Some places are reporting an unexpected phenomenon where healthy trees suddenly drop branches during the summer. Scientists are currently unable to confirm the link between heatwaves and this occurrence.
In 2003, as Paris endured an unusual heatwave, an ideal “oasis” for escaping the heat had to shut down. Approximately 400 public parks in the city were temporarily closed due to the risk of falling branches. Why did this happen?
People at Poblenou Park in Barcelona. (Photo: Bloomberg).
“(This regulation) essentially took away a small portion of green space where people could enjoy a somewhat cooler atmosphere,” remarked Richard C. Keller, a professor of medical history at the University of Wisconsin-Madison and author of *Fatal Isolation: The Devastating Paris Heat Wave of 2003*.
Falling trees and branch drops are typically associated with extreme weather events, such as storms or tornadoes. However, there is another risk that few are aware of outside of botanical experts: mature and seemingly healthy trees can suddenly drop branches during hot weather.
According to Bloomberg, this phenomenon is known as sudden limb failure or summer branch drop.
Reports of injuries caused by sudden branch drops are quite rare. However, in July, a woman was struck by a large branch that fell in a park in Washington D.C. (USA). This incident prompted a local tree maintenance service to send out emails warning residents of similar risks.
The cause of healthy trees suddenly dropping branches remains a mystery.
“The scientific community has yet to reach a consensus on the reasons for this phenomenon,” stated Spencer Campbell, Director of the Plant Clinic at Morton Arboretum in Lisle, Illinois (USA).
One hypothesis suggests that the phenomenon of summer branch drop is linked to extreme heat, particularly after a wet spring. A wet spring provides ample moisture for trees to thrive. However, when summer arrives with high temperatures, trees struggle to maintain the necessary water levels. As a result, trees may shed branches as a self-protective measure to reduce the amount of foliage they need to sustain.
Another perspective posits that the harsh winter conditions, such as low temperatures or the weight of snow on branches, damage the internal structure of trees. When spring arrives, these weakened branches cannot withstand the pressure of new growth, leading to branch breakage.
The cause of healthy trees suddenly dropping branches remains a mystery. (Photo: New York Times).
Instances of summer branch drop have previously been observed in regions with little snowfall, such as Australia, South Africa, or the Southwestern United States. However, since the 1970s, researchers have warned that this phenomenon is occurring in diverse climates, including cities in the USA (Illinois, New York) and the United Kingdom.
Moreover, trees that experience sudden branch drop may not be healthy, even if there are no obvious signs.
“Researchers can identify conditions such as trunk rot, cracks, and other disease symptoms in trees,” Mr. Spencer noted, adding that “it’s difficult to diagnose if symptoms manifest internally.”
According to Bloomberg, the most plausible conclusion regarding this phenomenon is that trees that are already unhealthy are then subjected to extreme weather, such as high temperatures.
A 2023 study conducted in four cities in Portugal found that in cases of sudden branch breakage during the summer, the affected trees were weak from within. However, the weakness of the trees is also influenced by environmental conditions. For example, trees may endure unusually warm nights followed by sudden cold snaps, or elevated temperatures that exacerbate fungal or insect-related diseases.
Regardless of the cause, Mr. Spencer advises not to worry excessively when standing under trees, even in the summer. “Such incidents are extremely rare. Typically, people are also less aware of this phenomenon,” he said.
