Olive Tree in Lebanon May Be Oldest in the World

Olive trees are common in Northern Lebanon, but in the village of Bshaaleh, a handful look downright ancient. Their branches grow in wildly unpredictable directions, and the trees’ gray, gnarled trunks are shot through with holes and crevasses large enough to hide a sleeping child. Many people believe these sentinel trees are thousands of years old. They are known as “Noah trees” because of some people’s belief that the trees are the source of the olive branch carried by the dove back to Noah’s ark.

Scientists have now established a more certain age for several of the trees of Bshaaleh (also spelled Bchaaleh) and found that most are about 500 years old. But one, a behemoth measuring roughly 14 feet in diameter, is more than 1,100 years old. That’s the oldest olive tree in the world, the team reported last month in the journal Dendrochronologia.

Not many other large olive trees have been reliably dated, despite the cultural, spiritual and economic importance of such trees in places such as the Mediterranean. Scientifically determining the ages of olive trees is challenging, said J. Julio Camarero, a dendrochronologist at the Pyrenean Institute of Ecology in Zaragoza, Spain, who led the study of the Bshaaleh trees. That’s because these trees often lack regular growth rings, he said. “The rings are not easy to see.”

Most species of trees form annual growth rings. Researchers can count the number of rings in samples of wood extracted from living specimens and precisely determine a tree’s age. That practice has spawned an entire subfield of scientific inquiry known as dendrochronology. Investigations based in large part on analyzing tree rings have shed light on, for example, the timing of the arrival of the Vikings in what is now Newfoundland, and the craftsmanship of Antonio Stradivari, the Italian artisan renowned for his stringed instruments.

But olive trees — which scientists have previously shown can live hundreds of years — often have irregular or even missing annual growth rings. Furthermore, older olive trees can exhibit multiple trunks, hollowed-out interiors and other vagaries of growth that make it difficult to determine their ages. Even professional tree-ring scientists have trouble with olive trees: An earlier study showed that when different tree-ring laboratories received samples of wood from the same olive trees, the labs reported tree-ring numbers that varied by as much as a factor of three.

Given the difficulties associated with counting olive trees’ rings, researchers have attempted to infer the ages of olive trees based on their diameters. However, such age estimates can be uncertain, given that soil fertility, climatic conditions and other factors can affect a tree’s growth. “Size is not the same as age,” Dr. Camarero said.

Olive trees are generally understudied as a result, said Peter M. Brown, the director of Rocky Mountain Tree-Ring Research, who was not involved in the new study. “No one has paid much attention to olives because of the difficulty in doing tree-ring research with them.”

In 2018, several of Dr. Camarero’s colleagues traveled to Bshaaleh, located about 50 miles north of Beirut. With permission from the village’s leaders, the researchers cut samples of wood from 11 olive trees. The team wouldn’t be counting tree rings, so getting a continuous sample of wood from the center of each tree out to its bark wasn’t necessary. Instead, the researchers planned to use carbon-14 dating to assess the oldest wood from each tree.

But even collecting just the innermost — and presumably oldest — wood from the trees was a challenge, said Ramzi Touchan, an environmental scientist at the Laboratory of Tree-Ring Research at the University of Arizona who led the sampling work. In many cases, the trees’ middles had rotted away over time. “You don’t see the center,” Dr. Touchan said. In other cases, the trees were multistemmed, and it was unclear where the oldest wood resided. In the face of all the uncertainty, Dr. Touchan said, “I was not optimistic.”

Back at the University of Arizona, researchers extracted the carbon from each of the roughly half-inch samples. By comparing the relative abundance of two isotopes of carbon — carbon-12 and radioactive carbon-14 — the team inferred how much time had elapsed since that wood was formed. Dr. Camarero and his collaborators obtained reliable age estimates for four trees: Three were likely between 500 and 700 years old, and one was roughly 1,100 years old.

Those ages make sense, said Concepción Muñoz Díez, an agronomist at the University of Cordoba in Spain who was not involved in the research.

But it’s important to consider that the olive trees of Bshaaleh might have been grown by attaching part of a tree onto an existing root system, Dr. Muñoz Díez said.

“They don’t know if the trees are grafted.”

The researchers might have inadvertently collected wood from that older rootstock, Dr. Muñoz Díez said, a possibility given that a grafted tree can grow a mixture of both rootstock and cultivar wood. In such a case, the determined ages would be overestimates, she said.

While Dr. Camarero and his team could not rule out the possibility that the trees had been grafted, he said the opposite conclusion could also be drawn: The ages he and his team recovered could also be underestimates if the samples were derived from the cultivar wood.

Whatever their true ages, the olive trees are living treasures for the people of Bshaaleh.

“They represent the cultural heritage of Bshaaleh’s residents, and they serve as a source of pride and a symbol of local identity,” said Rachid Geagea, who is the owner and caretaker of one of the trees and a former mayor of Bshaaleh.

Every fall, he said, residents of the village gather at the trees to harvest that year’s fruit. Working by hand or with devices that resemble fluttering rakes, the villagers collect hundreds of pounds of green- and purple-hued olives. Some of that fruit is preserved to be eaten, and some is pressed for oil.

People are sometimes disappointed when trees don’t turn out to be as old as expected, said Mauro Bernabei, a dendrochronologist at the Italian National Research Council who was not involved in the research. “It’s almost automatic that when you see these kinds of majestic trees to say that they’re millennial.”

But ascribing an age to a tree doesn’t change its value, Dr. Muñoz Díez said. “For those of us who already know them, cherish them and love them, age is a minor detail.”

Rachelle Alwan contributed reporting from Beirut, Lebanon.