What is a tree?

The thin line between a plant and a tree

Each of us works with trees, day after day. They are our profession, and the subject and the reason of existence of many arboricultural magazines. But when talking about trees and arboriculture, it is important to know what we are talking about. So we have to question ourselves what we are talking about when talking about trees.

Trees have a stem. Can trees have more as one stem? And how do multiple stemmed trees differ from shrubs? In Bretagne we saw this huge hazel with a spread of 20 meter. and several very thick stems. This is a real tree, however we usually would not call it a tree.

Plant or tree?

A tree, what is this what we call a tree? At first glimpse, it seems to be an easy question. A tree is a big plant that will reach high age with a fat wooden stem topped with a crown. But will this definition stand the check to reality? Do all trees have a stem? Can trees exist with more as one stem? And can trees exist without a stem? And if so, where will we point the line? And how we will decide where to point that line? Or, with other words, what makes a tree a tree, and when is a plant not a tree?

In Le Forêt Domaniale de Verzy in the vincinty of Reims about 800 of these odd shaped beeches can be found. Several among them are rather small. But beeches are trees, aren't they? So, this dwarf shaped beech es are trees as well.

Wood

But let’s dissect the definition of a tree. Trees are made out of wood. But what is wood? When does a cellwall contains enough lignine to call it wood? Take a look at maize, corn for an example. An annual grass. The stem is tight enough to keep standingupright when cut. The cellwalls are lignified. But it’s not a tree, that’s for sure. In fact, it is not that easy to separate herbal and woody plants. Those are two edges on a continious line, on which the only difference can be made by the amount of wood the cellwall contains. A straight line between wood, woody and herbal isn’t that easy.

Crown

And what about bamboo? Bamboo has perennial wooden stems, without secundary growht. Not an easy one. Shall we call those gigantic bamboo that can reach a height of up to 20 meter , trees, or not? Of course they do not have a real crown. But a freshly cut pollard neither has a crown. Can we see a pollard without a crown as a tree?

A coppice stool of beech (Fagus sylvatica) Is it a multiple stemmed tree? Or is it just a tree with an extremely short stem, so the crown starts close to the ground? And what is the difference with a shrub?

Secondary growth

So, bamboo does not form secondary growth. Would that be the answer? Secondary growth? So we can eliminate all palm trees at ones. We do not have naturally growing palm trees around here. Is this however an argument to decide whether a plant is a tree, or not?

Age

Let’s look at age. Trees can get really old, seem to be even immortal. Of course, due to catastrophes, they can die, or they get inhabited by organisms that can turn them down. Whenever they get the opportunity and circumstances are good, they get really old. But when looking around, we can find other plants that get real old. Very often they are colonies of what appears to be individual plants that are genetically identic as they originate from the very same seed. Lily-of-the-valley with an age of 300 years: check. Or tweeblaarkanniedood, Welwitschia mirabilis, a strange plant that grows in the desert of Namibia. Two leaves, just lying on the ground. And that grow for a thousand years. When age is an argument to call a plant a tree, we should also see these as trees. But that counts s well for those enormous fungal networks that exists. Some have proven ages of 700 years. In Oregon a fungal network of honey fungus is known to have an age of about 2 400 years. When age is an argument, we should count those in. Or not?

Lilly of the Valley can become old, real old, with a proven age of up to 300 years. When age would be an argument to call a plant a tree, this could also be a tree.

Own definition

What is the conclusion we can make after reading this? To start trees can become very large, bujt they can also be rather tiny. Tiny trees are small because they still have ot grow. Or they are tiny because cutting and vlipping keeps them small. Other trees stay small because they are dwarf forms of bigger trees. Or they can be tiny because being pollarded or even coppiced. Such trees can restart to grow and become bigger. While other trees, just do not grow that high.

Even within one and the same species, trees can look very different. That turns out to be problematic when given a proper tree definition. When talking about trees, we usually use For such a definition, we have the archetypical ‘tree’ in mind. But trees do not always bend themselves into this definition. We can also see a tree defined by the law of biology. For a lumberjack, an environmentalist, a carpenter,…. a tree is for each one of them something different. After all, a definition is just an agreement about the meaning of a word. In fact, we could make our own defitition.

Mother tree.

We still have no straight answer. We can give a linguistic definition for a tree, a straight biological definition for a tree is still not that easy. You should really try it for yourself. After reading the previous, it might have become more difficult, or even close to impossible.

That brings us tot he point where we would get, the question we should ask ourselves: “Why it is this hard to give a good biological definition for a tree?” All elements that are part of a tree, that all together form the tree as a living creature, are not that necessary or decisive. Plants that we don’t see as trees show the same elements. To answer our question, we have to go back in the evolution, looking for the mother tree, the tree all other trees originate from.

