What is a Tree?

We’ve learned many interesting things while writing and researching the Gardzen blog, including this: sometimes the simplest questions have the most complex answers. This particular blog is inspired by a very simple question asked by an eight year old at the local farmer’s market: “What is a tree, exactly?” In looking for an “exact” answer, we found out that this is a surprisingly complex question both agriculturally and botanically. So, as we often do, we thought we’d write a blog about it and share what we learned with the rest of you. Join us as we dig deep to find an answer to the question “what is a tree exactly” and learn how that answer affects how we garden.

Definitions of a Tree

We’ll start by looking at the various definitions of a tree that are in common use, and see what we can learn by comparing them. Checking a few different dictionaries, the Encyclopaedia Britannica, and (of course) Wikipedia, the popular definition of “tree” seems to be something along the lines of “A relatively tall and elongated perennial plant with a woody stem that produces secondary growth in the form of limbs, branches, and leaves”.

Most of us would probably agree with this; it’s a common sense/intuitive definition that fits with our lived experiences. However, things get complicated when we try to distinguish “trees” from other forms of plants like “bushes” and “shrubs”. What’s the difference between a tree and a shrub? Our friends at the Utah State University Forestry Extension were fairly direct in their answer: “The distinction between trees and shrubs is not always clear”, going on to point out that while there’s no strict scientific or botanical separation between trees and shrubs, a good working definition of “tree” might be a woody stemmed, self-supporting perennial with a defined crown of secondary growth (branches and leaves) with a trunk at least three inches/~8 cm in diameter and a minimal height of around 13 feet/4 meters.

 This definition may be helpful for many of us: it provides some basic measurements and characteristics that are both easy to understand and easy to apply in the real world. With these criteria we can fairly easily identify what species are “trees” both in the wild and in garden or agricultural spaces. Again, this is not a hard and fast definition and it’s not universally accepted by farmers, botanists, biologists, and foresters. It’s just a useful and easy to apply guideline that helps us separate trees from shrubs, bushes, woody vines, and the like.

The Parts of a Tree

To help us define a “tree”—and to help us all understand how they work—let’s take a look at the different parts of a tree per the definition above and what functions they fill in the life of the plant:

· Trees have trunks, tall woody structures with a bark of some type. The trunk is the central, self-supporting structure of the tree that reaches up from the ground.

· Most of the trunk is non-living tissue and serves only as a support structure. Trees grow outward every year, and once this year’s growth layer has done its job it “dies” and becomes part of the hard woody core that supports the rest of the tree.

· The outside layer is comprised of the “cambium” and the “phloem” and this is the living part of the trunk. The cambium is where the tree grows new wood and new bark, increasing the diameter of the trunk over time. The phloem transports the nutrients photosynthesized by the leaves from the crown of the tree to the roots, and as it dies off it becomes the bark of the tree.

· The layer inside the cambium is called the “xylem” which transports water and nutrients from the roots to the crown. As xylem ages, it dies off and becomes part of the wood of the trunk and new xylem grows to take its place.

· The branches and crown support vertical growth and the growth of leaves. This is important, as trees evolved to make use of vertical space in order to get more sun and thus better access to the solar energy needed for photosynthesis.

· The roots anchor the tree and gather nutrients and water from the soil. This is obvious but it needs to be said: the roots are part of the tree. They’re an extension of the trunk that helps both the structure of the plant and its nutrient/water gathering and they grow as the tree grows, reaching out further and deeper into the soil.

Defining Trees in Your Garden

So now that we’ve gone over several ways to define a tree and what the parts of a tree might be, how does this apply to us, the home gardeners? We would like to suggest that for purposes of our garden spaces a tree could be defined as a taller perennial with a woody, self-supporting stem that fulfills the function of a tree. Meaning that if it looks like a tree for the purposes of your garden or landscape design, supports itself, is a perennial with a woody stem, then you can call it a tree if you think it’s tall enough. Words belong to the people who use them, and while there are some good working definitions of a tree you can call your plants what you want.

We hope this helps, and we hope that you’ll start looking at the plants around you with these ideas in mind. Defining a tree turned out to be more complicated than we thought, but we’re glad we took the journey and learned more about the definition of a tree and how we can use those definitions to understand the plants around us.