In early autumn, longer nights prompt cells between leaf petiole and twig to begin dividing rapidly, producing a thin corky layer.
This corky layer of cells gradually blocks the flow of water and minerals coming up from the roots to the leaves and, also, blocks the transport of glucose, manufactured in the leaves, back to the woody parts of the tree.
The green color we see in leaves during the growing season comes from chlorophyll. Sunlight is absorbed by the chlorophyll in leaf cells and photosynthesis begins.
Photosynthesis is the process by which plants use the
energy of sunlight
Throughout the growing season of spring and summer, trees make their own food through photosynthesis, and provide additional glucose and oxygen vital to the web of life*.
As lengthening nights cause the corky layer of abscission cells to block the flow of liquid between leaf and woody twig, chlorophyll production slows, then stops… and the leaf colors that were hidden by green chlorophyll are revealed:
, red and purple
. (Additional anthocyanins are created during cool weather from sugars trapped in the barricaded leaf.)
Eventually, the corky abscission cells dry up, the petiole breaks off from the branch, and the leaf drifts to the ground. Within the loose leaf, the yellow, orange, red and purple colors gradually fade, leaving only brown tannins. The fallen leaves decompose on the ground to nourish the tree in future years.
Unique chemical compositions of different species of trees determine their potential range of colors. Location and weather conditions affect the expression and intensity of colors each fall. Ideal weather conditions for a colorful display would be a summer growing season with plenty of rain, followed by an autumn of sunny warm days and cool, but frost-free, nights.
Why lose leaves at all as cold weather approaches? Survival! Tender leaf tissue would freeze in winter, so trees must either have leaves that are tough and waxy like needle evergreens, hollies, and our
– or pull back their sap and discard their leaves, entering a dormant state for winter.
Winter survival preparation for the trees, glorious show of color for us!
*It is not just animals who need oxygen. Trees and all other plants require oxygen for breaking down glucose to release metabolic energy.