Autumn Leaves

by Allison Jones

 

Leaves are starting to turn yellow and red.  Autumn is in full swing.  In our area the peak time for brilliant leaf color is early to mid-October.  

But why do the leaves change color in fall?

The green color is caused by chlorophyll in the chloroplasts that are where photosynthesis occurs.  In the fall, chlorophyll is broken down as photosynthesis slows to a stop.  The building blocks of chlorophyll are stored and reused the next spring.  As the green disappears the other pigments present in the leaves begin to show through.  Xanthophylls produce yellows and beta-carotenes produce oranges.  These pigments were in the leaves all along but the green chlorophyll overpowered them.  

The brilliant reds of fall are a different story though.  The reds are produced by anthocyanins which are only synthesized once half the chlorophyll is gone.  These pigments are produced by a change in the sugar processing of the leaves.  Photosynthesis produces sugar from carbon dioxide and sunlight by using chlorophyll to capture the sunlight.  When the chlorophyll is being broken down, the process of sugar production and use changes.  At this point anthocyanins, which produce reds and purples, begin to be made.  Combined with the yellow and orange pigments already present in the leaf, this produces the darker oranges and fiery reds we so enjoy in the fall.

The autumn colors are not equally bright every year.  This is because the growing conditions are different each year and the weather during the changing of the leaf colors varies.  The temperature and amount of light effects the amount of pigment produced.  The brightest colors come when autumn is bright and cool with above freezing nights.  

Most of the broadleaf trees in our area produce yellow autumn leaves.  An example is quaking aspen which has brilliant yellow fall leaves.  Some trees found mostly in town like maples and oaks do produce the deep red leaves that are the expected colors of autumn.