It also marks the season for summer branch drop, a phenomenon in which seemingly healthy tree limbs can suddenly fall without warning. Summer branch drop occurs most frequently in black oak trees around Yosemite and is most common during particularly hot, calm afternoons when there is little or no wind. Older, less vigorous trees also appear to be more susceptible to summer branch drop. A tree that has already lost a limb to summer branch drop is also more likely to experience another branch failure. Infants, elderly people, and others who may have difficulty moving quickly should not be left directly beneath large oak trees.