Should oak caterpillars consume a large quantity of leaves in one year, the buds next spring will unfurl three days later. This postponement means that when the caterpillars emerge, their food source is unavailable, causing many to perish, which subsequently halves the amount of foliage eaten.
During springtime, longer and warmer daylight hours prompt trees to resume growth, opening their buds and releasing new leaves. Numerous species synchronize their life cycles with this, meaning some caterpillars, for instance, hatch precisely when the leaves are still tender and fresh, allowing them to commence feeding immediately.
Now, Sumen Mallik from the University of Würzburg in Germany and his colleagues have uncovered a countermeasure employed by oaks against this. They analyzed tree canopy conditions using images captured by the Sentinel-1 radar satellites across a 2,400 square kilometer area in northern Bavaria, Germany, from 2017 to 2021. The study’s findings were published in Nature Ecology & Evolution.
Two oak species dominate the forests in this region: the pedunculate oak (Quercus robur) and the sessile oak (Quercus petraea). Each pixel in the satellite imagery represented an area of 10 by 10 meters—roughly the size of a single tree’s crown—and the team examined a total of 27,500 pixels.
In 2019, a massive outbreak occurred involving the spongy moth (Lymantria dispar), whose hairy caterpillars feed on tree leaves, resulting in substantial damage when they are abundant.
The satellite data revealed which trees were defoliated and how they responded to this. If an oak had been heavily infested by caterpillars, its leaves emerged three days later the following spring compared to trees that had not sustained significant damage.
This delay reduced the damage inflicted by caterpillar feeding by 55 percent compared to the previous year. Mallik explains this happens because the caterpillars still hatch at the usual time but find themselves facing bare branches instead of a feast of young leaves, leading to the demise of many.
Oaks possess other defensive mechanisms too, including tougher leaves that are harder to chew or aromatic compounds which might attract predators of the caterpillars. “The bud burst delay appears to be more effective than all these other protective mechanisms,” states Mallik, who hypothesizes that other deciduous plants might employ this strategy as well.
“That is entirely plausible,” comments James Cahill from the University of Alberta in Canada, but he cautions that the delay in bud emergence following a caterpillar outbreak is a correlation, and direct evidence of causation is yet to be established. He suggests the delay could stem from reduced plant vigor caused by leaf loss, noting that data from multiple outbreaks would help clarify the situation. “It definitely warrants further investigation.”
Mallik posits that the delay might be attributed to physiological constraints, such as resource depletion, but given that it was observed across dozens of tree populations and was most pronounced in forests where the delay most effectively curtailed herbivory, he views it as an adaptation rather than merely an individual tree’s physiological reaction.
“The mechanisms are intriguing and represent a key area needing more research,” notes James Blande from the University of Eastern Finland.
Forests sometimes green up later in the spring than temperature-based computer models predict, especially as the climate warms, and this research offers an explanation, according to Mallik. “It’s very important to realize that plants are responding to more than just the changing climate,” Cahill adds.
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