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Over the past century, the growth of tropical trees has remained minimally affected by droughts. As a result, the rate of CO₂ sequestration in tropical wood has been fairly stable. These are the main findings of a global study, led by Wageningen ľ�ϸ���Ӱ�� & Research and to which Utrecht geoscientist Tom Wils also contributed. "Tropical trees are still fairly resilient to drought," says Wils. "But that resilience response has its limits – trees can't just tolerate everything." The research has now been published in Science.

The researchers used the largest collection of tropical tree-ring data to date: more than 20,000 tree-ring series from nearly 500 locations in 36 countries. They calculated that, on average across the tropics, trees grew 2.5% less during drought years compared to years with normal or above-average precipitation. This growth dip largely disappeared in the year following the drought. However, the researchers also warn that drought effects are much stronger locally, will aggravate as climate change progresses and have likely caused elevated tree mortality.

We are now only seeing a fraction of the climate chaos that awaits us.

Tree-growth archive

Trees take up CO₂ from the atmosphere through photosynthesis. A part of this CO₂ is stored in tree trunks for many decades. This long-term carbon sequestration in tropical forests can help mitigate climate change. However, stem growth usually slows down during periods of drought, causing less CO₂ to be sequestered in wood. Lead author Prof. Pieter Zuidema of Wageningen ľ�ϸ���Ӱ�� & Research: “Until now, we didn’t know to what extent stem growth in tropical forests decreases during droughts. Tree rings are an excellent tool to quantify this, as they contain an archive of tree growth over the past century. With our new network of tree-ring records, we were able to calculate the effect of droughts on stem growth across the tropics for the first time.”

Resilience

The tree-ring data were collected at 483 locations spread across the tropics: from wet and warm Amazonian forests to dry forests in southern Africa and cooler Asian mountain forests. The Utrecht contribution, by Dr Tom Wils, concerned data on the only African conifer in the dataset: a juniper species from the cypress family. "This species, Juniperus procera, is very sensitive to drought," says Wils. First, the researchers determined the driest years since 1930 for all locations. They then calculated how much narrower the tree  rings were during those years compared to normal years. They also measured the width of tree rings in the two years following a drought. Zuidema: “We observed that – on average – stem growth of tropical trees decreased by 2.5% during the 10% driest years. If we only considered the 5% most extreme years, average stem growth decreased by 3.2%.” The researchers also found an almost complete recovery in the year following the drought. They attribute this to sugar reserves in trees, which can be used to produce new leaves and wood.

Local impacts stronger

The effects of droughts differed significantly between the research areas, however. Stem growth slowed down a lot more than the global average (by over 10%) at a quarter of the locations. Zuidema: “This happened mainly in hot and dry regions, such as in northeastern Brazil and southern Africa. The effects of droughts can be more extreme there because trees shed their leaves quickly and the soil retains less moisture. In wetter areas, such as Amazonian forests, the effects were weaker.”

Will the effects remain limited?

The researchers warn that stem growth resilience could weaken when droughts occur more frequently and become more intense under aggravating climate change. They already witnessed this at the research locations during the past decades. Zuidema: “Recent droughts have already caused stronger reductions in stem growth compared to earlier droughts. It gets harder for trees to recover. We expect that ongoing climate change will only exacerbate this effect.” This could substantially lower the CO₂ sequestration rate in tropical forests. Wils: "The results of our research are in line with what I often discuss with students: we are now only seeing a fraction of the climate chaos that awaits us."

Droughts may also result in more tree mortality. “We have made an initial estimation by combining our data with those from long-term forest monitoring studies. We think that each dry year may cause approximately 0.1% additional tree mortality, resulting in extra CO₂ emissions from rotting dead wood. This 0.1% may seem insignificant, but summed across the world’s tropical areas, roughly as much CO₂ could be released as Germany’s annual emissions. It will take decades before that amount will be taken up through tree growth again.”

Learning from the past

The study is the result of a recently formed . Coauthor Dr Peter Groenendijk of the ľ�ϸ���Ӱ�� of Campinas in Brazil is one of the network’s initiators: “Hundreds of tree-ring studies have been conducted on tropical trees over the past few decades. However, this is the first time those data have been compiled for large-scale analysis.” The network brings together data and expertise. Groenendijk: “We are trying to understand the past in order to predict the future."

Publication

Zuidema, P.A., Groenendijk, P., Rahman, M. et al. (2025), 'Pantropical tree rings show small effects of drought on stem growth', Science. DOI: 10.1126/science.adq6607