Why Do Trees Change Color in the Autumn?

Autumn is the time when the leaves on your trees turn beautiful colors—red, yellow, orange, purple, and so on—and you start gearing up to sweep them off your front porch and rake them out of your yard. The leafy fireworks display may be pretty, but it can be a big pain when they drop all over your yard. Why can’t the leaves just stay on the trees? Why do trees change color at all? How hard could it be to stay green all year long?

Here are five facts about your trees changing color and shedding their leaves in the fall.

1. Why can’t the leaves just stay on the trees?

Deciduous trees, a fancy word for trees that lose their leaves every year, don’t really have a choice in the matter. They have adapted to go dormant for a period of time every year—generally in the wintertime—and drop their leaves as part of that process. If they kept their leaves through the winter, they would expend an exponential amount of energy to keep producing food. This would be made more difficult by the low temperatures and the shorter daylight hours (which might be made even shorter with cloud cover). By dropping their leaves, trees also reduce water loss, since without their leaves, they have nothing to act as a vehicle to evaporate that water.

If you look at evergreen trees—such as junipers or pine trees—you may notice that their needles or leaves are tougher and may have a waxy cast to them. This is to help them with water conservation. Their leaves or needles are also tougher and more able to withstand the weather through the winter months. Evergreens have done well for themselves. Conifers have been around since the Cretaceous period, from about 145 million years ago to 66 million years ago. If it ain’t broke, don’t fix it.

2. Okay, but why do the leaves change color first?

The leaves of these trees are as important as any other part of the tree, and part of their role is to be jettisoned in order to keep the tree healthy.

The purpose of leaves is to create food for the tree. From the spring to the fall, these leaves catch the sun’s rays and create energy from that light, such as sugars and starch. This process takes place in the cells of the leaves, which contain chlorophyll—also known for giving the leaf its green color. If trees suddenly received their nutrients from something other than chlorophyll, we could expect to see orange, yellow, or red leaves all year long. The green masks the other colors in the tree’s leaves.

If you notice your tree’s leaves are turning brown and dropping off before they’re supposed to—such as in the middle of summer—consider how much water you’re giving them. In the extreme heat, make sure that you’re deeply watering your tree every few days, rather than watering lightly every day.

3. Your trees are winterizing.

When the leaves change color, you know the tree is starting to wrap up its nutrient-creating powers for the winter so that it can rest through the cold weather. In the same way that you have chores to winterize your home before the cold temperatures hit (such as turning off your external water faucets or installing storm windows), a tree winterizes by shutting off its food-producing systems so that it doesn’t waste that energy. A tree’s way of turning off the lights on the way out the door is dropping those leaves.

A tree also requires leaves to protect itself in the summertime in the same way that you might want a hat. With the tree’s leaves protecting its bark, the tree won’t overheat and the bark won’t burn—this is especially true of young trees. However, in the wintertime, the sun is much less powerful, and the danger of a sunburn on the tree is much lower. Therefore, it can drop those leaves without worry.

4. Your trees are protecting themselves from winter mishaps.

If those leaves stayed on the tree, they could cause some damage to the tree itself. Have you ever seen a tree in an unseasonable snowstorm? The branches and leaves of the tree could be weighed down with heavy snow, breaking off limbs and possibly causing problems, such as creating stress cracks in the trunk. In the worst cases, these stress cracks could cause the tree to lose major limbs or even split in half. If there’s a heavy windstorm, the leaves could act like the sails on a ship and break those branches off or tip the tree over.

5. Your trees are maintaining the soil.

Once a leaf has been dropped, it can be recycled back into the life cycle of the tree. The leaf breaks down into smaller components, which help the tree and the surrounding environment in a few different ways. During the winter, the dead leaves will protect the roots of the tree from the colder temperatures by acting as a blanket. Also, dead leaf matter feeds the soil around the tree. In the spring, those nutrients will provide organic matter, which all living organisms in the soil require. Remember, though, that if you want to protect your lawn, you’ll rake the leaves up first for shredding and compost. Leaving those leaves on your lawn as-is could create problems for your yard. They’ll do the most good if you put them in the right place at the base of your trees.

To shred the leaves a little, you can run your lawn mower over them a couple of times or use a shredder to break the leaves down. Then pile them in a ring around your tree—make sure not to pile right against the trunk but to make a little doughnut around the base of the tree with the leaf matter. You can also add a little fertilizer here as well.

Mr. Tree has provided excellent tree service to the Portland, Oregon, area since 2000. If you have any questions about your trees, contact us today for a chat with one of our certified arborists.