Each year, as the days get shorter and cooler, certain types of trees begin a spectacular transformation. They slowly begin to shed the green from their leaves and turn different colors. This is a process called senescence, which is induced at certain times a year by photoperiodism, the process by which a tree regulates its life cycle and prepares for winter.
Broadly speaking, there are two major types of trees: evergreens and deciduous trees. Evergreen trees, like firs and junipers, keep their needles all year round. Many of these trees grow needles or scale-like leaves. Because they don’t lose their needles in the fall, they stay green, thus the name evergreen.
Deciduous trees, like maples, oaks, and beech trees, operate in the opposite way. The word “deciduous” means “tending to fall off,” and as you might guess, these trees lose their leaves during the colder months of the year. Unlike evergreen trees, deciduous trees don’t typically grow needle-like leaves, but larger, broader, flat leaves.
There are exceptions to this rule, however. Oaks, a deciduous tree, do eventually shed their leaves each year, but not generally until new growth occurs in spring. Tamaracks, an evergreen tree, sheds its needles every fall.
What does a leaf do?
Leaves may seem like a relatively small part of a tree, but without leaves, a tree can’t grow. Leaves are responsible for producing the food that a tree requires in order to survive. Leaves engage in a process called photosynthesis, which turns light energy from the sun into food.
Each leaf has a series of stomatas, pore-like structures, that allow the leaves to “breathe” in carbon dioxide, a necessary ingredient for photosynthesis, and “breathe” out oxygen, a waste product of photosynthesis. Leaves also exhale excess water, sort of like sweating. This exhaling of water increases humidity near the tree and can help cool the air in the summer. Trees are like giant green air conditioners!
Why do trees lose their leaves?
Even though the leaf is one of the most important parts of a tree, it is important for a tree’s health for those leaves to be shed each year. Unlike evergreen trees, whose needles tend to be small and narrow, deciduous trees generally have large, broad leaves. During windier winter months, these leaves would undoubtedly become damaged and could increase the likelihood of the tree being toppled.
Shedding leaves also helps conserve both water and energy during dry, colder months. Instead of relying on leaves all year long, it stores some of the food the leaves produce during spring and summer and subsists on that through winter.
When the time comes for the leaves to fall, the tree begins a process called abscission. During abscission, the tree grows specialized cells at the point where the leaf’s stem meets the branch of the tree. Eventually these cells will sever the leaf from the tree.
What trees lose their leaves seasonally?
Any tree that falls under the ‘deciduous’ umbrella will lose its leaves at some point in its annual photoperiodism. Some trees, like maples, beech, and ash trees, will lose their leaves during the autumn season. Oak trees are a bit different, often holding onto their leaves until new growth begins in the spring. Ash trees are often the first to lose their leaves, while sycamores will usually wait until midwinter to drop their leaves.
