Oak trees delay spring leafing after caterpillar damage

Spring should be a banquet in the forest. For caterpillars and other insects that feed on leaves, it is the point when conditions appear to fall perfectly into place.

They emerge just as young oak leaves begin to unfurl: tender, new and full of nutrients. Under normal circumstances, the timing is so well matched that the insects can start feeding almost at once.

Yet oak trees are not simply passive targets waiting to be consumed.

New research finds that oaks heavily damaged by caterpillars in one year alter their behaviour the following spring. Rather than producing leaves on schedule, they postpone leaf emergence by around three days.

That might seem a minor delay, but it can entirely upset the caterpillars’ plans. They hatch in anticipation of food, only to discover that the leaves remain enclosed within their buds.

The research indicates that this brief postponement has a major effect. It substantially reduces caterpillar survival and lowers the damage inflicted on the tree by about 55 percent.

A less costly delaying tactic

The study’s lead author, Soumen Mallick, is a postdoctoral researcher at the University of Würzburg.

“The delaying tactic is more effective for the oak than a chemical defense, such as bitter tannins in the leaves,” said Mallick.

For the tree, boosting tannin production would require a considerable expenditure of energy.

Put simply, delaying leaf growth proves less costly than mounting a chemical defence.

Oak trees respond to biological pressure

Trees are commonly assumed to respond chiefly to temperature, rainfall and daylight. This study, however, points to a more dynamic process.

Oak trees are reacting not only to weather conditions, but also to biological pressure.

“This discovery fundamentally changes our previous understanding of the onset of spring in the forest,” Mallick said. It demonstrates that trees can respond flexibly to biological threats.

This represents a significant change in how spring in a forest is understood. Rather than being an event driven only by climate, it is also influenced by a subtle exchange between plants and the insects seeking to eat them.

Watching the forest from space

To demonstrate this, the researchers adopted an approach far broader and more advanced than the conventional practice of observing individual trees from the ground.

Rather than manually following a small number of trees, they used Sentinel-1 satellite data to monitor a 2,400-square-kilometre area of northern Bavaria.

These radar satellites are particularly valuable because they can identify changes in tree canopies even beneath heavy cloud cover, a considerable benefit during spring.

Across five years, from 2017 to 2021, the team examined 137,500 observations. Each satellite image had a resolution of 10 by 10 metres per pixel, approximately the area covered by a single tree crown.

In total, the scientists assessed 27,500 of these pixels across 60 forest areas.

This scale was important because it allowed the team to examine responses across whole landscapes, rather than in only a small group of trees.

Caterpillar outbreak reveals the strategy

One particular year provided an ideal natural experiment. In 2019, a severe gypsy moth outbreak struck the region.

The caterpillars defoliated many trees, creating precisely the sort of stress needed to establish whether oaks altered their timing in response.

“The radar sensors recorded exactly which trees were stripped bare and how they reacted in the following year,” said co-senior author Jörg Müller.

The results were unambiguous. Oaks that had suffered the heaviest attacks delayed leaf emergence in the following spring.

This helps account for a question that has long puzzled scientists. Forests can sometimes remain brown for longer than rising temperatures alone would imply.

What forest models overlook

The findings carry important implications for ecology and conservation.

Many forest models continue to concentrate mainly on what the researchers call “lifeless” factors, including temperature and rainfall, while giving far less attention to relationships between living organisms.

However, if trees change their seasonal schedules in response to insects, such models are leaving out part of the picture. That omitted factor may become increasingly important as the climate changes.

The researchers characterise this as an evolutionary tug-of-war. Warming temperatures are encouraging trees to leaf out progressively earlier, while insect pressure gives them an incentive to delay.

This conflict may influence the appearance of spring in forests of the future.

A smart, adaptable strategy

The oak’s approach is particularly ingenious because it is temporary. Rather than permanently altering its seasonal schedule, the tree postpones leafing only after a genuine infestation.

As a result, insects cannot readily adjust to a fixed revised timetable, since the response remains flexible.

“This dynamic interplay is an example of the forest’s high resilience and adaptability in a changing world,” said Andreas Prinzing from the University of Rennes.

Perhaps the most remarkable aspect is that, although forests may appear motionless from outside, they contain countless small negotiations of this kind.

It offers a different view of spring: not merely a season arriving on cue, but a living contest formed by weather, memory and the continual pressure to survive.

The research was published in the journal Nature Ecology & Evolution.