Story highlights
Trees read shorter days as the signal to start preparing for winter
Warm sunny days followed by cool nights create good leaf peeping opportunities
Drought can force trees to lose their leaves earlier than usual
Red, orange, yellow or brown?
Why do some leaves burst into various shades of red while others turn orange or brown – or some combination of all three? Ed Sharron, a science communication specialist with the National Park Service’s Northeast Temperate Network in Vermont, explains the science behind the beautiful colors of fall.
Why do leaves turn red?
The more favorable warm sunny day/cool night temperature cycles that occur in early autumn, the more likely that fall season is to experience vibrant colors with lots of reds.
Leaves that get the most sunlight will develop red leaves, as the sugars inside them are “baked” into the red anthocyanin pigments (the same process which causes many apples to only be red on the side facing the sun as they grew).
Why don’t all leaves turn red?
Trees that don’t receive as much sunlight will reveal the orange, yellow and brown colors, caused by the carotenoid and xanthophyll pigments that are already present in the leaves, but are hidden under the green until the chlorophyll breaks down.
Extensive drought or other factors may cause a tree to go dormant for the winter sooner than during a typical year, which could cause the leaves to fall off sooner or be browner than normal.
When do leaves know when to call it quits?
The primary signal to trees is the length of day: shorter days tell a tree it is time to stop photosynthesizing (the process by which sunlight and water combine to make food for the tree) and to shut down for the winter.
Why do they turn such spectacular fall colors before they fall off?
When photosynthesis shuts down, the chlorophyll in leaves that makes them appear green breaks down. What colors come next depend on the following: Relatively warm sunny days promote the creation of sugar within the leaves. Relatively cold, but not freezing, nights begin to slowly create a layer between a leaf’s stem and branch, which traps some of the sugars within the leaf. Sugars that escape being trapped in the leaf are stored in the tree trunk and roots and act as a kind of natural antifreeze that protects deciduous trees during winter.