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Taking Shade Under Yosemite Oak Trees Comes With Risks

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Yosemite, CA – With the triple-digit temperatures, Yosemite National Park is warning visitors about a summer phenomenon.

Black Oak tree in Yosemite and an example of Summer Branch Drop
Yosemite National Park photo

While many visitors like to cool down by taking a break under the shade of a nice oak, rangers caution that it can be hazardous to your health due to what is called “Summer Branch Drop (SBD).” Park officials detailed, “It is when seemingly healthy trees suddenly lose large or horizontal branches without warning. This unexpected arboreal event commonly happens on hot and calm days in the summer.”

While there is no clear consensus from arborists and scientists on this phenomenon, one theory revolves around moisture building up. Park officials explain that the moisture building up within branches on warm and calm days when evapotranspiration, the release of moisture through leaves, is limited, can cause the branches to fall. They added, “SBD may also be related to drought stress and very dry soil conditions.”

The California black oaks in the park are most prone to suddenly dropping their branches. Park officials give this warning to visitors, “Generally, it is a good idea to avoid sitting or setting up a tent directly underneath a large oak branch. Be aware of your surroundings and do not leave immobile people, such as infants or the elderly, directly under large oak trees. If you hear a loud crack from a tree, quickly get away from the area.”

Visitors that witness SBD are asked to report it to a nearby ranger or through the Yosemite website.