Trees have been in existence and evolving for millions of years. There are over 100,000 species today and they come in all shapes and sizes. Their variation in appearance comes from various adaptations to their local environments. In areas that experience 4 distinct seasons, such as the northern United States, a beautiful adaptation has come about.
Fall foliage occurs once a year, for a short period during the transition from summer to winter. In order to understand why leaves change color, you must understand the life cycle of a tree.
The temperature begins to rise and the days become longer. The rise in temperature begins to melt any remaining snow and soften the earth. The warmer weather signals the trees to come out of winter dormancy. The melting snow provides water to the roots until the rainy season begins (April).
During the spring, trees begin a rapid growth period. Leaves bloom, roots branch out, existing branches elongate and new branches are formed as the tree stretches out to absorb the sunlight. Many trees send out flowers during the spring which will get pollinated and create seeds or fruit. It is the chlorophyl in these newly formed leaves that create the energy that fuels tree growth.
If the tree has dropped any seeds during last fall or winter, the warming spring sun and wet soil will cause the seed to germinate in the hopes of becoming a tree itself.
Throughout the course of the summer, a tree will gradually slow its growth. The sun becomes more intense and water is less abundant. The tree will begin to shift its energy to formalizing its spring-time growth and to producing seeds and fruit. The trunk will thicken to support the bigger limbs and the branch shoots of last season will turn to bark. Fruit or seeds will begin to form and fatten where the flowers used to be.
Fall is the season of beauty and good eating. Sensing the change in temperature and the change in the amount of daylight, a tree will begin to prepare for a harsh winter ahead. Just like all living organisms, the tree’s main concern is continuation of the species.
During this period, its fruit and seeds will ripen (a perfect time to go apple picking) and fall to the ground where its outer shell will protect it through the winter and provide nutrients for it in the spring – a hopeful attempt that a baby will be born to carry on the parent’s genes.
Since the tree has stopped its growth and has finished producing its seeds, it will no longer need to make energy until the spring time. The tree stops producing energy and the chlorophyll in the leaves begins to break down. Chlorophyll gives leaves the shiny, green color apparent most of the year; but as the chlorophyll slowly dissipates, the other chemicals in the leaves begin to show through such as carotene and xanthophyll. These chemicals give us the beautiful yellows, oranges and reds that we see.
Eventually the leaves fall off and new buds are formed waiting for the spring time to bloom.
The tree enters a dormant period to deal with the lack of light and cold temperatures. Growth comes to a stop as the tree awaits the coming thaw.
Then, as the temperature warms and spring approaches, the cycle starts all over again.
It comes from this adaptation of surviving through the winter that brings us the beautiful leaf colors of autumn. In areas where seasons are not so varied, like along the equator, plants stay green all year around and do not get to experience this once a year phenomenon.
Be sure to take a trip this fall and check out the foliage!