Droughts have minimal effect on tropical tree growth – but climate change worsening tree mortality
20,000 tree ring samples shows remarkable growth resilience to droughts, but tree death could result in equivalent of Germany’s annual CO2 emissions
Tropical trees are only minimally affected by drought conditions, but climate change threatens to affect a fine balance according to new research.
In a global study published in Science, a research team led by Wageningen University and including the University of Birmingham have used the largest collection of tropical tree-ring data to date, using more than 20,000 tree-ring series from nearly 500 locations in 36 countries and using data since 1930.
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. 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 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.
These tropical trees have weathered drought conditions across nearly 300 years and have shown a remarkable ability to recover in a year following a drought.
Dr Bruno B L Cintra from the University of Birmingham, and a co-author of the paper said: “This global research collaboration shows us how resilient trees are to climate conditions. These tropical trees have weathered drought conditions across nearly 300 years and have shown a remarkable ability to recover in a year following a drought.
“As with many other aspects of trees which provide a crucial role in taking up CO2 in the atmosphere, there is a real risk that climate change may affect how these trees play this protective role. We have seen how droughts can increase the risk of mortality in tropical trees, which may have a significant effect on total global emissions, and our research underscores the need to get to grips with human emissions and protect trees.”
The study is the result of a recently formed network of tropical tree-ring studies. Coauthor Dr Peter Groenendijk of the University 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.
“We are trying to understand the past in order to predict the future."
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.
Professor Pieter Zuidema of Wageningen University & Research, and lead author of the study said: “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.”
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. 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.
Professor Pieter Zuidema said: “[Stronger local impacts] 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. 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.
“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.”
Notes for editors
Notes for editors
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