Fall is for the Trees

A look at an orchard in early fall. Notice the variety of colors present.

Fall is a special time of year. It’s a time to enjoy the beauty of nature, but it’s also a time for preparation. We prepare our cars, homes, and yards for the coming snow, ice, and freezing temperatures. We insulate our homes and switch out our bald tires for studded snow tires. Once the cold sets in we try to avoid going outside. And if we do have to venture out we dash from one heated location to another, out the door of our home into the car and from the car into the store. We have the options and the abilities to protect ourselves from the winter weather by removing ourselves from it. Trees don’t have that luxury. A tree can’t simply uproot and walk into a warm house. It’s stuck where it’s rooted. It must prepare itself for winter.

A maple tree girded in full fall foliage. 

Perhaps the most visible preparation is the changing colors and eventual loss of leaves on deciduous trees. Nothing quite brings to mind fall as seeing the multitudes of colors across the different species of trees. Reds, oranges, and yellows dot our cities and forested hills. The landscape becomes a kaleidoscope of colors for a short time. Slowly the kaleidoscope fades as the once colorful leaves fall to the ground. Our cities and hills lose their vibrant colors. All that remains are leafless trees, bare until the spring.

Trees lose their leaves for a very basic reason. It’s better for their survival. They wouldn’t necessarily die if they kept them throughout the winter, but it would cost them more energy than they would take in. Photosynthesis uses sunlight to create carbohydrates. Plants use these carbohydrates for almost everything. They need them to grow. For photosynthesis to work, however, there needs to be sun and heat. However, winter days are short, often cloudy, and cold. Opposite of the ideal conditions. So the trees drop their leaves.

Why do the leaves change first though? Why aren't the leaves just dropped, still green, to the ground? Nutrients. The tree spent energy and resources to grow the leaves. There’s no reason to just throw them away. So first the tree resorbs what material it can from the leaves. It uses enzymes to break down complex molecules into their base amino acids or carbohydrates. These are then funneled into the branches, trunk, and roots of the tree for storage until the new growth in the spring. Of course this process isn't free. The tree must continue photosynthesizing as this occurs.

An aspen grove with its brilliant yellow foliage.

Where does the red, orange, and yellow come from? For the most part the colors are already present in the leaves. They’re just not seen because the chlorophyll is so much more prevalent during the growing season. The color change occurs in the fall because the chlorophyll stops getting replaced as it degrades under constant sun exposure. This allows the carotenoids (orange), xanthophylls (yellow), and anthocyanins (red) pigments to show through. Oddly enough production of some of these pigments can often increase in the fall as the leaves get closer to dropping. Leading to brilliant colored leaves in some trees. One reason that production of these pigments might increase is because of nutrients available in the leaf. As fall progresses the pathways out of the leaf eventually close. This leads to a buildup of nutrients. These nutrients are then used in the production of lesser pigments. But eventually even these pigments degrade from light exposure. At this point the only color left is brown, which comes from tannins.

What about internal changes? What goes on inside a tree to protect it from freezing temperatures? The first thing to know is that the cold temperatures themselves aren't necessarily harmful to the plant. The bigger issue is that water expands as it freezes. Fill a water bottle full of water and put it in the freezer. When it finishes freezing you'll see that the water has either burst out the cap or through a side. The same thing would happen if water froze inside a cell.

So why don't tree cells burst when the temperature drops below freezing? The simple answer is that all water doesn't freeze at 32º. The freezing temperature varies depending on the concentration of solutes in the water. Increase the solutes, decrease the freezing temperature. Trees make use of this property by increasing the concentration of solutes in their cells as winter approaches. These solutes function as an antifreeze for the tree.

Another benefit that all plants have are cell walls. Cell walls are rigid structures that surround the softer cell membrane. They provide support, strength, and protection. They're one of the reasons that trees can grow so large. They allow the individual cells to flex and move while relying on the cell wall to maintain integrity. If water does have a chance to freeze in a plant it often occurs outside of a cell. As the ice crystal expands it can punch through the cell wall and push on the cell membrane. The cell membrane is flexible. It can wrap itself around the encroaching ice. As the cell shrinks it forces water out, increasing the solute concentration inside the cell. Further protecting the cell from internal freezing.

The downfall of increasing the solute concentration is that it takes up space in the cell. Space that otherwise would be used by the cell for normal operations. This forces the cells to go dormant while they're full of this antifreeze. The cells are protected, but they can't keep working. They must wait until it warms in the spring to flush the antifreeze and resume normal cellular operations.

Just as the trees spend weeks and months preparing themselves for the coming cold, so must we. We can't prepare overnight for a temperature change of 70º and freezing weather. We must acclimate ourselves to it slowly. We can't pump antifreeze into our blood and cells, but we can insulate our bodies by increasing the number of layers we wear. We can sip warm drinks and funnel heat into our core. We prepare our bodies and homes in different, but similar, ways to the trees. And we do it for the same reasons. Survival. We survive the cold by preparing for the cold in a series of steps. 
What else can we takeaway from the fall trees? The simplest idea might be that even in beauty there is purpose. Fall leaves are pretty, but that is not their primary purpose. Nor is any of the other beauty in nature simply for our benefit. All beauty serves a purpose, whether that beauty is a tree preparing for fall, or melting snow funneling into a cascading waterfall. All of these things are part of the natural process of our environment. Go out and enjoy the beauty. 

As always tune in next week for another amazing article. I hope to continue with the fall theme for a few more weeks.

Josh