Cooling off underneath an oak tree is one of the greatest pleasures after a long trek in the summer sun. However, a delightful picnic or siesta in the shade can be dangerously disrupted by summer branch drop. Summer branch drop (SBD) 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. Arborists and scientists have different theories on the cause of SBD, but there is no clear consensus. One theory is that SBD is a result of moisture building up within branches on warm and calm days when evapotranspiration, the release of moisture through leaves, is limited. SBD may also be related to drought stress and very dry soil conditions. Any tree can be affected by sudden branch drop. In Yosemite, California black oaks are most prone to suddenly dropping their branches. Generally, it is a good idea to avoid sitting or setting up a tent directly underneath large oak branches. 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. If you happen to see a tree or branch fall in the park, please report it to a nearby ranger or through our website. #FindYourPark #RecreateResponsibly #Tree #Safety