Oak trees delay leafing to outmanoeuvre caterpillars

Trees in temperate oak forests delay their time of budburst and leaf emergence to protect against insect attacks, scientists have found. This helps them escape the peak activity of caterpillars feeding on them.

The study published in the journal Nature documented the changes in 60 oak forest sites spread across 2,400 km using high-resolution satellite radar data over five years between 2017 and 2021. 

The researchers found that intense leaf infestation by caterpillars on oak trees in a given year was followed by consistent delayed budburst in the spring season the following year by an average of three days. 

This strategy makes it unfavourable for the caterpillars to survive immediately after hatching by making food unavailable. The insect relies heavily on the soft fresh leaves and faced food scarcity as oak leaves remained hidden in the buds.

The delaying of leaf emergence by just three days, was adequate to drastically reduce the insects’ survival rate and findings showed that damage on the tree shrunk by 55 per cent.

“The delaying tactic is more effective for the oak than a chemical defence, such as bitter tannins in the leaves”, says Soumen Mallick, a postdoc at the University of Würzburg’s Biocentre and lead author of the study in a press statement. This is because the tree would have to expend a great deal of energy to increase tannin production.

He added that the 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,” Mallick said.

The three-day delay in phenology is sufficient enough to cancel out phenological advance noted during a decade of climate warming which is observed to be an advancement of 2.5 days per decade. 

This phenomenon helps explain why these trees have not advanced their spring leafing as erected in the climate models.

“Hence, our data suggest that a single year of elevated herbivory can counteract, and even exceed, the phenological effect of a decade of climate warming. This antagonism may help explain a persistent puzzle: budburst advancement often lags behind the rate of warming,” the study noted.

The study noted that that trees are caught between two opposing forces of global change- climate warming, which selects for earlier budburst to maximise the growing season, and herbivory, which selects for a delay to avoid pests, creating a potential “evolutionary trap”.

“Whilst rising temperatures caused by climate change are pushing them to sprout leaves ever earlier, the pressure from insect feeding is forcing them to hold back. A key advantage of this delaying tactic is that it is temporary and reversible. As the trees only sprout later following an actual infestation, the insects cannot adapt permanently,” the statement noted.

“This dynamic interplay is an example of the forest’s high resilience and adaptability in a changing world,” said Andreas Prinzing, University of Rennes (France), the other co-senior author of the study. Future experiments are intended to help understand these mechanisms even more precisely, Prinzing added.

Observing possible implications, the study stated, “In forests where relative budburst timing is fixed, early-bursting trees would consistently face higher herbivory, reducing their fitness, depleting early-bursting genotypes and eroding phenological diversity within populations. Phenological mosaics disrupt this asymmetry by introducing a delayed negative feedback: trees that are heavily defoliated in one year tend to postpone budburst the next year, thereby escaping peak herbivore pressure.”

It added that this redistribution of herbivory across individuals and years could buffer extreme outcomes, prevent localised perennial overexploitation and maintain within-population variation in phenology between currently delayed and non-delayed plastic trees and between plastic and non-plastic trees.