Study Questions the Accuracy of Olive Tree Dating Methods

A new study pub­lished in the sci­en­tific jour­nal Dendrochronologia is mak­ing waves in the olive tree age dat­ing com­mu­nity.

Researchers from the United States, Spain and Lebanon con­cluded that an olive tree in north­ern Lebanon, esti­mated to be approx­i­mately 1,100 years old, is the old­est liv­ing olive tree accord­ing to radio­car­bon dat­ing.

They added that most olive trees believed to be more than 1,000 years old have been dated incor­rectly using less pre­cise meth­ods.

“Most mon­u­men­tal olive trees are cen­ten­nial but not mil­len­nial, with very old trees prob­a­bly restricted to harsh sites where trees show slow growth rates,” the researchers wrote.

However, some sci­en­tists and his­to­ri­ans believe numer­ous olive trees across the Mediterranean basin are much older. In the West Bank, Crete, Sicily or Sardinia, to name a few, one can eas­ily find large, majes­tic olive trees that are often con­sid­ered to be between 2,500 and 4,000 years old.

Local com­mu­ni­ties revere these trees as nat­ural his­tor­i­cal mon­u­ments. This rev­er­ence is due to their deep and unique inte­gra­tion into their regions’ social, cul­tural, reli­gious and eco­nomic fab­ric.

The sig­nif­i­cant role played by these remark­able trees may influ­ence a non-sci­en­tific approach to deter­min­ing their age. For exam­ple, a tree that is 500 years old may eas­ily be regarded as sig­nif­i­cantly older.

The researchers wrote, ​“Published age esti­mates indi­cate that most ages of old olive trees range between 200 and 700 years.”

They argue that one of the com­mon mis­con­cep­tions is the size of the world’s old­est olive trees. Large, wide trunks, impos­ing canopies and strong branches sprawl­ing out in ran­dom direc­tions give form to a wild and untamed appear­ance. Onlookers’ imag­i­na­tion about the age of such impres­sive crea­tures is eas­ily trig­gered.

“The majes­tic appear­ance of some old olive trees con­veys strong feel­ings, and it hap­pens quite often that mon­u­men­tal olive trees are also mis­taken as mil­len­nia-old trees,” Mauro Bernabei, a den­drochro­nol­o­gist at the Trees and Timber Institute of the Italian National Research Council (CNR-IVALSA), told Olive Oil Times.

“For nearly all of these olive trees, this is not the case,” he added. ​“However, this in no way dimin­ishes their mag­nif­i­cence, his­tor­i­cal impor­tance, or cul­tural sig­nif­i­cance.”

Researchers believe olive trees might live longer than any other broadleaf species. These trees are char­ac­ter­ized by wide, flat leaves that dif­fer from the nee­dle-like or scaled leaves found in conifers.

“We know of conifers liv­ing up to 4,000 years. The well-known Pinus Aristata in North America can live up to 4,800 years. When it comes to broadleaf, though, sci­en­tists tend to con­sider 1,000 years as an approx­i­mate limit,” Bernabei said. ​“The olive tree dated in Lebanon is prob­a­bly reach­ing the very limit of broadleaf lifes­pan.”

“Several fac­tors deter­mine the size of a tree,” he added. ​“Examples are the envi­ron­ment in which the tree is located, the soil fer­til­ity, the expo­si­tion to the sun or the height of its loca­tion, and more. Among all fac­tors influ­enc­ing olive trees devel­op­ment, age is the one affect­ing it less.”

The his­tor­i­cal method of deter­min­ing the age of trees – count­ing the rings in the trunk – remains the most accu­rate way to esti­mate the age of many species.

“There are cita­tions of this in the Bible; Leonardo da Vinci wrote about it; it is ancient knowl­edge,” Bernabei said. ​“All chil­dren are told that one ring for one year is the rule.”

The devel­op­ment of these rings and their num­ber and dimen­sion depend on many fac­tors, includ­ing plant species and cli­mate.

Tree rings allow sci­en­tific analy­sis to pin­point with some pre­ci­sion how many years passed since the tree was born. Usually, a tree cor­ing oper­a­tion can reveal the growth rings and sus­tain a cred­i­ble date esti­mate.

