Why Trees Shed Their Leaves in Autumn

Written by Jeanne Grunert and published on https://garden.lovetoknow.com/.

Autumn is a favorite season for many of us here in Chicago. After all, who doesn’t appreciate the cooler weather, football-filled weekends, and pumpkin-spiced everything? These aren’t the only reasons to appreciate fall, though.

Nothing beats going on a walk and noticing all the fall foliage around the city. But have you ever wondered why trees even shed their leaves in the first place? We answer that question and more in our latest post.

What Causes Plants to Shed Their Leaves in the Fall

What causes plants to shed their leaves in the fall? It is a complex
interaction among genetics, light and temperature. Starting in late
summer, many species of deciduous plants, including trees and shrubs,
turn brilliant colors and shed their leaves. To understand the mystery
behind this annual fall show is to uncover the magical factories inside
the plant’s leaves.

Factors That Signal to Plants That Fall Is Here

What causes plants to shed their leaves in the fall? The answer lies in the plant’s genetics and reaction to its environment.

Chlorophyll

Within each cell of a plant’s leaves is a substance called
chlorophyll. That’s what gives leaves their green color. The chemical
called chlorophyll interacts with water, carbon dioxide and sunlight to
create the simple carbohydrates plants need to grow and thrive.

During the spring and summer when sunlight is abundant and
temperatures are warm, the plants’ leaves contain plenty of chlorophyll.
It masks other colors or pigments found within the leaves. Depending
upon the plant, the leaves may contain varying amounts of two other
chemical pigments: carotenoids and anthocyanins.

Sunlight

As the summer days wane, the duration of daylight and the angle of
the sun’s rays change as the earth moves through space. Plants can sense
these minute changes day by day. As the days grow shorter, the lack of
sunlight starts to signal a slowdown of food production.

Temperatures

Along with less sunlight, temperatures begin to cool. As the
nighttime temperatures grow cooler, this also signals the plants to stop
or slow production of food. As chlorophyll production stops altogether,
the carotenoids and anthocyanins inside the plant’s leaves become
visible.

Falling Leaves

This combination of stopped chlorophyll production, less sunlight,
and cool temperatures acts like a switch within the plant’s genetic
system. It flips to the “off” position and signals the leaves to stop
growing and manufacturing food. First, chlorophyll production stops. The
masked anthocyanins and cartenoids are now visible, revealing the
leaves’ hidden coats of scarlet, crimson, ochre, and golden yellow.
However, as time goes by and no energy is produced in the leaves, the
plant releases them and leaves fall to the ground.

Leaf Differences in Evergreens

Deciduous trees and shrubs lose their leaves in the fall as a protective measure. Their leaves are tender, and cold temperatures would kill them. Water flowing through their tender leaves would freeze, stopping energy production. Evergreen trees and shrubs, or those that retain their green leaves through the winter, maintain a thick, waxy coating on each needle. This waxy coating protects the leaves against cold.

There’s a difference inside the leaves, too. Special chemicals act as a sort of antifreeze within evergreen needles to keep the liquids flowing through the plant from freezing. Thus evergreens can maintain their leaves (needles) throughout the harsh winter months while deciduous trees must shed them.

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