The Fluctuating Foliage of Fall

Near Fish Creek - October 2021

"The tints of autumn...a mighty flower garden blossoming under the spell of the enchanter, frost."   ~John Greenleaf Whittier, American Quaker poet, advocate for the abolition of slavery, best-remembered for his anti-slavery writings

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“The colors are brighter when we’ve had a drought.” “Color is best with lots of rain.” “Oh, the colors start when it freezes.” “It has something to do with the angle of the sun.” “The trees just know.” 

Having heard these and many more conflicting theories on the fall leaves in past weeks, I thought it was high time I read up on the matter. Here are a few things I learned.

Deciduous leaves are luscious with the green of chlorophyll during spring and summer, their growing season. In the process of photosynthesis, the chlorophyll absorbs sunlight’s energy to transform carbon dioxide and water into oxygen and sugar, thus providing the majority of food for the plant.

In the fall, shorter days and cooler temperatures signal the end of food production, so the chlorophyll begins to break down. As the chlorophyll fades, the colorful carotenoid and anthocyanin chemicals already present in the leaf are no longer hidden by the intense green. Carotenoid pigments, also found in corn, carrots, pumpkin and tomatoes, are yellow, brown, red and orange. The red, blue and purple pigments of anthocyanin can also be found in berries, grapes, and red apples.

Since the tissues of tree leaves are tender and can’t survive cold winters, the leaves are dropped. In this process a layer of cells forms where the leaf stem attaches to the tree. This seals off the leaf, and when the leaf eventually drops those cells have sealed the cut on the tree. This is known as a leaf scar.

Most of us will gather up our piles of leaves to keep things tidy. Or to stuff giant plastic pumpkin bags in preparation for Trick or Treat. But those fallen leaves are vital to the health of the forest floor. They break down, adding nutrients to the soil and help provide the sponginess to absorb moisture.

Here’s the lowdown on leaf color:
Temperature and moisture are the main influences.
Yellows and golds are fairly constant from year to year.
The combination of warm days and cool (not freezing) nights causes the sugars (anthocyanins) to build in the leaves, to give the most brilliant reds and purples - and these vary from year to year.
Some moisture, especially in spring, can boost colors in the fall, but some sources say too much rain can lessen color intensity.

My favorite takeaway is that each autumn is unique in terms of color. With so many variables in temperature and moisture throughout spring, summer and fall, the colors are amazing in their own way each year. 

As I write this, I’m looking out my office window at a large tree branch covered in…dead brown leaves. Hmm. Perhaps not so amazing. But I notice how the evergreen in front of the less-than-lovely leaves is a beautiful deep green in contrast. I also think what a satisfying crunch those brown leaves would offer underfoot. They, too, make their own contribution to these gorgeous - and fleeting - fall days.


 Informative Articles 
College of Environmental Science and Forestry: Why Leaves Change Color
U.S. Dept of Agriculture/Forest Service: Science of Fall Colors