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Giant trees in tropical forests, witnesses to centuries of civilization, may be trapped in a dangerous feedback loop, according to a new report.

The biggest trees store half of the carbon in mature tropical forests, but they could be at risk of death as a result of climate change—releasing massive amounts of carbon back into the atmosphere.

Researchers at the University of Birmingham and Smithsonian Tropical Research Institute (STRI) today published their findings in in Nature Plants.

Adriane Esquivel-Muelbert, lecturer at the University of Birmingham, studies the effects of climate change in the Amazon. Evan Gora, STRI Tupper postdoctoral fellow, studies the role of lightning in tropical forests. 

The two teamed up to find out what kills big tropical trees, examining evidence from hundreds of research papers to discover that nearly nothing is known about the biggest trees and how they die because they are extremely rare in field surveys.

Adriane Esquivel-Muelbert commented: “Big trees are hard to measure. They are the pain in a field campaign because we always have to go back with a ladder to climb up to find a place to measure the circumference above the buttresses. It takes a long time. Studies focusing on the reasons trees die don’t have enough information for the biggest trees and often end up excluding them from their analysis.”

Evan Gora commented: “Because we generally lack the data necessary to tell us what kills trees that are above 50 centimeters in diameter, that leaves out half of the forest biomass in most forests.”

Only about one percent of trees in mature tropical forests make it to this size. Others wait their turn in the shade below. The other thing that makes tropical forests so special—high biodiversity—also makes it difficult to study big trees: There are so many different species, and many of them are extremely rare.

“Because only one to two percent of big trees in a forest die every year, researchers need to sample hundreds of individuals of a given species to understand why they are dying and that may involve looking for trees across a huge area,” added Dr. Gora.

Imagine a study of blood pressure in people who have lived to be 103. One would have to locate and test seniors from cities and towns around the world: a time-consuming, logistically complex, expensive proposition. A large body of evidence shows that trees are dying faster in tropical forests than ever before. This is affecting the ability of forests to function and in particular, to capture and store carbon dioxide.

“We know the deaths of largest and oldest trees are more consequential than the death of smaller trees,” said Dr. Gora. “Big trees may be at particular risk because the factors that kill them appear to be increasing more rapidly than the factors that seem to be important for smaller tree mortality.”

In large parts of the tropics, climate change is resulting in more severe storms and more frequent and intense droughts. Because big trees tower above the rest, they may be more likely to be hit by lightning, or damaged by wind. Because they have to pull ground water higher than other trees, they are most likely to be affected by drought.

Hoping to better understand what is happening to big trees, the researchers identified three glaring knowledge gaps:

  • Almost nothing is known about disease, insects and other biological causes of death in big trees;
  • Because big trees are often left out of analyses, the relationship between cause of death and size is not clear; and
  • Almost all of the detailed studies of big tropical trees are from a few locations like Manaus in Brazil and Barro Colorado Island in Panama.

Understanding how big trees die involves a trade-off between putting effort into measuring large numbers of trees and measuring them often enough to identify the cause of death.

The researchers agree that a combination of drone technology and satellite views of the forest will help to find out how these big trees die, but this approach will only work if it is combined with intense, on-the-ground observations.

Dr. Esquivel-Muelbert commented: “I hope the impetus for this research will come from a shared appreciation for these mysterious living monuments, which are fascinating to everyone. When you see one of those giants in the forest, they are so big.

“My colleague Carolina Levis says they are the monuments we have in the Amazon where we don’t have big pyramids or old buildings - they have been through so much. They are fascinating, not just in the scientific sense but also in another way. It moves you somehow.”

  • For more information, interviews, please contact Tony Moran, International Communications Manager, University of Birmingham on +44 (0)782 783 2312. For out-of-hours enquiries, please call +44 (0)7789 921 165.
  • The University of Birmingham is ranked amongst the world’s top 100 institutions. Its work brings people from across the world to Birmingham, including researchers, teachers and more than 6,500 international students from over 150 countries.
  • ‘Implications of size-dependent tree mortality for tropical forest carbon dynamics’ - Gora, E.M and Esquivel-Muelbert, A is published in Nature Plants, doi: 10.1038/s41477-021-00879-0
  • The Birmingham Institute of Forest Research (BIFoR) supports natural science, social science, and cultural scholarship investigating forested landscapes now, in the past, and in the future.
  • Funding for this study was from the Smithsonian Tropical Research Institute, the U.S. National Science Foundation and the TreeMort project as part of the EU Framework Programme for Research and Innovation.
  • Smithsonian Tropical Research Institute, headquartered in Panama City, Panama, is a unit of the Smithsonian Institution. The institute furthers the understanding of tropical biodiversity and its importance to human welfare, trains students to conduct research in the tropics and promotes conservation by increasing public awareness of the beauty and importance of tropical ecosystems.