is still resisting,\u201d Esquivel-Muelbert said, \u201cbut that doesn\u2019t mean it will resist forever.\u201d<\/p>\n \u201cThese forests are resilient, but they\u2019re irreplaceable. If we lose them, they don\u2019t come back in our lifetime.\u201d<\/p>\n Domingues noted that 30 years of observations, though impressive for tropical fieldwork, still capture only a short moment in ecological time. \u201cFor the forest, 30 years is nothing,\u201d he said. \u201cThese trees live for centuries. We need to keep watching.\u201d<\/p>\n Despite the unknowns, both researchers are clear: Protecting mature, intact forests is crucial if we want to fight climate change. Reforestation won\u2019t replace the carbon storage capacity of old-growth trees. \u201cThese forests are resilient, but they\u2019re irreplaceable,\u201d Esquivel-Muelbert said. \u201cIf we lose them, they don\u2019t come back in our lifetime.\u201d<\/p>\n The study\u2019s main message, Esquivel-Muelbert added, is not that the Amazon is thriving under climate change. It\u2019s that the forest is still holding on, at least for now. <\/p>\n \u2014Meghie Rodrigues (@meghier.bsky.social<\/a>), Science Writer<\/p>\n Citation: Rodrigues, M. (2025), As CO2 levels rise, old Amazon trees are getting bigger, Eos, 106, https:\/\/doi.org\/10.1029\/2025EO250413<\/a>. Published on 5 November 2025.<\/p>\n