Each Autumn (Fall) there is a great changing of the guard in Northern climates around the world where green is replaced by contrasting reds, oranges and yellows.
What are leaves?
Before you can understand why leaves change their colors in the fall, you need to know what they do. Leaves are essentially the engine of the tree, the roots gather up water and other trace minerals / nutrients from the soil and bring them up to their leaves. The leaves then convert this water and into glucose (sugar) by mixing it with the carbon dioxide absorbed from the air, a process known as photosynthesis. As a byproduct of this interaction, trees release oxygen so that you and I can breath along with extra water.
6 CO2 (gas) + 12 H2O (liquid) + photons → C6H12O6 (aqueous) + 6 O2 (gas) + 6 H2O (liquid)
carbon dioxide + water + light energy → glucose + oxygen + water
Why are leaves green?
Photosynthesis is the conversion of water and carbon dioxide into sugar and water, it’s these sugars that power the plant and this entire process happens within the chlorophyll cells. It just so happens that chlorophyll is green!
Why do leaves change color in fall / autumn?
Although trees don’t have a brain, the shortening of the days as they go from summer to winter signals them to begin storing and limiting resources going into their chemical factories, leaves! They begin producing less and less chlorophyll! During winter, liquid water and the short days make sustaining a tree just about impossible due to the shape and structure of leaves. How can they absorb carbon dioxide if there is a layer of ice over them? The added problem is that sap freezes, making any kind of work impossible for the tree. This process causes chlorophyll cells to die off within the leaves, allowing the colors that where already there to become more prominent and the green to go away.
Type of tree by color
You will notice that maple trees tend to go from green to red, this is because some glucose gets trapped in the leaves while oak trees usually go brown because of wastes left behind. These combinations glucose and byproducts being left in leaves are what lead to different colors you see in the forests this time of year. Here is a summary of the combinations you may find of interest:
Birch – bright yellow (paper and yellow)
Dogwood – purple, red
Hickories – gold, bronze
Maple – orange and red (sugar maple), extreme yellow (black maple), red (red maple)
Oaks – red, brown, rust
Why do leaves fall in the autumn?
Now you know why leaves change color, but why do they fall? Well, although leaves weight very little, add onto them some ice and snow and the added weight is simply too much for many trees to support. This is why, when you have bouts of freezing rain, many tree branches break and fall. To avoid this problem, trees have developed a specialized layer of cells, called separation layer, which gradually seal the branch from the leaf, once complete, it falls.
Changing of the Guard: Evergreens
You will notice in many forests, that although many of the trees change colors, there are some that stay green year round! Well, evergreens such as pine, fir, cedar and spruce have very short and narrow leaves, called needles, with a layer of wax over them. The wax enables water to simply slide off of them which is important when that water may freeze and the needles themselves have fluids within them that act like an anti-freeze. This enables it to continue the photosynthetic production cycle mentioned above year round. Evergreen trees are also handy in any survival situation as the wax acts as a powerful fire propellant and the ground under these trees is usually dry and very comfortable to sleep on, but that is another story.
The Bottom Line
Leaves are like humans, each with their own, unique story to tell.