Why Leaves change color and Fall Festival next week, Oct 11!

What & Why?
These tree hormones lead to something called leaf senescence (basically, leaf drop) which is a complex process that allows the tree to halt sugar production and results in leaves falling from trees. This process allows the leaves seal up their connection to the tree and detach cleanly without creating any wounds in the tree. Ultimately, this prevents damage to the tree which would occur if leaves were to freeze while attached to the tree.

This creates an assortment of fall color!
The orange and yellow colors we see are due to carotenoids that are present in the leaf through the year. Carotenoids are also what give many brightly colored vegetables their coloration (like carrots, sweet potatoes and tomatoes).

During most of the year, the green chlorophyll in the leaves, busy taking in sunlight and producing sugars and storing energy for the tree, cover up these carotenoids which are always present. As the plant slows down for winter, the process of leaf senescence takes hold, the chlorophyll fades, and we can see leaves true underlying coloration! Oak leaves turn Brown in the fall due to brown tannins and pigments which are similarly present throughout the year and also covered up by the active green chlorophyll during summer.

While oranges and yellows from carotenoids are present in the leaves throughout the year, the deep reds and purples we see in the leaves can actually vary in a tree from year to year. 

A group of antioxidants called Anthocyanins are responsible for the red coloration (these are also present in some vegetables and flowers); only some trees produce Anthocyanins. As the tree gets ready to drop leaves, the leaves continue storing up energy by producing sugars; however, the tree will not be able to absorb all of this energy. This leads to the production of Anthocyanins as sugars accumulate in the leaves. The more Anthocyanins present in a leaf, the deeper red it may become, and also, the more frost tolerant, as anthocyanins promote frost tolerance.

This is a highly variable process that the trees enter into in preparation for winter, but at the end of the season the flow of the weather and our trees working as natural clocks is what is responsible for the beautiful tapestry that we find on our lawns, sidewalks, and streets.

But don’t just take my word for it!  Below are a few additional links to reputable education based articles on this topic.

• https://ipm.missouri.edu/MEG/2022/10/fallColor-DT/
   
• https://planttalk.colostate.edu/topics/trees-shrubs-vines/1728-leaves-change-color-fall/
   
• https://www.canr.msu.edu/resources/purple-leaves#:~:text=The%20purple%20color%20in%20leaves,a%20reddish%20color%20in%20autumn.