The autumn season offers a pop of colors in foliage that is undeniably gorgeous.  The Eastern United States provides possible combinations of colors unmatched in the world.  Climate, sun exposure, and tree species of the Eastern US are responsible for an explosion of hues just before the leaves fall and winter creeps upon us.

sunset on fall foliage

Bright fall foliage begins with spring.  A warm and wet spring followed by a dry and sunny summer are prerequisites for brilliant fall colors.  Then an autumn filled with sun, little precipitation and cool temperatures that stay above freezing will give rise to decorated deciduous trees that would make a Punky Brewster outfit jealous.

So what makes deciduous tree leaves change colors?

Deciduous trees have leaves containing chemicals which give it color.  Chlorophyll, a biomolecule that captures light to provide energy for photosynthesis, is responsible for making leaves green.  Chlorophyll provides such a dense green pigment that it dominates other pigments in the leaves.  The less dense pigments that exist within a green leaf are orange, yellow, and brown.

Orange and yellow pigments come from two classes of carotenoids; carotenes and xanthophylls.  Both pigments assist in photosynthesis by capturing energy from light that chlorophyll does not absorb.  Carotene, a hydrocarbon, is an orange pigment.  yellow fall foliageXanthophyll, a hydrocarbon containing oxygen, is responsible for the yellow color.  The brown pigment is from the common biomolecule tannin. 

Long nights or lack of sun inform deciduous trees that the growing season is over.  Reduced sunlight and colder temperatures trigger the trees winterizing methods.  Hormones guide compounds down to the roots for storage.  An abscission layer or membrane begins to grow at the stem of leaves.  The abscission layer grows to the point which blocks the exchange of water, minerals, and sugars between leaves and the rest of tree.  Needed nutrients and water from the tree do not reach the leaves; causing an inability to produce more chlorophyll.  The remaining chlorophyll breaks down and disappears.  The lack of the dense green pigment allows the orange and yellow pigments to become visible.  Sugars manufactured in the leaf but did not transfer to the tree prior to the abscission layer growth becomes product that triggers anthocyanin growth.  Anthocyanin is a group of red and purple pigments. 

The loss of chlorophyll production and the addition of anthocyanin pigments result in the beautiful fall foliage.  Eventually, sunlight and freezing temperatures will break down the pigments.  The leaves will fall and become brown.  The bare deciduous trees bunker down for the winter and wait for the return of spring.  Hopefully, a wet and warm spring will occur so next autumn deciduous trees can once again pop with color and brighten the landscape for a few weeks.