Let’s explore why leaves are green, why they change color in the fall, and why they fall off. There’s a lot here, so we’ll unpack it.
bigleaf maple, sugar maple, red maple
First, why are leaves green in the summer?
Leaves are green because they contain chlorophyll. Chlorophyll is the “secret sauce” for leaves’ solar-energy factories: Using chlorophyll, leaves combine (1) light, (2) water, and (3) air to store energy in the form of sugar molecules.
Chlorophyll happens to be green. What does it mean for chlorophyll to “be” green? It means that chlorophyll reflects green light and absorbs light of other colors. Because chlorophyll is green, it is called a pigment.
What happens to the green color in the fall?
Like many complex molecules, chlorophyll gradually breaks down over time. It does this at a steady rate. Plants must continually replace their chlorophyll.
Plants’ production of new chlorophyll is regulated by light. In response to shorter days in the fall, plants produce less chlorophyll. But existing chlorophyll molecules break down at a steady rate. Therefore, in the fall, the amount of chlorophyll in plants decreases and the green color fades.
Where do yellows and oranges come from?
As chlorophyll decreases, less-strong pigments that are already in the leaves appear. These include carotenoids, which are yellow and orange pigments. One carotenoid is beta-carotene, a pigment in carrots, sweet potatoes, and pumpkins. These yellow and orange pigments exist in leaves all summer. But their color is masked by great quantities of green chlorophyll, which is the star of the show.
When chlorophyll lessens, these other colors become visible. This is what happens to the leaves of bigleaf maple, black cottonwood, and many other PNW native trees and shrubs that turn bright yellow or orange in the fall.
Where do reds come from?
Some leaves display various shades of red in the fall. The most notable is red maple, a tree native to eastern North America that is widely planted in the PNW for home landscaping and along streets. Trees and shrubs that are native to the PNW and that turn red in the fall include dogwoods and blueberries.
Red leaves are the result of a different group of chemicals called anthocyanins. The peels of red apples are rich in anthocyanins. When these chemicals are present, they are usually found in the outer layers of plant structures.
Some anthocyanins exist in leaves during the summer. And some trees and shrubs, under the right conditions, manufacture more anthocyanins in the fall. This is what happens inside leaves that turn red.
Why do trees spend energy generating red colors in the fall?
Trees that manufacture additional anthocyanins in the fall are so common that there is probably a reason for making it—i.e., an evolutionary advantage to the tree. But scientists do not yet understand exactly what this reason is.
Here are a few of the current theories:
(1) Anthocyanins protect the leaves from sunburn / drought after the chlorophyll fades.
(2) When red leaves fall down, the anthocyanins are absorbed into the soil, providing a key ingredient to ward off pests or competitors at the base of the tree.
(3) Red colors scare off certain insects, such as aphids, that find yellow more desirable.
(4) Red colors attracted certain beneficial insects that have since died out—i.e., the red is an evolutionary relic.
Further research may reveal the correct answer.
What makes leaves fall off?
Leaves of broadleaf trees are very thin and are subject to freezing in the winter. To avoid damage, deciduous trees have evolved to shed their leaves in the fall. How does this work?
Where a leaf stalk joins a twig, in the fall a layer of corky cells develops. This layer gradually separates the leaf stalk from the twig. Water from the roots can no longer get in, and sugar from the leaves can no longer get out. This layer of corky cells is called the “abscission” layer (or “separation” layer).
The tree then secretes enzymes that digest cell walls within the abscission layer. The cells weaken. Soon, wind or rain breaks the leaves off, leaving tiny scars. Thus, deciduous trees idle their solar-energy factories for the winter.