“Tree of life” facing an outbreak of deaths

 arstechnica.com  6/14/2018 3:42:33 PM  2  Diana Gitig

Baobabs have super-thick trunks and branches that look like root systems reaching for the sky. African bushmen said that when the god Thora created the world, he took a dislike to the baobab growing in his garden so he threw it out over the wall of paradise onto Earth below; it landed upside down and continued to grow that way. Others claim that Thora had to uproot the trees and replant them upside-down, because when they were initially planted the right way, they kept on walking. Tribal elders and kings used to hold meetings under baobab trees since they believed that the tree's spirits would guide them in decision-making.

On a more practical level, the baobab is known as the Tree of Life because it can store water in its enormous trunk during the rainy season and bear fruit later in the dry season, when other food is scarce. But associating them with life may be temporary; the oldest and largest among these trees are dying.

Baobabs periodically produce new stems the way other trees make branches, which is why they appear so thick. They start off with one stem and grow additional ones in a ring shape. Sometimes, the multiple stems are obvious, when they are only fused at their bases and each grows outwards like an inverted cone. But sometimes the stems fuse to look like one giant circular trunk, with a hollow cavity inside that can be used as a shelter, for grain storage, as a water reservoir, or even for a burial site. Or a bar.

Since the stems fuse over time, the tree’s age cannot be determined by counting rings of fallen specimens as is done for other trees. In 2005, a group of scientists developed a method for measuring the age of live baobabs. They use accelerator mass spectrometry radiocarbon dating of small samples collected from the inner cavities and from other areas on the trunk. This is in fact how they determined that all baobabs are actually multistemmed. They found that the wood in the middle of the stems is the oldest, with younger wood radiates outward toward the environment and also inward toward the cavity.

The researchers provided images of some of the dead baobabs.
Enlarge / The researchers provided images of some of the dead baobabs.

Since 2005, the group has dated more than 60 trees, all of the largest and potentially oldest baobabs on the planet. To their surprise and chagrin, they found that the largest and oldest trees—or surviving trees’ largest and oldest parts—have died. These trees all had girths exceeding twenty meters around, grew an average of twenty-one meters tall, and ranged in age from 1,100 to 2,500 years. They all lived in southern Africa. And starting in 2006, their oldest stems—in some cases all of their stems—toppled over and died.

“The deaths of the majority of the oldest and largest African baobabs over the past 12 years is an event of an unprecedented magnitude,” the scientists write. “These deaths were not caused by an epidemic and there has also been a rapid increase in the apparently natural deaths of many other mature baobabs.” After a century of humans using their insides as living spaces, maybe they were just done.

Nature Plants, 2018. DOI: 10.1038/s41477-018-0170-5 (About DOIs).

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