Large Trees Die First In Drought Says LANL Study


Researchers from the Los Alamos National Laboratory have found that large trees are affected more by droughts after reviewing patterns across 38 forests around the world.

In a study published in the journal Nature Plants, researchers pointed out that while smaller trees also suffer during times of drought, the survival and growth of larger trees are more severely affected and that the death of larger trees affect ecosystems because of the role trees have in the carbon cycle.

"Our theory suggested this should be a globally consistent pattern, but this project was the first to test this hypothesis globally," said Nate McDowell, one of the authors of the study, and plant physiologist and forest ecologist from the LANL.

For the study, the researchers examined different kinds of forests, ranging from tropical rainforests to semi-arid woodlands, to see how the size of a tree affects how it responds to a drought. Similar research have been carried out before but only in a handful of sites, nowhere near the 38 forests and 40 drought events tackled by the study.

Tree death is unpredictable but the study helped clarify some processes. For instance, large trees are vulnerable to droughts because of how they are built. Because they are bigger, it's more difficult for them to bring nutrients and water to their leaves to aid in photosynthesis. It also doesn't help that because they are also usually taller, larger trees face higher evaporative demands because they are closer to the sun.

The researchers also found that tree-death percentage was directly proportional to trunk diameter, meaning that the bigger a tree's trunk is, the likelier it is to die during a drought event. They report that 65 percent of the drought events assessed showed that drought-related tree deaths were related to tree size.

The demise of larger trees is seen to impact ecosystems more because, alive, trees soak up a lot of greenhouse gases and store them within their woody tissues. When they die, trees start releasing the greenhouse gases they have stored, turning a carbon sink into a carbon source.

Aside from a team from the LANL, researchers from the U.S. Geological Survey, the University of New Mexico and the Smithsonian's Tropical Research and Conservation Biology Institutes also contributed to the study.

Photo: Jayel Aheram | Flickr

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