what causes leaves to change color in the fall season

Leaves of trees are green in color most of the year because of chlorophyll. Chlorophyll is used during the photosynthesis process to absorb energy from sunlight which is then converted to sugars that provides trees with the nutrition it needs much like humans do

In the fall, as the days gets shorter and temperature drops trees get less direct sunlight and also not much heat elsewhere which starts to break down the chlorophyll. This lack of chlorophyll causes pigmentation in the leaves which makes it turn yellow & orange