Genes

Scientists have been searching for the genes that make a plant a tree. They couldn’t find them. The same genes that are responsible for growing up, primary growth are also responsible for thickening, secondary growth. Probably, trees used the genes that were available to form secondary growth and make wood. In the evolution some genera of herbal plants, evolve to trees. The Canary giant snowthistle (Sonchus canariensis) is a fine example of this evolution. Other islands hold still other examples. They are able to do so, because the ability to form wood and become trees, are found in the same genes as those that form upright growth. Plants became trees at different occasions, starting form all kinds of plants.

Black locust is a tree of the same family as Chinese wisteria or clover. In the kingdom of plants, trees can be found in nearly every group of plants. Trees do not form their own closed group of plants. During evolution, different plants at different momnets solved a similar problem by growing above other plants.

Convergent evolution

There is no such thing as a mother tree. No tree from which all others developed. Evolution is not a targeted process. Evolution follows a pathway of trial and error, of coincidence and survival by coincidences. A small mutation can result in a complete new species. At the other hand, evolution can’t find an endless range of solutions for the same problem. On the contrary. Most often, we see that a similar problem will lead to a similar solution.

When two species developed a similar solution without having a common ancestry, that is what we call a convergent evolution. Trees are a perfect example of such a convergent evolution. In evolution we see different occasions on which plants growth over other plants and developed into trees. That finally resulted in the current situation, where most groups of plants contain trees.

Struggle for light

There is a common problem that plants solved by growing into trees. So we should ask ourselves what that problem was that plants had to deal with that could be solved by growing into trees. That question can easily be answered by asking another question : “What advantage does plants have when being a tree?” The answer is quite simple: trees grow tall to win the concurrence with other plants. Light, that is where it is all about. The struggle for the best place under the sun, made some plants grow over others. That way they can catch the sunlight as much as possible and photosynthesise more as any other plant around.

Plants become trees to grow over other plants and to maximize the amount of sunlight they can capture for photosynthesis.

Plants become trees to grow over other plants and to maximize the amount of sunlight they can capture for photosynthesis. We could say that trees won the struggle for light. But, they have to pay a (often very high) price for their victory. To put their leaves high up in the sky, trees have to build a structure. Stem and branches are formed to put he leaves up high, above other plants. And the enormous root system serves among other things for stability so that all efforts to grow over other plants won’t be in vain. Building those huge structure takes both time and energy.

Trees need time to build their structure. A seedling has to compete for its survival at the level of herbal plants. In the shade of a wood, in a grassy meadow or the heathland dunes, seedlings do not have the advance of large trees. But even when the tree grows on, it still isn’t the giant that grows above all other trees. The sapling still is shaded by many other trees. That has consequences for the way the tree will shape its body and form a trunk.

The huge structure trees have to build, gives them an advantage. But they need to build this structure by the carbon they can capture themselves in photosynthesis. Maintenance of this structure requests a lot of energy. there is no way to gather extra energy when necessary. This has an important influence on the way tees live and survive.

Plant food

A second problem is the energy that will be needed to build this vast structure. Trees have just one resource for energy: photosynthesis. In the photosynthesis carbon is fixed to form chains of sugar and starch the tree uses as the building material for cells as well as to provide the system with energy. Carbon captured in photosynthesis is used as a building material. When these carbon chains will be broken down, the energy will be released that the tree will need to run he machine.

You can fertilize plants, you can’t give them extra energy. The minerals you add to the soil or even on the leaves, are not ‘plant food’ sensu structu. Feed are substances that provide energy when burnt. Minerals can’t be burnt. They are part of the elements that make plant cells. The energy the tree needs for all of its processes come from breaking down the carbohydrates the plant captured in photosynthesis. For its energy supply, trees depend on their own production? The carbon trees can capture can be used for growth, maintaining the biological processes of the cells, propagation, through photosynthesis, maintaining the microcosmos around the roots. Part of it is stored in the wood as reserves.

Maintaining the structures as animals and humans do, is for plants far from possible. When animals or humans get ill, White blood cells will attack pathogens and destroy them. This as an energy consuming process. After disease, animals and humans are weakened and have to strengthen again. They need food to do so.

Own production

Plants have no possibility to take extra energy in with ‘plant food’. When getting confronted with pests and diseases, they need another way to deal with it. Plants need to spend their energy very efficiently. Parts that ask more as they produce will be divested. And maintenance of this enormous structure is impossible.

We can summarize all this in a few lines. Trees are plants that grow vast. By being this vast, they can grow over other plants. For their energy supply they depend exclusively on their own production. That makes them vulnerable. All, really everything, from how they grow, how to develop, how they decline and why they decline, it can all be brought back to this few simple principles. Understanding this few principles is the first step to a deeper understanding of trees.

Trees and other plants are autotrophic. They capture energy in photosynthesis. This has an enormous impact on all processes, starting at development in their youth, to the way trees decline, that happen in the tree. Trees can survive as long they can capture enough carbon to provide all processes with the energy needed.