Olive trees have grown on the sunny hillsides of Andalusia, Spain’s southernmost region, since the days of the Roman Empire. Today, 300 million olive trees planted in neatly arranged rows cover 2.5 million hectares of land, forming a unique landscape known as mar de olivos—the sea of olive trees. They produce one third of the world’s olive oil.
A group of researchers from the University of Cordoba in Spain and the Research Center for Geo-Space Science (CICGE) in Portugal has tried to determine how well the olive industry will adjust to the effects of climate change. They found that increased drought and warmer winters will limit the amount of land available to grow commercially relevant olive varieties, reducing production by 30% before the end of the century. Their work appeared online on 17 December in Science of the Total Environment.
Olive trees are very well suited for the Andalusian climate. They are resistant to heat and drought, which are common during summers. The winters are cold, but temperatures rarely dip below −8°C, the tree’s lower tolerance limit. Olive trees also need a bit of a chill during winter to flower during spring, a physiological requirement known as vernalization.
To determine how current olive plantations will respond to climate change, the researchers used an algorithm-based computer method known as species distribution modeling (SDM). This tool is commonly used to predict which areas are suitable for the presence of a certain species based on environmental features. Combining SDM with high-resolution climate projections and extremely detailed satellite imagery, they were able to predict the future evolution of the most common olive varieties.
According to Salvador Arenas-Castro, a biologist at the CICGE and first author on the new paper, this is the first time such a method has been developed with this level of detail and tailored to the specific olive varieties that are grown commercially. “We have the GPS coordinates of each olive tree and its specific variety, and we can relate them with layers of future environmental conditions,” Arenas-Castro said. “We can correlate each tree’s specific location with its predicted levels of precipitation, temperature, evaporation, and so on.”
The researchers found that the most important factor that will reduce olive production is rainfall reduction and loss of soil humidity. Five of eight Andalusian provinces were predicted to lose olive production, with reductions in suitable land for olive production of 29% in Seville, 25% in Cadiz, and a lesser proportion in other regions. They also found that some mountain areas will become better suited for growing olive trees, as these usually colder regions will become more temperate. However, these areas are, for the most part, natural reserves or currently occupied by other crops.
Emphasis on Versatile Varieties
The team also predicts changes in the current distribution of different olive tree varieties. Currently, two varieties represent 80% of all the Andalusian olive crops: picual (60%) and hojiblanca (20%), which are the most productive and versatile. However, over the centuries olive growers have selected other varieties more adapted to certain local climates, soil types, or difficult terrain.
The new research found that these local varieties have more risk of disappearing. “These varieties are more restricted because they have specific needs that are also spatially restricted,” Arenas-Castro said. He cites nevadillo, a variety currently grown in Sierra Morena, near Cordoba. The right climatic conditions to grow it will have completely disappeared by 2100.
Other experts aren’t so sure about the fate of these trees under climate change. “Olive trees are extremely hardy; it’s unlikely they will just die off,” said Diego Barranco, an expert in olive cultivation and olive genetics at the University of Cordoba who wasn’t involved in the new study. “Such drastic climatic changes aren’t expected in the medium term.”
However, it’s likely that growers will replace struggling varieties with more versatile ones. Barranco anticipates that these changes will be slow-paced, making it unlikely that any variety is completely lost.
From Prediction to Action
Despite the potential impact of these findings on the local economy, the researchers say their work isn’t likely to guide any meaningful action in the short term. “We add this knowledge to the scientific literature, but it’s up to those guiding the decision-making process to pick it up and transform it in incentives for the olive growers,” Arenas-Castro said. “Most policy makers tend to think in the short term, whereas this tool works in the mid- to long-term, but precisely to help assessing those short-term decisions.”
These incentives aren’t likely to arrive any time soon. “Currently, olive growers aren’t making decisions based in future projections,” Barranco said. “If they start having problems, for instance, if the winters get too warm for a certain variety, they will slowly replace the troubled varieties with others that won’t have that problem, but at the moment they are not seeing enough changes to alter their plantation patterns.”
However, choosing the right varieties might prove crucial if temperatures rise faster than expected. In the most common plantation system, olive trees need 8 to 10 years to reach full production. “Drought is currently the main problem [for olive plantations], and it affects all the varieties, but at the current change rate, growers are still choosing the most productive ones,” Barranco said.
—Javier Barbuzano (@javibarbuzano), Science Writer