These may be the oldest olive trees in the world

Olive trees are common in northern Lebanon, but in the village of Bshaaleh a handful look downright old. Their branches grow in wildly unpredictable directions, and the trees’ gray, gnarled trunks are pierced with holes and crevices large enough to hide a sleeping child. Many people believe that 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 specific age for several Bshaaleh (also spelled Bchaaleh) trees and found that most are around 500 years old. But one, a colossus about 15 feet in diameter, is more than 1,100 years old. That is 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 like the Mediterranean. Scientifically determining the age of olive trees is a challenge, says 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 don’t have regular growth rings, he said. “The rings are not easy to see.”

Most tree species form annual growth rings. Researchers can count the number of rings in wood samples extracted from living specimens and accurately determine the age of a tree. This practice has spawned an entire subfield of scientific research known as dendrochronology. For example, studies based largely on analyzing tree rings have shed light on the timing of the Vikings’ arrival in what is now Newfoundland, and the craftsmanship of Antonio Stradivari, the Italian craftsman known for his stringed instruments.

But olive trees – which scientists have previously shown can live for hundreds of years – often have irregular or even missing annual rings. In addition, older olive trees may exhibit multiple trunks, hollowed interiors and other growth defects that make it difficult to determine their age. Even professional tree ring scientists have difficulty with olive trees: a previous study found that when different tree ring laboratories received samples of wood from the same olive trees, the laboratories reported tree ring numbers that varied by no less than a factor of three.

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

As a result, olive trees are generally under-researched, says Peter M. Brown, director of Rocky Mountain Tree-Ring Research, who was not involved in the new research. “No one has paid much attention to olives because of the difficulty in conducting proficiency testing with them.”

In 2018, several colleagues of Dr. Camarero to Bshaaleh, about 80 kilometers north of Beirut. With the permission of the village leaders, the researchers cut wood samples from eleven olive trees. The team would not count tree rings, so there was no need to continuously take a sample of wood, from the center of each tree to the bark. Instead, the researchers planned to use carbon-14 dating to assess the oldest wood from each tree.

But even collecting just the inner — and believed to be oldest — wood from the trees was a challenge, said Ramzi Touchan, an environmental scientist at the University of Arizona’s Laboratory of Tree-Ring Research, who led the sampling effort. In many cases, the centers of the trees had rotted away over time. “You can’t see the center,” Dr. Touchan said. In other cases, the trees were multi-stemmed and it was unclear where the oldest wood was located. Despite all the uncertainty, Dr. Touchan: “I wasn’t optimistic.”

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

Those ages make sense, says Concepción Muñoz Díez, an agricultural engineer at the University of Cordoba in Spain, who was not involved in the study.

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

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

The researchers may have accidentally collected wood from that older rootstock, said Dr. Muñoz Díez, a possibility considering that a grafted tree can grow a mixture of both rootstock and cultivar wood. In such a case, the established ages would be an overestimate, she said.

Although Dr. Camarero and his team couldn’t 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 an underestimate if the samples came from the cultivar wood. .

Whatever their actual age, the olive trees are living treasures for the people of Bshaaleh.

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

Every fall, he said, village residents gather near the trees to harvest that year’s fruit. By hand or with devices that resemble fluttering rakes, the villagers collect hundreds of kilos of green- and purple-tinged olives. Some of that fruit is saved to be eaten, and some is pressed into oil.

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

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

Rachelle Alwan contributed reporting from Beirut, Lebanon.