Enjoying the constant bedazzlement of trees
Every October, any naturalist worth their salt must be able to explain why and how leaves turn color and fall off trees. The label “naturalist” is used loosely here for this writer, but here goes.
Let’s start with seasonal signals. As Earth turns to winter mode, it gets colder with less light in a day. Less light means less opportunity for photosynthesis, the process used by trees to make food to grow and reproduce.
Photosynthesis happens primarily in the leaves, so if there is less light, the leaves are no longer necessary. The trees begin shutting down food-making capacity.
Now if that’s not amazing enough, with more cold, less light, and less water, the tree is prompted to produce a hormone which causes some special cells to swell. These “abscission” cells are located in a layer where the leaf is attached to the tree. Swelling eventually seals off the flow of water to each leaf. That causes green chlorophyll to disappear and food making to cease. When the green color disappears, voila!
One sees the yellows and oranges hiding underneath. Bright reds and purples come from glucose trapped in the leaves when sealed, and brown color comes from waste left in leaves. Sunny days and cold nights might make colors brighter.
Eventually, abscission cells form a “tear here” layer, loosening the leaf from the tree, causing it to fall or be torn away by the wind. Maybe we should call this season “Throw” or “Pitch” instead of Fall, because that is what every deciduous tree is doing.
You may be wondering why deciduous trees bother getting rid of their leaves only to have to grow more come spring. After all, evergreens don’t shed needles for winter. It’s because deciduous leaves do not have the waxy coating that evergreen needles use to better insulate them from freezing.
Evergreens also have some antifreeze in their liquid.Water drawn up through a deciduous tree and then freezing is harmful to the tree. If you have ever had lettuce freeze in your refrigerator, you know that a leaf once frozen is worthless.
Besides, a leaf gets a lot of wear and tear and perhaps insect damage in a year, so it’s better to push them off to the ground where they can decompose and replenish the soil.
If you think about it, we should be enormously impressed with trees. Redwoods, the tallest trees in the world, can stand more than 350 feet tall. Here locally, some of our deciduous tree species can reach 140 feet.
Yet trees, with solar help, can hoist water up to their very top leaf without any of the machinery humans would need.
We live in a semi-tropical rain forest. That is to say that left on its own over time, most land around here will become forest. We need to save our forest as a whole ecosystem, not only for aesthetics and recreation, but for even more utilitarian reasons: They are a hedge against rapid climate disruption because they hold carbon. They also produce oxygen, clean the air, stop loss of soil, support our unique flora and fauna, and shade hot places.
Most importantly, trees know how to photosynthesize, turning solar energy to food and beginning the chain that, along with all plants, supports all life on earth. Scientists have been trying to do the same with little success.
Now that you know the complex process trees perform, take time to enjoy a forest walk. Pause to watch as a tree sends it leaves away. It’s a miracle worth assuring repetition. Hold on to our existing trees—and plant more.