Why Do Trees Lose Their Leaves?

Red maples with fall color and water in background.

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Most of us know that certain types of trees (deciduous ones) are going to lost their leaves in the fall. We look forward to the colorful display they put on during the fall foliage season just before the leaves drop, but we spend the next several months looking at stark, bare branches. These are all important phases in the annual life cycle of deciduous trees and are necessary for the tree to thrive. But why would you want to grow a tree that is going to suffer such a loss, when there are other perfectly good trees, such as the pines (Pinus genus), that retain green needles all year, thereby affording winter interest? Besides beautiful fall color, deciduous trees are added to our landscapes for form, shade, and wildlife habitat.

Find out exactly what is happening when a deciduous tree loses its leaves, why this is beneficial to the tree, and why some trees don't lose their leaves.

What Does Deciduous Mean?

A deciduous tree drops its leaves at the end of the growing season. By contrast, trees that are evergreen retain foliage throughout the year. Most coniferous trees are evergreens, but deciduous conifers do exist, such as golden larch (Pseudolarix amabilis) and dawn redwood (Metasequoia glyptostroboides).

What's Happening When a Tree Loses Its Leaves?

We are accustomed to saying that leaves "fall" off the tree, but, in a way, this is misleading. It makes the process sound totally passive, when, in fact, there is an active element to it. A deciduous tree does not so much lose its leaves as it gets rid of them by cutting them and letting them drop.

To do this, the tree grows a cell layer between the leaf stem and the branch called the "abscission" layer (the word derives from the Latin root for "cut"). The abscission layer behaves like a pair of scissors, pushing against the stem and cutting the leaf off. The cut is then sealed so that no moisture can be lost during the winter.

Fun Fact

Foliage changes color in the fall because of a subtraction, not an addition. Colder weather signals that it's time for the leaves to cease making chlorophyll—the chemical that makes leaves green all summer. Chlorophyll masks colors that reflect the presence of other chemicals in the leaves. The fall color we enjoy so much is the result of an unmasking. Since chlorophyll is necessary for photosynthesis, the leaves become useless to the tree long before they fall off.

Why Trees Lose Their Leaves

Humans see the process of a tree dropping its leaves as a loss. But that's not how the tree "sees" it. For the tree, leaf shedding is a strategy for surviving the winter.

Winter is a difficult season for plants in the North, but not necessarily because they mind the cold temperatures. After the ground freezes, your plants can get quite thirsty. The frozen ground acts as a barrier, preventing any water that's above ground from traveling down to the roots. Above-ground water is also locked up in the form of snow and ice. To survive, plants must develop strategies that allow them to conserve moisture and survive the winter drought.

During other times of the year, the relatively large surface area of the leaves of broadleaf, deciduous trees is helpful: It aids their photosynthesis. It allows them to absorb as much sunshine as possible and to convert light energy into chemical energy for growth. Some moisture is lost through the leaf surface during this period, but it isn't significant: The moisture is readily replenished (unless there's a drought and you fail to irrigate).

However, the large surface area that is beneficial during warm weather would become a disadvantage during cold weather: It would allow moisture to escape through the broad leaf surface at a time when it is not being readily replenished. Rather than let this happen, deciduous trees, as part of the process of going dormant for winter, get rid of their leaves altogether, sealing themselves off against moisture loss.

What Is Dormancy?

Dormancy is a state of temporary metabolic inactivity or minimal activity in a plant. It is a mechanism used by some plants to cope with adverse growing conditions, such as a lack of water. Deciduous trees become totally dormant in winter; while the growth rate also slows in winter for evergreen trees, they become only semi-dormant.

Why Some Trees Don't Lose Their Leaves

As effective as this strategy is, not all trees need to use it. Evergreens don't. It is no coincidence that as you travel farther and farther north in the Northern Hemisphere, deciduous trees begin to drop out of the picture: All you see are needled evergreens. Why?

The needle on an evergreen is much tougher than the leaf on a deciduous tree. This is partly due to its shape and size. With less surface area, there is less chance for moisture loss. Moreover, these needles have a waxy coating that makes them better able to withstand freezing temperatures. As a result, evergreen trees more efficiently conserve moisture and can survive the harsh winter climate without resorting to dropping their leaves.

Tip

If some of the needles on your evergreens turn brown and drop in the fall, there is usually no cause for concern. Evergreens also drop some of their needles annually or every few years in order to make way for new growth.