I looked outside my window yesterday and was greeted by two trees already showing their spectacular yellow color.  Yep, autumn is here already.  It seems a bit early for the colors to change, but I seem to recall thinking in previous years that the leaves were turning later than usual so maybe this is the actual normal.  Remembrances of specific events that happen once a year is a tricky thing to do.  This is partly why it’s so difficult to get people to believe that climate change is happening.  Things happen in gradations instead of all at once.

But that’s not what I want to talk about.  I want to talk about colors!  Or colours for the one person in the U.K. that reads my blog.  Leaves change color in the fall.  Duh.  But why do they change different colors?  The answer is science!

Green – You all know this from your basic biology course, I’m sure, but leaves turn green because they’re filled with chlorophyll (a word that I’m sure has denied many a student a spelling bee championship).  Chlorophyll is what plants use to help them absorb light which they use to perform photosynthesis which converts light to energy.  Yes, plants eat light for breakfast, lunch, and dinner.  Yum!  But chlorophyll is kind of like a mask that hides the true color of the leaves.  In the fall, as trees prepare to hibernate for the winter, the chlorophyll slowly drains away and the plant’s true colors shine through.

Yellow – The “true” color of most tree leaves. Year round, most trees produce carotenoid which is responsible for the vast array of yellow colors you see in trees during the autumn.  During the spring and summer, the yellow color is just overridden by the green of the chlorophyll.  What color of yellow a leaf appears in the fall is a result of differing amounts of carotenoid in the leaf.

Red – Some of the most beautiful trees are those that turn a brilliant red (or purple) color.  Like chlorophyll, though, the red color is due to a special production of the chemical anthocyanin.  Scientists don’t know for sure why anthocyanin is produced in some trees.  One theory is that anthocyanin is produced in because it helps protect the leaves from the light so they can continue producing food for the plant a little while longer.  Another is that it’s a warning sign for insects to let them know that they probably don’t want to choose this tree to live in for the winter.  Many trees that turn red will turn another color after the anthocyanin production stops.

Orange – If you know your colors, you know what’s coming here.  The orange of leaves is a mix of both anthocyanin and carotenoid.  So when you see an orange leaf, you know that there’s a bit of anthocyanin being produced but not enough to override the yellow of the carotenoid.  As with the red leaves, you will often see orange leaves change to yellow before falling off the tree.

There you have it, science!