Two oak trees in the spring, with varying degrees of leaf growth
Sven Finnberg
When caterpillars consume a significant portion of an oak tree’s leaves in one year, the trees delay the opening of their buds by three days in the following spring. This postponement results in caterpillars lacking food upon hatching, leading to many dying and reducing leaf consumption by half.
During spring, the lengthening and warming days prompt trees to resume growth, leading to the opening of buds and the unfolding of new leaves. Many species, including some caterpillars, align their life cycles with this period, ensuring they hatch when leaves are tender and suitable for consumption.
Soumen Mallick from the University of Würzburg in Germany and his team have uncovered a defensive mechanism in oak trees. They examined tree canopy conditions using images from Sentinel-1 radar satellites, covering a 2400-square-kilometre area in northern Bavaria, Germany, from 2017 to 2021.
This region’s forests are primarily composed of two oak species: the pedunculate or English oak (Quercus robur) and the sessile oak (Quercus petraea). Each pixel in the satellite images represented a 10 by 10-metre area, comparable to the size of a single tree’s crown, with the team analyzing 27,500 pixels in total.
In 2019, a severe outbreak of gypsy moths (Lymantria dispar), whose caterpillars feed voraciously on tree leaves, caused substantial damage.
According to the satellite data, trees heavily affected by caterpillars had their leaves emerge three days later in the subsequent spring than those less affected. This delay reduced the damage from feeding by 55 percent compared to the previous year. Caterpillars hatch at the same time but find no young leaves to consume, leading to widespread mortality, explains Mallick.
A caterpillar on an oak leaf bud
Sven Finnberg
Oak trees also employ other defenses, such as making leaves harder to chew or producing aromatic compounds that attract predators of the caterpillars. The delay in bud opening is considered more effective than these other defenses, according to Mallick, who suggests that other deciduous plants may employ similar strategies.
James Cahill from the University of Alberta in Canada finds the possibility plausible but points out that the connection between bud delay and caterpillar outbreaks is correlational, lacking definitive causality. He mentions that decreased plant vigor due to leaf loss could cause the delay, but more data from multiple outbreaks would provide clarity. He asserts that further research is warranted.
Mallick speculates that the delay might be due to physiological constraints like resource depletion. However, since this pattern was observed across numerous tree populations and was most effective in forests where herbivory reduction was greatest, he believes it represents an adaptation rather than an individual tree’s physiological response.
James Blande from the University of Eastern Finland describes the mechanisms as intriguing and in need of further study.
Mallick notes that forests often turn green later in spring than computer models predict based on temperatures, especially as the climate warms. This study offers an explanation for this phenomenon.
Cahill emphasizes the importance of recognizing that plants respond to factors beyond just climate change.
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