Rangers at Yosemite National Park are warning visitors to be extremely careful when sitting under trees during late summer days. At this time of year, healthy-looking oak trees can undergo something called 'summer branch drop', which is exactly what it sounds like – limbs suddenly breaking off and crashing down.
As the National Park Service explains, if you hear a cracking sound near a tree, you should get out of the way immediately. Any trees can be affected, but Yosemite's black oaks are the most likely to suddenly shed branches, so it's best to avoid sitting or camping underneath their large limbs.
Summer branch drop (also known as sudden limb drop or sudden limb failure) isn't related to wind, and often happens in the afternoon on hot, calm days. Rather than coming away right at the trunk, branches tend to break off a few feet along, and are usually long, horizontal limbs.
According to the University of California's Master Gardeners, summer branch drop may be caused by drought stress, which limits the flow of water within the branch. This raises its internal temperature, and increases the concentration of ethylene. This can weaken the plant cell walls, which together with increased sap pressure, can result in the limb breaking away.
"Old wounds and decay hidden inside a limb (possibly resulting from improper pruning) occasionally contribute to branch drop, but this does not account for the majority of summer branch drop failures," say the Master Gardeners.
"Be aware of your surroundings and do not leave immobile people, such as infants or the elderly, directly under large oak trees," says the NPS. "If you hear a loud crack from a tree, quickly get away from the area. If you happen to see a tree or branch fall in the park, please report it to a nearby ranger or through our website."
