It’s Not About The Frost
When I lived in Vermont a favorite autumn activity was leaf peeping. There were places we’d visit every Fall because the panorama of colors was so spectacular.
Memories of those places come to the fore as summer morphs into autumn. I happened to see an article about how and why the leaves turn. Below is the bulk of that article. Thanks once again to the Old Farmer’s Almanac.
“Did you know that fall’s vivid colors are actually hidden underneath summer’s green color? Also, the main reason for color change is not weather, but light, or actually the lack of it.
First of all, not all leaves turn vivid colors in the fall. Only a few of our many species of deciduous trees—notably maple, aspen, oak, and gum.
Several factors contribute to fall color (temperature, precipitation, soil moisture), but the main agent is light, or actually the lack of it. The amount of daylight relates to the timing of the autumnal equinox.
As the autumn days grow shorter, the reduced light triggers chemical changes in deciduous plants causing a corky wall to form between the twig and the leaf stalk. This corky wall eventually causes the leaf to drop off in the breeze. As the corky cells multiply, they seal off the vessels that supply the leaf with nutrients and water and also block the exit vessels, trapping simple sugars in the leaves. The combination of reduced light, lack of nutrients, and no water add up to the death of the pigment chlorophyll, the “green” in leaves.
Once the green is gone, two other pigments show their bright faces. These pigments, carotene (yellow) and anthocyanin (red), exist in the leaf all summer but are masked by the chlorophyll which helps plants absorb sunlight. (The browns in autumn leaves are the result of tannin, a chemical that exists in many leaves, especially oaks.)
Sugar trapped in autumn leaves by the corky wall (the abscission layer) is largely responsible for the vivid color. Some additional anthocyanins are also manufactured by sunlight acting on the trapped sugar. This is why the foliage is so sparkling after several bright fall days and more pastel during rainy spells.
What Brings the Best Fall Foliage? In general, a wet growing season followed by an autumn with lots of sunny days, dry weather, and cold, frostless nights will produce the most vibrant fall colors. If freezing temperatures and a hard frost hit, it can kill the process within the leaf.
Also, drought conditions during late summer and early fall can trigger an early “shutdown” of trees as they prepare for winter, causing leaves to fall early from trees without reaching their full color potential.
For some inexplicable reason we grew up under the misconception that a night of frost affected the change in leaf colors. It is always a good thing to uncover the truth and learn how things actually work.
Comments are welcomed.