Does heat cause branches to fall from trees? Scientists aren’t sure

In 2003, when Paris went through a heat wave that would kill an estimated 15,000 people in France, one oasis of cooler air remained off limits: the city’s roughly 400 public parks. They were temporarily closed due to the danger of falling tree limbs.

“This basically deprived people of the one little bit of green space they might have had to find a slightly cooler atmosphere,” says Richard C. Keller, a professor of the history of medicine at the University of Wisconsin-Madison and the author of the book. Fatal Isolation: The Devastating Paris Heat Wave of 2003.

Trees losing limbs are usually associated with severe weather such as tornadoes, hurricanes and storms. But a little-known risk outside arborist circles is that mature, apparently healthy trees can suddenly lose branches in the summer. It is a phenomenon known as sudden limb failure, sudden limb drop or summer branch drop.

While reports of injuries from sudden limb failure are rare, last month a woman was killed by a falling tree branch in a Washington DC park. The incident prompted at least one local tree service to send out an email warning people about the risk.

Why how healthy looking trees suddenly lose their limbs is a bit of a mystery. “There is no scientific consensus as to why this happens,” says Spencer Campbell, plant clinic manager at the Morton Arboretum in Lisle, Illinois. “It’s hard to predict.”

But at least one theory links summer branch drop to extreme heat — especially if a hot summer is preceded by a wet spring. Wet springs allow for extensive tree growth, which can lead to water stress when warmer summer temperatures arrive. Trees may drop limbs as a form of self-pruning.

Another idea holds that extreme winter weather—from cold temperatures to the pressure snow puts on branches—causes trees to weaken internally, so the pressure of new limb growth in the spring tips them over the edge.

Summer branch drop has historically been considered a warm-climate phenomenon, seen in places like Australia, South Africa, and the southwestern US that typically receive little snow. But since at least the 1970s, arborists have also reported it in places with a wider range of climates, including Illinois, New York and Great Britain.

Trees experiencing sudden limb drop could also be unhealthy, even if it’s not visible. (The Washington tree was inspected, pruned and found healthy in 2022, according to news reports.) “Professional arborists are trained to identify trunk rot, cankers (and other) physical symptoms,” Spencer says, adding that “it’s more difficult to diagnose if it is an internal crack or weak point.”

The most likely conclusion is that the sudden limb drop stems from a confluence of circumstances: trees teetering on the brink of being unhealthy, along with conditions like extreme heat pushing those trees to a breaking point.

This was the conclusion of a 2023 case study that looked at sudden limb drop in four cities in Portugal. The researchers determined that the summer branch-fall incidents were related to internal tree decay, but noted that the decay itself could also be related to environmental conditions.

“I think for a lot of the trees in our landscape, it’s not just one thing; there are a lot of stressful events,” says Campbell. Possible culprits range from unseasonably warm evenings to cold spells, fungal diseases and insect damage exacerbated by rising temperatures

Whatever the cause, Campbell says it’s not worth worrying too much about staying under the trees, even in the summer. “This event is exceptionally rare,” he says. “It happens, but most of the time it happens without anyone noticing.”

Photo: Visitors sit in the shade in Barcelona’s Poblenou Park during a heat wave in July 2023.

Copyright 2024 Bloomberg.