There are two kinds of trees: those that shed their leaves and those that do not.
Trees that lose all their leaves once a year are called deciduous whereas trees which lose their leaves continuously and not all at once are called evergreen.
Deciduous trees, or those that lose their leaves, begin to change with the seasons.
Each season is characterized by differences in temperature, amounts of precipitation, and the length of daylight.
In the autumn months of temperate zones, the dropping of the outside temperature and the reduction of the amount of sunlight each day causes trees to change color and then to drop. (The amount of rainfall also affects deciduous trees, as mentioned below.)
These zones extend from 23.5 degrees North (and South) latitude to 66.5 degrees North (and South) latitude.
The fall (or autumn) in the northeastern portion of the United States and Canada, for example, generally starts some time in October and is considered by many to be one of nature's most beautiful displays, with trees that turn from green to colors that range from yellow to orange to deep red. When all of the leaves have dropped and the season turns cold, deciduous trees enter a period of dormancy.
However, there are also trees that lose their leaves due to a shortage of water. This takes places in hotter zones:
[T]ree species in many sub-tropical and tropical regions which experience a strongly seasonal rainfall, also lose their leaves once a year with the onset of the dry season e.g. the African Acacia trees.
Evergreen trees may drop foliage or "needles" sometimes, but do not completely drop leaves in the fall and grow new leaves in the spring. These kinds of trees make up vast areas of forest (including the Pine Barrens of New Jersey), and are also used in the U.S. for Christmas trees; evergreens include fir, blue spruce, pine, etc.
Autumn is also praised by many authors because of the change of the seasons, such as "Ode to the West Wind" by Percy Bysshe Shelley, and "The Autumn" by Elizabeth Barrett Browning. In horror movies or Halloween flicks, the setting is traditionally on a brisk fall night, with leaves from deciduous trees floating about, swirling in the air and skittering across the street.
For one who has never seen the change in deciduous trees during the autumn months, it is something worth experiencing at least once.