Oct 13, 2022
What is a tree?
We know trees are made of wood and have a trunk, branches and leaves, but how do we define what makes a tree… a tree?
How do we define a tree and what makes it different from other forest plants and organisms? In this post, we look a little closer at the common characteristics of a tree and some of the science behind their shape, size and structure.
Definition of a tree
As you know, there are many different tree species. However, they all share some key features that meet the criteria for what we understand to be a tree.
Here's our simple definition:
A tree is a tall plant that can live for a very long time. It has a single stem or trunk and branches that support leaves. Beneath the ground, a tree has a root system that acts as an anchor and stores the water and nutrients the plant needs to grow.
One of the ways we distinguish trees from other plants is their thick and rigid ligneous tissues, which we know as wood. And of course, different tree species are made up of different types of wood.
Ultimately, while it's useful to have a basic tree definition, as with all nature we need to delve deeper to understand the sheer variety of trees in the forest.
Let's explore some of the other features that make trees such a special and important part of the natural environment.
Some organizations prefer to define what makes a tree a tree by measuring height. The idea is to keep things simple, considering trees come in a variety of forms and species, with diverse and often localized characteristics.
The definition from the Food and Agriculture Organization of the United Nations (FAO) says that for a plant species to be a tree, it should reach 5m tall if the growing conditions are good. That's in developing countries – in developed countries the height goes up to 7m.
The National Forest Inventory (IFN) takes a similar approach. It defines a tree as a woody plant that has a bare stem at its base and can grow to a height greater than or equal to 5m when it reaches maturity.
Of course, some trees grow to truly incredible heights. The redwoods in California are the tallest trees on Earth and attract visitors from all over the world. They grow to heights of more than 90m – the tallest is named Hyperion and reaches a massive 115m into the sky.
Anatomy of a tree
Trees are perennial plants, which means they live for many years, sometimes centuries and for some species, several millennia. The life cycle of a tree is different from species to species, but the basic structure of a tree remains the same.
A tree is also a plant that has aerial (above ground) parts, including the stem, leaves and flowers, and underground parts, its roots. Like any complex living organism, a tree is born, breathes, grows and reproduces if conditions and circumstances allow, before it eventually dies.
In botany, trees and other woody plants are characterized by their secondary growth, which means their layers of tissue expand and their trunk gets wider. Basically, a tree is a plant that grows outwards as well as up. The extra width gives the tree the strength it needs in its stem and roots to support itself.
Wood, leaves, fruit and flowers
Most people associate trees with wood, leaves, fruit and flowers – the things we see on the outside. We generally have a good idea of what a tree looks like and we teach children to recognize trees from a young age.
On the inside, wood is also called xylem, which are dead, thick cells that transport water and nutrients from the soil to the leaves. Xylem is connected to all other parts of the tree to form a continuous system that reaches throughout the plant and allows it to live and grow.
While other plants do produce wood, trees are defined by their large size and single, load-bearing trunk. Deciduous trees have leaves or needles that fall every winter or dry season, whereas foliage on evergreen trees stays green and functional for multiple years.
A tree can grow flowers or fruit, which contains seeds that spread and allow it to reproduce. Each species has its own unique features, which is part of what makes trees such a crucial part of a rich, biodiverse habitat.
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