The day on which this annual event occurs is September 21st. The boldest phenomena we associate with this time of year is the changing colors of deciduous tree leaves.
Many environmental factors affect the leaves of trees such as climate zones, atmospheric conditions (cloudy skies), lack of sufficient rainfall, presence of toxic substances in the air, and soil fertility. But the overwhelming factor is darkness or its absence thereof. The steady, by about three minutes a day, increase of darkness giving longer nights and the dropping of night time temperatures institutes the change.
The sap (much like the human blood stream) that carries the green chlorophyll becomes heavier and begins a slow retreat down into the roots for winter safety and no longer reaches the leaves. If you can spot a single leaf and study it over a period of several weeks you will notice that the outside edge of said leaf turns first. Then as each day passes it gradually acquires increasing color from the outside in to the stem. The higher the leaf is on the tree or the furthest it is from the trunk the sooner its color will change. Unless the wind and squirrels extend a helping hand the very topmost leaves on the limbs of a tree will be the first to fall.
This makes photographing a tree in all its fall glory difficult because except for maybe one day of perfection there will be bare branches sticking out around the body to the tree.
The first native trees to lose their leaves in our area (along the 38th parallel) are the black locust and black walnut. It is not unusual to see their leaves gone by the middle of August almost as if they evaporated. The dogwood begins its show of burgundy in the early/middle of September. Elms are the no shows as their leaves seem to shrivel to a drab brown and fall.
Maples play the vibrant strings of fall color, each species sounds its own footnote. Sugar maples an orange-red, black maples - brilliant yellow, and the red maple produces a blazing scarlet. Yellow poplars play true to their name and sport a burnish yellow.
Next in line are the hickories with golden bronze and sourwood with its deep crimson. Beech trees produce a golden yellow for a few days, then turn dusty tan with many of its leaves remaining through the winter until they are pushed off by fresh buds in the spring.
The mighty oaks open the last stage with color of brilliant red or golden bronze depending on their family. There is a story told that during our early days the oaks took allegiance with either the settlers or the Indians. Those who sided with the settlers have round edges to their leaves like the bullets and the oaks that supported the Indians have sharp points on their leaves like arrowheads. Hence we have white oaks and red oaks.
Two transplants close out the final chords. The Bradford Pear's leaves turn a sturdy burgundy and have a texture like leather which cling to the limbs well into winter.
The final crescendo is the ginkgo, for a brief time it is a blazing
The symphony of fall color has ended for the season, not in a puttering muting, but in a thundering climax.
Nash Black, author of Games of Death http://amzn.com/B011F3SCPK a collection of ghost stories.