As the days get shorter, starting around the autumnal equinox, there is less light, heat and water for photosynthesis.
Photosynthesis utilizes chlorophyll as it provides energy for the tree. With less hours of sunlight, the chlorophyll content of the leaves decreases and the green color no longer hides the yellows, oranges and reds that are also in the leaves, so we see bright autumn colors before the leaves fall.
Trees with a northern exposure often will change color and lose their leaves earlier than the same types of trees that are nearby but getting more hours of sunlight.
Trees respond to the duration of daylight, and will start to lose leaves when the days are shorter even if temperatures have not yet fallen. Different species respond to slightly different lengths of days. A tree living under a street light will have its cycle disturbed by the light.
