Backyard Nature: What causes trees to change their colors and shed their leaves in the fall

by Marlene Berlin

Almost as soon as trees start to wear their fall colors, they begin to slough them off.

Around the neighborhood and on Rock Creek Park trails, flashes of yellow, orange and red are appearing. And the leaves – some brilliantly colored, some not – are also dropping from trees at a steadier and faster rate. We know this is what leaves do in the fall. We expect it. But how does this happen? And why?

How do trees drop their leaves?

The cells that connect the leaf to the branch are called the abscission layer. Through this connection, water passes from tree to leaf, and energy passes from leaf to tree. When the temperatures begin to drop and the days become shorter, trees reduce their production of hormone auxin, and the abscission layer degrades. This loosens the leaf’s bond to the tree. It will eventually fall to the ground below, or be blown away by the wind.

Why do trees do this?

The main theory is that it’s a self-preservation mechanism. Still-leafy branches hold on to more snow, and under that weight, are more apt to snap when the wind blows through.

Also, by the end of summer, leaves are often damaged by insects and disease. Dropping leaves allows the trees to start each spring with a fresh batch of leaves. And the fallen leaves fertilize the soil and feed the tree.

The brown spots on these fallen leaves can be signs of disease and insect predation.

Some trees do not drop their leaves. Why?

According to Ned Friedman, director of Arnold Arboretum at Harvard University, “No one has a clue.” Keeping the leaves once they have turned brown and no longer produce chlorophyll is called marcescence. The hormonal changes that cause leaves to drop do not occur until spring. This happens to the beech tree in my backyard.

Green leaves still abound in Rock Creek Park.

And why do leaves change their colors?

Actually, in many cases, the colors are there all along, but they’re covered by all that green. They get their green color from chlorophyll, and when shorter days and cooler nights cause leaves to stop producing the chemical, the green degrades. Then, the hidden pigments emerge. The carotenoids – the source of the yellow, orange and brown colors – develop in the spring. The red and purple pigments, called anthocyanins, are produced in the fall.

From the U.S. Forest Service, here are trees commonly found in our area, and their fall colors:

Oaks: red, brown, or russet
Hickories: golden bronze
Aspen and yellow-poplar: golden yellow
Dogwood: purplish red
Beech: light tan
Sourwood and black tupelo: crimson
Red maple: brilliant scarlet
Sugar maple: orange-red

What is not explained is why there is a range of colors in a particular tree. Maybe it is how the sun hits the tree during the day, and the different rate of degradation of the abscission layer.

When the leaves fall

They end up on our yards, sidewalks, trails, streams… and well, everywhere. They are ubiquitous. And according to Master Naturalist and Forest Hills Connection contributor Kathy Sykes, they are valuable. They provide fertilizer and mulch for trees and garden beds, and winter homes for insects. Leaving them where they fall, or raking them from sidewalks to tree boxes, will save you time and money.