Autumn Leaves – The Chemistry Behind the Color

Welcome to Fall, the most beautiful time of year.  A drier, cooler nip to the air, flurries of leaves, and the smell of a wood stove all come to mind. And as we dust off our rakes and leaf blowers and ready ourselves to clean up all of that fallen ‘litter’, do you ever wonder what exactly causes the trees to change, to display their fiery colors?  Well I do, and I decided to do a bit of reading to learn about the mechanisms behind leaf season –  lo and behold, you can thank chemistry!

During summer, the leaves of deciduous trees work to create sugar and water from carbon dioxide and light.  They do this using chlorophyll, an internal compound which also causes leaves to appear green. As water and nutrients flow from the basal roots, through the branches, and into the leaves, sugars produced in the leaves by photosynthesis flow to other parts of the tree and are used and stored as energy. This process occurs throughout leaf-on in the spring and summer months.

However, as we’ve experienced recently, the shorter days and cooler nights of autumn cause a shift in the tree’s physiology and chemistry. One of the first changes to occur is the development of a membrane within the ‘abcission zone’ between the branch and the leaf stem, interrupting nutrient flow within the leaf.  As a result, the the amount of chlorophyll drops off, causing the green color to start to fade.  Here’s where things get colorful: if the leaf contains carotene (birch, hickory, and sugar maple, for example) it will turn bright yellow as the chlorophyll disappears. In other tree species, as the proportion of sugar to chlorophyll increases, the sugar forms anthocyanins, pigments that cause the yellowing leaves to turn red. In the Southern Appalachians you can count on sumac, red oak, and red maple to show off their fiery hues come autumn.

But while the above does explain why it happens, it doesn’t explain differences in vibrancy, which is something we can all notice from year to year.  Some years you may see a duller autumn, and that’s because the range and intensity is significantly affected by weather. Chlorophyll is lost quicker in both brighter sunshine and colder temperatures; if above freezing, anthocyanins are produced more abundantly than in milder conditions.  Relative humidity also plays a big role, as drier weather brings the proportion of water down within the sap, sugar becomes more concentrated within solution and subsequently produces more anthocyanins. So, to sum it all up, in autumn you can expect to see the most vibrant leaf changes when the days are bright and sunny and the nights are cool and dry.

Wait, that means with all of the recent bright days and cool nights, the season should be starting, right?  Well have a look and decide for yourself!

-Owen Carson, Field Technician

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