However, the method does not work for olive trees. ​“There is an issue with the olive tree, as its rings are not clearly vis­i­ble,” Bernabei said. ​“When you look at olive wood, you can spot dark shapes which are often mis­taken for rings. They are not.”

Some long-liv­ing trees, such as conifers, develop dis­tinct rings that can be directly linked to the sea­sonal changes of win­ter and sum­mer.

Many trees pull back all their flu­ids dur­ing win­ter in prepa­ra­tion for the oncom­ing below-freez­ing tem­per­a­tures. This process helps pre­vent the expan­sion and freez­ing of liq­uids, which could oth­er­wise cause sig­nif­i­cant dam­age to the tree.

Year after year, this self-pro­tec­tive behav­ior leaves a clear imprint on the wood’s growth process and con­tributes to the for­ma­tion of rings.

However, olive trees — Mediterranean plants that thrive in a warmer cli­mate — do not expe­ri­ence this process. Instead, dry sea­sons might leave ring-like marks on the olive tree, which are not par­tic­u­larly use­ful for deter­min­ing the tree’s age.

“This type of ring might mean some­thing about the dif­fer­ent moments of a sea­son the tree lived through but can­not tell much about the years that went by,” Bernabei said.

“The only way to observe the rings in the olive tree is to make slides, so to observe the wooden tis­sue at a micro­scopic level,” he added. ​“Even in that case, though, such analy­sis may not con­vey the desired results.”

Other issues in dat­ing olive trees’ age come from their shape. ​“The trunk shape is rarely per­fectly cir­cu­lar in very old trees,” Bernabei said.

“Those trees have beau­ti­ful shapes made of turn­ings, inden­ta­tions and defor­ma­tions that pre­vent a clear view of the wood ring,” he added. ​“This mor­phol­ogy also makes sam­pling com­pli­cated.”

Sampling and tree cor­ing to date the age of a tree are car­ried out with spe­cial­ized equip­ment. The process involves extract­ing use­ful tis­sue from dif­fer­ent parts of the tree.

Bernabei cited pre­vi­ous research based on sam­ples, show­ing how dif­fer­ent sam­ples of the same olive tree were sent to dif­fer­ent lab­o­ra­to­ries for analy­sis, which yielded highly vari­able dat­ing results.

“There were no two sim­i­lar results, and that is due to the fact that the rings sim­ply can’t be seen,” he said. ​“When it comes to olive trees, sam­pling is hin­dered by the fact that after the first exter­nal layer of five to ten cen­time­ters, the inter­nal por­tion is dry and super-hard.”

“The cor­ing equip­ment can­not go far­ther with­out being bro­ken,” he added. ​“If you use a driller, then it devel­ops such heat that it burns both the sam­ple and the inter­nal wood.”

One method used in Lebanon, ini­tially devel­oped by Bernabei, involves radio­car­bon dat­ing olive tree wood.

Radiocarbon dat­ing tech­nol­ogy mea­sures the amount of car­bon-14 in organic mate­r­ial. By com­par­ing the remain­ing amount of the radioac­tive iso­tope of car­bon to the known time of its decay, researchers can esti­mate its age.

By apply­ing these tech­niques to mul­ti­ple sam­ples taken from dif­fer­ent olive trees, Bernabei esti­mated the age of three olive trees in the Gethsemane gar­den in 2015.

The chal­lenge for dat­ing these trees was that the tree trunks were hol­low, so the cen­tral, older wood was miss­ing. As a result of this lim­i­ta­tion, many more trees could not even be ana­lyzed.

“In sev­eral trees, a large void in the trunk’s cen­ter has led to it diverg­ing into dif­fer­ent stumps, which makes it very dif­fi­cult to recon­struct the shape of each one’s basal area,” Bernabei wrote. ​“In the end, only three from a total of eight olive trees could be suc­cess­fully dated.”

Based on his years of research, Bernabei believes it is impos­si­ble to know the true age of the world’s old­est olive trees.

He con­cluded, ​“A sci­en­tif­i­cally sound esti­mate of the age of olive trees can­not cur­rently be deter­mined.”