Monday, October 24, 2016

Fall colors explained

Fall colors at Sleeping Bear Dunes National Lakeshore

Have you ever wondered why leaves change colors?

It's a simple question with a complex biological answer involving masking, sugars, and pigments.

Normally the leaves of most plants appear green to people with normal color perception.  They are green because they are filled with a pigment called chlorophyll.  Chlorophyll is responsible for photosynthesis - the sugar production in plants and some bacteria (and one weird sea slug that incorporates chlorophyll into its body from an algae). It is also responsible for releasing oxygen in a form that we can use.

When leaves change color in the fall, they do so because the tree stops production of chlorophyll.  During the growing season, chlorophyll is so abundant in leaves that it masks all other colors found in the leaves.  As the level of chlorophyll decreases, the other colors begin to show through the mask.

Leaves, unmasked!

Yellow and orange colors are caused by a group of pigments known as carotenoids.  These colors were present throughout the growing season, but couldn't be seen because of the abundance of chlorophyll.

Thank carotenoids for the golden fall color of Quaking Aspen

Red and purple colors are different.  They are caused by by a group of pigments known as anthocyanins.  These chemicals are produced in the fall when chlorophyll production ceases.  The sugars that are produced by chlorophyll require the presence of several groups of chemicals to help break them down so they can be used as fuel at the cellular level - one of these groups is known as phosphates.  Phosphates are molecules that form around an atom of phosphorus, an important micronutrient.  Because phosphorus is present in limited quantities in the soil, trees can't afford to lose their phosphates when they drop their leaves - instead they transfer the phosphates back into the branches, trunk, and roots. 

The red in these Sumac leaves comes from anthocyanins

When the phosphates are shuffled away from the leaves, the leaves have to use a different process to break down the sugars - this process results in the production of anthocyanins.  When the level of anthocyanins becomes high enough, and the level of chlorophyll low enough, the leaves will appear red or purple.

Sometimes leaves appear red in the early spring.  This happens because the tree also produces anthocyanins in the spring before chlorophyll production ramps up to summer levels.  Once chlorophyll production begins at full scale the anthocyanin production ceases and the chlorophyll masks the reds and purples.

Anthocyanins at work - fall color in a spring leaf of Red Oak

Brown leaves are produced by an entirely different process.  The brown is the color of the cell walls within the leaves - some trees do not produce large quantities of either carotenoids or anthycyanins, so we see the brown of the cell walls instead.  This also the reason why all fallen leaves eventually turn brown - the pigments fade away and we are left seeing the brown cell walls of the leaf.

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