Autumn is in full swing now. Everywhere I look is a sea of nearly unbroken gold. According to this map, it’s at or past peak autumn color in most of Colorado. There are so many leaves falling that they remind me of large, yellow snow flakes. But what makes them yellow? What makes the trees in New England orange or red or burgundy?
From bud break until just before it leaves the tree or shrub (not just trees change color), a deciduous leaf is green. Deciduous means “shedding at maturity.” For deciduous trees and shrubs, this means once a year during the fall. Did you know that your “baby teeth” are also known as deciduous teeth because they fall out after a certain point? Deciduous is the opposite of “evergreen,” which is the term used to refer to trees or shrubs that retain their leaves, like pines (what we call “needles” are just really flattened and elongated leaves).
A leaf is green in the spring and summer because of chlorophyll. Chlorophyll is a molecule that allows the plant to harness sunlight. In the presence of water and carbon dioxide, the sunlight captured by the chlorophyll can then be used to produce the sugar (glucose) that the plant needs to survive. Oxygen is actually only a waste product of this process, which is known as “photosynthesis.”
When days start getting shorter, this signals to the tree or shrub that it will soon be too dark and dry to keep photosynthesizing. So, similar to hibernating animals, deciduous trees and shrubs also prepare to save energy during the winter by going into a kind of dormancy where they live off of stored glucose. During this time the chlorophyll in the plant’s leaves begins to break down and other pigments in the leaf become visible. These pigments, called “carotenoids,” are responsible for the bright yellows and oranges you see in trees like aspens or cottonwoods. Carotenoids are also responsible for the color of vegetables like carrots.
When I lived on the east coast, there were many trees, particularly maples, that turned amazing shades of crimson some years. The red color of these and other trees is due to a different pigment called anthocyanin. Anthocyanins are what make blueberries blue or cherries red.
Unlike carotenoids, anthocyanins are not present in the leaf year round. According to a fairly recent study, anthocyanins are produced exclusively in the fall as a way to protect the plant from excessive light exposure, which may cause damage to leaves that, in turn, could make it more difficult for the plant to recover nutrients. Evidence to support this is that leaves in more shaded areas do not produce anthocyanins. Whatever the reason for their production, anthocyanins produce the most vivid colors in years where the autumn has been sunny and somewhat dry with cool, but not freezing nighttime temperatures.
Do you like to travel to watch the leaves turn?