Study finds oak trees delay leaves to outwit hungry caterpillars

By Stephen Beech

Oak trees deliberately delay sprouting leaves by three days to avoid being infested by hungry caterpillars, reveals new research.

The trees' clever tactic to outwit potentially deadly predators was detected by scientists in Germany using data from satellite.

They found that many insects, particularly caterpillars, hatch in spring just when the trees' nutrient-rich leaves are still young and soft, providing them with ample food.

But if oak trees are heavily infested by caterpillars in a given year, they react the following spring by delaying their leaf emergence by three days.

That means the caterpillars have nothing to eat after hatching because the oak leaves are still firmly hidden in the buds.

The strategy is "highly effective" as the three-day delay is sufficient to drastically reduce the insects' survival rate, say scientists.

And it reduces the damage caused by feeding on the tree by 55%, according to the study published in the journal Nature Ecology and Evolution.

Lead author Soumen Mallick, of the University of Würzburg, Germany, said: "The delaying tactic is more effective for the oak than a chemical defense, such as bitter tannins in the leaves.

"This is because the tree would have to expend a great deal of energy to increase tannin production.

"This discovery fundamentally changes our previous understanding of the onset of spring in the forest.

"It shows that trees do not merely react passively to the weather in timing their leaves emerge but also respond flexibly to biological threats."

The research team used state-of-the-art interdisciplinary methods from ecology and remote sensing.

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Previously, scientists had to laboriously observe individual trees on the ground.

For the new study, a 2,400-square-kilometer (925 square mile) area in northern Bavaria was monitored continuously using Sentinel-1 satellite data.

The radar satellites provide precise data on the condition of tree canopies even in thick cloud cover.

The research team analyzed a total of 137,500 individual observations spanning five years, from 2017 to 2021.

The satellites provided data at a resolution of 10x10 meters per pixel, which roughly corresponds to the crown of a single tree. A total of 27,500 such pixels were analyzed across 60 forest areas.

The researchers said the year 2019 proved particularly revealing as the region experienced a massive gypsy moth outbreak.

Study co-senior author Jörg Müller, from the Julius Maximilian University of Würzburg, said: "The radar sensors recorded exactly which trees were stripped bare and how they reacted in the following year."

The researchers say their findings conclusively explain for the first time why, in some springs, the forest does not turn green as quickly as temperatures would suggest.

Previous computer models often calculated the state of the forest inaccurately because they took into account almost exclusively "lifeless" factors such as temperature and ignore biological interactions between plants and insects.

The researchers explained that trees find themselves in an "evolutionary tug-of-war" with rising temperatures caused by climate change pushing them to sprout leaves ever earlier, while pressure from insect feeding is forcing them to hold back.

They say a key advantage of the delaying tactic is that it is temporary and reversible.

Study co-senior author Andreas Prinzing said as the trees only sprout later following an actual infestation, the insects cannot adapt permanently.

Prinzing, of the University of Rennes, France, added: "This dynamic interplay is an example of the forest's high resilience and adaptability in a changing world."

The team plans further experiments to help understand the delay mechanisms more precisely.