For example, at the solstice, trees growing in cold places slow down the creation of new wood cells and focus their energy on finishing already formed but still incomplete cells. Along similar lines, trees use the solstice to fine-tune the “winding down”, or senescence, of their leaves in preparation for autumn. For decades we have known that beech mast events happen in the year after a warm summer. Scientists have long puzzled over how beech trees across Europe seem to use the same seasonal window to control mast events. As soon as the days start to shorten after the solstice, beech trees across Europe seem to simultaneously sense the temperature.