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.

Leaves fall because of chemical changes that occur within the tree sap. First  the veins transporting sap into and out of the leaves slowly close down.  Deciduous trees produce chemicals, mainly ethylene and abscisic acid, that  basically cut the link between the leaf and the tree’s nutrient system.  As the  veins close down, a layer of cells, the abscission layer, forms at the base of  the leaf stem, and when it is complete, the leaf falls from the  tree.

Environmental factors also have an impact on when trees lose their leaves.   Trees in cities experience a variety of man-made stressors such as air  pollution, salt damage (from both home water softeners  and use of salt to melt snow and ice), industrial pollution (heavy metals  introduced into the soil) and herbicides used to kill weeds and unwanted plants  near the roots of the tree. All of these factors can influence when an  individual tree will lose its leaves. For example, a tree planted closer to a  busy street may lose its leaves earlier than a tree of the same type in a park  few blocks away.

All deciduous trees shed their leaves.  Some species like maple, beech and  aspen are noted for their spectacularly bright fall color.  Others, like the oak  and chestnut are less colorful.  Different species form the abscission layer at  different rates, causing the leaves to fall at different times. The oak is  unusual because it does not completely form an abscission layer, so oak leaves  often remain attached to the tree throughout the  winter.

Common environmental factors also affect when leaves fall.  The amount of  water the trees get, early or late frost, high winds and unseasonable  temperatures (both warm and cold) can change the pattern of leaf loss in  deciduous trees.  Trees that are watered regularly and stand in sheltered areas  often hold on to their leaves longer than others of the same species that are in  a more exposed setting.