Why do trees drop their leaves? Because they no longer serve a purpose and weigh the branches down. All summer leaves photosynthesize – twigs send water and nutrients; leaves return sugar. With less light and colder temps, leaves produce less energy and still require maintenance; in an ice storm they cause broken branches. Through an intentional process the tree seals off the ends of the twigs, freeing the leaves to fall away. The tree enters dormancy, saving its energy for spring.