Aaron Roth, a biologist at Northeastern University explained, “What you are actually seeing is the trees conserving energy”. This is not a problem in the summer when trees can pull water up from their roots and there is enough sunlight to be absorbed by the hungry plants.
In the autumn when the weather cools, something amazing happens in the trees. All the green leaves begin to change colors yellow, red and brown but what makes the leaves change color and what makes them fall? For these question we need to learn a little bit about how leaves work. Leaves are the food factory of the plant. Trees take in water through their roots, sunlight and carbon dioxide through their leaves. They use energy from the sunlight to turn water and carbon dioxide into oxygen and glucose. The oxygen is released back into the atmosphere where it can be used by animals. The glucose is used to feed the plants. The process plants use to make their food is called ‘photosynthesis’ and it is possible because plants have a bright green chemical inside of them called ‘chlorophyll’ which allows them to absorb the energy from sunlight. Chlorophyll is what makes leaves look so green! As summer ends and fall begins, the days begin to grow shorter and the nights longer, this is a signal to the tree that the growing season is at an end and the winter is coming. During the winter there is not enough light or water for the broad leaves trees to make their food. Instead, they will rest and live off the food they stored during the summer, so they begin to shut down food factories. The trees stop sending water to the leaves. All the chlorophyll in the leaves gets used up, the green color fades and other colors become visible in the leaves. Once the leaves have dried up, they begin to drop off and soon the branches of the tree are bare. Trees that lose their leaves every winter are called deciduous trees.
Now you know why the leaves change color and fall in the autumn.