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What Makes a Tree a Tree?

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Greenery grows across our entire world. How do we discern a tree from a shrub from a plant? While there are many characteristics common amongst trees, there is no scientific or DNA method to differentiate. For us to truly determine what is a tree is, we will need to look at specific characteristics. While many living plants will share some of these characteristics, only trees can check all the boxes. Below, we will break down the key characteristics that botanists assign to all trees. We will also highlight the history of our understanding of trees.

It may seem like we all know what a tree is. After all, we have been drawing trees in school since we learned how to draw. However, truly pinpointing it is not a child’s task. With such a wide variety of shapes, colors, and sizes, trees are a diverse species.

Let’s deep dive into the real origins of trees.

What is a tree?

A tree is a plant living inside the Plant Kingdom, along with all the flowers, bushes, and shrubs. Overall, trees are tall plants, towering over every other plant in the kingdom. Trees live exceptionally long lives compared to other plants. In fact, they live at least a few decades, but can even live for hundreds of years.

As we know, trees grow branches and stems to support leaves. Unlike many other plants, trees grow deep and complex root systems to support their weight and height. Trees grow in layers. The heartwood is at the center with the sapwood surrounding it. Continuing outward is the cambium, phloem, and the bark.

Trees are perennials and re-grow their leaves every spring if they are deciduous. Most of the evergreens keep their needle-like leaves year-round.

Why is a tree called a tree?

The word “tree” comes from an ancient word. From Old English, the word “treow” and “treo” defined all large woody plants. We use these words to refer to wood or timber. With the actual plant being the source of the wood or timber, the plant was also given this designation. Though we differentiate the timber from the plant, it was unnecessary in ancient cultures. The word covered both the use and function.

What characteristics are the most tree--like?

All Trees Grow Indeterminately

Trees are always growing. Prior to reaching maturity, a tree will grow in both height and spread. Every growing season will add more stems and leaves. Once maturity is reached, a small amount of height and spread will continue to grow. This is especially true for spread as the branches grow more stems. After all this, the diameter of the stems and branches will increase with time. Also, the root system of a tree is always growing. It will spread further in the soil.

Botanists consider primary grow as the growth in height and length of buds and branches. While secondary growth is the thickening of branches, roots, and stems.

All Trees Live Long Lives

Most plants live brief lives. In fact, some plants will only grow seasonally. Trees, however, are not like this at all. Even the shortest-lived trees will last over 15 years. These trees are abnormal, though. Most trees will live at least 50 to 100 years and many more will live over 1000 years.

There are some examples of trees living 5000 to 6000 years old. With such long lives, they must withstand the harshest of conditions. Some trees live right at the base of active volcanoes. Additionally, a tree living through generations will face drought, floods, huge thunderstorms, hurricanes, and frigid temperatures.

Although an individual tree may live within your generation, many tree species grow in colonies. These colonies survive through tens of thousands of years. When one tree dies, another tree grows in its place. This cycle continues. Notably, scientists estimate a colony of trees in Utah to be over 80,000 years old.

You will not find any other plant that is even close to matching this longevity.

All Trees Have Trunks (and Rings)

If you look at any child’s drawing of a tree, the most prominent feature of the drawing is the trunk.

Large tree trunk

Though some plants grow on a singular stem like trees, no other plant grows on such a substantial, woody stem.

Some trees grow in a clump of stems, but the trunk is formed from this clump. Overall, the shape of the trunk is irrelevant to the definition. Some species grow in vertical towers that seem almost perfectly straight. On the other hand, other trees grow with trunks that seem contorted and twist into an irregular shape.

The girth of a trunk varies widely from species to species. The species growing in clump-like bases sport smaller girths. However, most trees grow with a true singular trunk ranging in size from a few inches to hundreds of feet.

All Trees Develop a Deep Root System

With the immense size of trees, a strong support system is necessary. Almost all plants grow roots for the same reason. However, trees need a much deeper root system to support the weight. Wind is a much bigger factor for trees than it is for smaller plants.

Furthermore, a plant that grows to the size of a tree demands significant water. Without a deep root system, a tree could not survive. Also, as it stretches deeper into the ground, the tree can take more water in.

The depth of the root system is not nearly as impressive as the length. In fact, therefore, we know roots to do so much damage to infrastructure we build nearby.

Tree colonies interweave their roots together not only to add support, but to depend on each other. All in all, the colonies survive and thrive because of this growth method.

Finally, trees are the only plant in the kingdom to develop rings annually based on the seasonal ebb and flow of weather and nutrients.

All Trees are Covered in Bark

Tree trunks and branches are covered in bark. Over time, the bark changes from smooth to fissured for most species. However, some may stay smooth throughout their lifetime. Many young trees are missing their bark. It develops thickly as the years go on.

Bark protects trees from all kinds of damage. Disease, pests, and weather are all deterred by thick bark. The other characteristics of bark such as functional uses, colors, and thicknesses all vary between species. Trees are the towering giants of the Plant Kingdom. We can put together many key characteristics to determine if a plant is a tree or shrub. While most trees seem easy to identify, there are some that look very similar to shrubs. Using the characteristics above, you will have no problem with this classification anymore.

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