A Mediterranean future too hot to handle for olive trees

Now, David Kaniewski at the University of Toulouse, France, Carla Khater at the National Council for Scientific Research in Beirut, Lebanon, and co-workers have analysed the growth dynamics of wild and cultivated olive trees in relation to past climate trends in the Mediterranean. The study is the first phase of an international effort to characterize the impact of climate change on the economic resources of Mediterranean countries.

“The agricultural economy in Lebanon depends largely on olive production and related by-products,” says Khater. “Cross-disciplinary studies are immensely helpful for informing future sustainability strategies.” 

The team analysed pollen from a four-metre-long sediment core, taken from the ancient city of Tyre in Lebanon, enabling them to reconstruct 5,400 years of olive tree dynamics and climate variations. 

“A core sample reveals a wealth of information that has been stored in the sediments for thousands of years and can be read like a book, layer by layer, to reconstruct the environmental history of a place,” says Kaniewski. 

They also used climate and growth data from 325 olive growing areas across the Mediterranean. Their results show that the trees’ optimal fruiting capacity is closely tied to seasonal temperature fluctuations, particularly during flowering in spring. Temperatures below -5 ºC damage olive tree tissues, and summer temperatures above 30 ºC weaken the trees’ ability to photosynthesize, affecting fruit formation. Optimal flowering, fruiting and olive oil production requires an annual average temperature of around 17 ºC.

“This study shows for the first time that there is an optimum temperature for olive growing that has not changed in thousands of years,” says Kaniewski. 

Rainfall is another important factor. While olive trees are acclimatised to dry conditions, the researchers found they require around 128 mm of rain in spring to produce new shoots, buds and flowers, and around 46 mm in summer for fruiting. Given these insights, it seems likely that certain countries such as Lebanon will become too hot and dry to grow olives. 

“It may be necessary to use drought-tolerant olive varieties, but these may not have the same yields or the same taste,” says Kaniewski. “We will probably witness a ‘migration’ of olive growing – a watershed moment for the Mediterranean.”

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