Why do Leaves Change Colors in the Fall?

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The fall is a great season to learn more about the trees around you. As we paddle, we are realizing that we can learn a lot about the forest by the colored leaves we are seeing. As we look at a hillside of trees with white bark and yellow leaves, we can tell that those are most likely birch or aspen trees. Many hilltops here have trees with red leaves. They are most likely maples. Sometimes we see red leaves that are darker– almost brown. These are oaks. Then there are trees that are still green. They have needles instead of leaves. Do you know why some trees lose their leaves in the fall? What is the difference between trees that lose their leaves and trees that keep their needles?

Why do some trees lose their leaves?
Basically, some trees lose their leaves in the fall to save water and energy. Losing their leaves helps these trees survive the cold, dry  winter weather. Did you know that trees use sunlight to make their food, or energy? Plants use a process called photosynthesis to convert sunlight into energy. Leaves are kind of like their solar panels, playing an important role in capturing the energy from the sun.
In the warm seasons, trees use sunlight, air and water to make their food. During that process, a tree loses water through tiny holes in the leaves. The air is cold and dry in the winter. If trees kept their leaves, they would not be able to get enough water to replace what they lose through the leaves. So trees seal the spots where leaves grow. No water can pass in or out. This causes the leaves to dry up and fall off. When warm air and water return in the spring, trees will grow new leaves.
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Why do leaves change from green to yellow, orange, or red before falling off?
Chlorophyll is a special chemical in the leaves that helps with photosynthesis. Chlorophyll is what makes leaves green. When leaves turn yellow, orange or red in the fall, that means there is no longer any chlorophyll in the leaves. There was a small amount of these other colors in the leaves before the fall, but the chlorophyll covered those colors up. You might want to do more research to figure out why a maple leaf turns red, but a birch leaf turns yellow!

How do trees “know” when to lose their leaves?
In a northern climate like this one, the amount of sunlight changes throughout the year. During the fall, this part of the Earth is getting less daylight. This means that the days are getting shorter and the weather is getting colder. It is the change in daylight that causes trees to start losing their leaves.
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Why do some trees stay green all year?
The trees that are still green have needles instead of leaves. Trees that lose their leaves in the fall are called deciduous trees. Trees that have needles and stay green all year are called evergreen or coniferous trees. The special needles of evergreen trees help the tree make food like the deciduous trees. The difference is that these needles are weather resistant.

In the Boundary Waters, we are on the very southern edge of the boreal forest. If you travel north from here, the trees of the boreal forest are mostly evergreens. If you travel south from here, you would see more deciduous trees. This explains why the forest we are in is called the Laurentian Mixed Forest. The forest is a mixture of deciduous and coniferous trees.

What about a tree with needles that fall off?

Did you know that there are some trees called deciduous conifers? We have one type of tree in this forest that has needles that turn yellow and fall off this time of year. This is actually my favorite type of tree. It is called a tamarack tree. Tamaracks like to grow in wet, boggy areas. Right now their needles are bright yellow. Soon, they will fall off. In the spring, they will grow new green needles.

Types of trees
There are so many interesting type of trees here. The deciduous trees are birch, aspen, maple, ash, and oak. The coniferous trees are white pine, red pine, jack pine, cedar, spruce, and fir. The one deciduous conifer here is the tamarack. What kinds of trees do you have near your house? Are they deciduous or coniferous? If they are deciduous, what color are their leaves now? The fall is such an interesting time to learn about trees!
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