The changing leaves of autumn are really awesome to look at, but they're also a really striking example of nature taking extreme measures to protect itself.
You’re probably familiar with photosynthesis — it's the process plants use to turn carbon dioxide, water, and light energy into sugars and oxygen. And you probably also know that photosynthesis depends on a pigment - a colored compound called chlorophyll.
You may not realize that plants contain lots of other pigments, as well. Some of the most important are;
- Carotenoids - yellow, orange, and brown pigments that give color to things like corn, carrots, pumpkins, and sweet potatoes.
- Anthocyanins - which give red and purple color to cherries, berries, pomegranates, and red apples, to name a few.
However, many trees are less active in the winter because they grow at northern and southern latitudes that get less sunlight during those months. These trees are called deciduous, from the Latin word that means “to fall off”. Since deciduous trees don’t do much photosynthesis in the winter, it doesn’t really make sense to spend a bunch of energy maintaining big green leaves.
So when the days get shorter and the temperature gets cooler, they send less of their limited resources to the leaves, and start using what water and nutrients they have to keep the rest of the tree alive.
The chlorophyll in the leaves breaks down, and the green color gradually goes away. And when that happens, the other pigments - which were there all the time - are better able to show off their colors, before the leaves die entirely and fall off the tree.
So, the leaves aren’t really changing pigments - they’re just losing their strong green pigment to reveal the other colors in the tissue. After the tree stops the supply of food and water to the leaves, all that’s left is for the tree to cut them off.
The tree forms a special layer of weakly-bound cells near the base of the leaf’s stalk. Then another layer of cells at the very bottom of the stalk expands to push the leaf away. Eventually the leaf can be knocked off easily, even by light wind. And then it’s your job to rake them up!