Olive trees are common in Northern Lebanon, but in the village of Bshaaleh, a handful look ancient. Their branches grow in unpredictable directions, and the gray, curled trunks of the trees are riddled with holes and crevices large enough to hide a sleeping child. Many people believe that these sentinel trees are thousands of years old. These are known as “Noah trees” because some people believe that the trees are the source of the olive branch that the dove brought back to Noah’s ark.
Scientists have now established a more precise age for some of the Bshaaleh (also spelled Bchaaleh) trees and found that most are about 500 years old. But one, a behemoth measuring about 14 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 areas such as the Mediterranean. Scientifically determining the age of olive trees is difficult, 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 don’t have regular growth rings, he said. “Rings are not easy to see.”
Most tree species form annual growth rings. Researchers can count the number of rings in wood samples taken from living specimens and determine the age of a tree with certainty. That practice gave rise to an entire subfield of scientific inquiry known as dendrochronology. Investigations based in large part on the analysis of 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 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 growth rings. Furthermore, older olive trees may exhibit multiple trunks, hollow interiors and other growth changes that make it difficult to determine their age. Even professional tree-ring scientists have problems with olive trees: An earlier study showed that when different tree-ring laboratories received wood samples from the same olive trees, labs reported tree-ring numbers that varied by as much as a factor of three.
Due to the difficulties associated with counting the rings of olive trees, researchers have tried to infer the ages of olive trees based on their diameters. However, such age estimates can be imprecise, as soil fertility, climate conditions and other factors can affect the growth of a tree. “Size is not the same as age,” says Dr. Waiter.
Olive trees are generally not studied 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 of doing tree-ring research on them.”
In 2018, some of Dr.’s colleagues Camarero traveled to Bshaaleh, located about 50 miles north of Beirut. With permission from the village leaders, the researchers cut wood samples from 11 olive trees. The team will not be counting tree rings, so taking a continuous sample of wood from the center of each tree to its bark is unnecessary. Instead, the researchers plan to use carbon-14 dating to analyze the oldest wood from each tree.
But even collecting just the innermost — and presumably oldest — wood from trees is 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 core of the trees has rotted over time. “You don’t see the center,” said Dr. Touch. In other cases, the trees are multistemmed, and it is unclear where the oldest wood resides. In the face of all the uncertainty, Dr. Touchan, “I’m not optimistic.”
Back at the University of Arizona, researchers extracted 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 was able to determine how much time had passed since that wood was formed. Dr. Camarero and his colleagues 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, said Concepción Muñoz Díez, an agronomist at the University of Cordoba in Spain who was not involved in the research.
But it is important to consider that Bshaaleh olive trees can be grown by attaching part of a tree to an existing root system, said Dr. Muñoz Díez.
“They don’t know if the trees are joined.”
The researchers may have accidentally collected wood from older rootstock, Dr. Muñoz Díez, a possibility given that a grafted tree can grow a mixture of both rootstock and cultivar wood. In such a case, the specified ages would be overestimates, he said.
Although Dr. Camarero and his team the possibility that the trees were grafted, he said that the opposite conclusion can also be made: The ages obtained by him and his team could also be underestimated if the samples came from the cultivar wood.
Regardless of their true age, olive trees are living treasures for the people of Bshaaleh.
“They represent the cultural heritage of the residents of Bshaaleh, 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 autumn, he said, village residents gather at the trees to harvest that year’s fruit. Working by hand or with devices similar to fluttering rakes, villagers collect hundreds of kilograms of green and purple-colored olives. Some of that fruit is preserved for eating, and some is pressed for oil.
Sometimes people are 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 when you see these kinds of majestic trees that you say they’re millennials.”
But giving a tree its age doesn’t change its value, says Dr. Muñoz Díez. “For those of us who already know them, appreciate them and love them, age is a minor detail.”
Rachel Alwan contributed reporting from Beirut, Lebanon.
