Centuries-old giant trees in tropical forests may be trapped in a dangerous feedback loop and are at risk of dying from climate change -induced events, according to a new report.
What is Happening?
- According to the study published in the journal Nature Plants, the biggest trees store half of the carbon in mature tropical forests, but they could be at risk of dying 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) 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, lecturer at the University of Birmingham, says, “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, STRI Tupper postdoctoral fellow, 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 1% 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. Dr Gora adds, “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.”
- 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.
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Dr Gora says, “We know the deaths of largest and oldest trees are more consequential than the death of smaller trees. 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. Additionally, because they have to pull ground water higher than other trees, they are most likely to be affected by drought.
- 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.