I’ve written a fair bit about fall foliage, including the best resources for finding peak foliage; how to take great foliage photos, especially with a polarizing filter; and how to find better foliage using a geologic map.
Over the course of researching these articles, I often got distracted deep into the underlying science of what causes leaves to start changing in the first place, and why different colors appear when they do. So here are my two favorite tidbits of the science behind this colorful season.
Abscission: No More Nutrients for You, Leaf
At a certain point in the fall, the cells at the base of a tree’s leaf stems start dividing rapidly. Interestingly, as the number of cells starts proliferating, the growing collection of cells does not expand or take up any more space. That is, they divide into an increasingly dense layer of ever-smaller cells.
This process, known as abscission, soon reaches a point where the cell layer effectively blocks the passage of nutrients from the tree to its leaves. (It also creates a thick layer of cells that seal the site of the leaf stem once the leaf drops.)
Once abscission is complete and the leaves are deprived of sustenance, they begin a steady transformation lasting approximately two weeks that culminates when they fall from the tree.
The Chemistry Behind the Colors
Following abscission, the chlorophyll that gives leaves their green color soon begins to break down. As it fades away, other compounds in the leaf make a brief appearance. As the graphic below explains in chemical detail, the primary ones are flavonoids (yellow), carotenoids (yellow, orange, and red), and anthocyanins (red).
Different trees and plants have different combinations of these chemicals, which results in the varying color palettes of different species.
Now’s the time to see them all in full glory, as the most recent foliage report shows. Happy fall!