Fall is the perfect time to watch the foliage around us explode in color. Deep reds, bright yellows and cozy oranges shimmer in the wind for just a few weeks before the branches are once again bare and the cold weather ensues.

But why do leaves change color in the fall, and while we’re at it, why do they fall off their trees at all? 

Why do leaves change color and fall?

What leaves do for a tree

In a nutshell, leaves make food for the plant they are attached to, whether that be a tree, a shrub or a flower. Leaves are connected to its plant through a vascular system that carries these nutrients around the plant, similar to how our human vascular system carries blood around our body. They do this all through a process called photosynthesis, a system you probably remember learning about way back in middle school. 

During the warmer months that make up spring and summer, leaves work constantly to convert sunlight into nutrients for the plant. Chlorophyll, the chemical that gives leaves that green color, absorbs light energy and contains it in the leaf, which then interacts with the natural enzymes found in plant cells found in the leaf. That reaction between the light energy and the enzymes helps to break down the chemical components in the water (supplied to the plant from the roots), breaking it down into oxygen and hydrogen. 

The hydrogen reacts with the carbon dioxide found in the plant enzymes to create a form of sugar, and that sugar is what’s funneled through the plant’s vascular system to provide the nutrients the tree needs to grow. Any remaining oxygen is released through miniscule pores found on the leaf’s surface. 

Leaves are falling all around…

Not all trees are able to handle the colder weather later in the year, so they drop their leaves until spring peaks its sunny head around the corner a few months later. 

Interestingly enough, leaves don’t just fall off with a strong breeze or a cold wind; the trees actually shove the leaves off! If anything, this season should be called “Push” rather than “Fall,” if we want to be scientifically accurate. But anyway… 

If we think of leaves as solar-powered cooks, then during the summer months these tiny cooks are working all day every day to make the most of that bright sunlight, all the while depositing an abundance of food and nutrients into the plant. It’s a great system, and those leaves make it well-worth the effort of keeping them around full-time.

In the colder months, though, the days are much shorter and the sun is not nearly as direct. The quality and quantity of sunlight decreases with each passing day, and the tree has to decide if it wants to keep its staff of full-time chefs all winter long, or if it wants a break. It takes a lot of nutrients to keep leaves alive, and if the leaves themselves aren’t able to make a lot of nutrients in the first place, then the tree can be quickly sapped of its reserve. Plus, if the water in those leaves gets cold enough to freeze, then the leaves could die completely and damage the tree’s vascular system.

For these trees, then, it makes more sense to get rid of its leaves and go dormant over winter, rather than hang onto the leaves and run the risk of a bad freeze. 

The process by which trees drop their leaves is a fascinating one, and it all starts with the decreasing sunlight.

Hormones inside the leaves can sense when there is less sunlight and lower temperatures, so those hormones will activate a process called abscission that starts to shut down the production process. This means that the chlorophyll stops working, changing the leaf’s color back to its natural state of yellow, orange or red (yes, leaves are not naturally green!). It also means that tiny cells begin building a wall between the leaf and the twig it’s attached to, slowly cutting off the flow of nutrients and water from the leaf. 

These abscission cells eventually grow thick enough that the leaf is shoved completely off the twig with just the slightest of breezes, falling to the ground in that magnificent display that we just love to watch. And, because those cells formed such a thick wall, there is no open wound on the branch where the twig was, and the scab of plant cells keeps the twig protected all through winter as the tree lies dormant.

This process is purely self-preservation, and we see this happening in other adverse conditions, too. When faced with a particularly horrific drought, the tree may cut off nutrients to leaves or branches in order to strengthen the more vital parts of the tree. The tree will reactivate these dormant pieces when there are enough nutrients available, and the process starts all over again!

Food for spring

Wading through a forest full of fallen leaves can feel like trudging through snow, as there are just so many leaves! Yet when we return in spring, those blankets of leaves are gone! Where do they go?

Fallen leaves decay through natural processes, as fungus, bacteria and invertebrates cover the forest floor and provide the vital processes of decay and deconstruction that keeps the circle of life running smoothly. 

Webs of fungus called mycelium send tiny strands of hyphae into organic matter around them, where those hyphae secrete enzymes that break down the chemical structure of the fallen leaves. The nutrients extracted from those leaves are transported through the mycelium to the trees and plants all around it, feeding and strengthening the forest through this massive underground web. 

Finally, small critters and bacteria eat through the remaining leaves, breaking down the physical structure and secreting even more nutrients into the soil via their waste. 

In the end, all the nutrients stored in those bright green leaves returned right back to the ground, where they will feed the tree yet again come spring. Then, the cycle continues once again.

Fall is a magical time for many reasons, but seeing how this brief interval of time can so drastically affect the livelihoods of deciduous trees makes it all the more fascinating.

Enjoy the beauty of the changing color of the leaves. Take a short rode from your home at http://www.starloftspgh.com to the Laurel Highlands this weekend.

Happy Fall! Best, Janet.