How trees grow

Although trees are all around us, there are many misunderstandings about how they grow. Studying how a real tree grows reveals surprising mechanisms that together evolve these complex plants.

Plumbing

Think of trees as a big collection of annual plants, each of them growing at the ends of an interconnecting plumbing system. We call these plants twigs. They create the leaves, flowers and fruit of a tree. By themselves, they are much like any other plant. Their connection to the larger system of branches is what sets them apart. The branching system distributes flows of hormones, water and sugars – to and from the twigs – regulating growth and evolving trees into the majestic structures we all admire.

Flow of water

On older branches, only a thin outer layer just beneath the bark is actually living tissue. The rest of the branch consists of dead old cells. Their cell walls provide the structural strength a tree needs to stand tall. But more importantly, their sponge like structure sucks water from the roots up through every branch, all the way to the leaves.

A tree uses this water in several ways. As the water moves up through the branches, it is filtered to extract all its nutrients and minerals. To keep a steady flow going and give the tree enough nutrients, each leaf’s surface evaporates water in the heat of the sun. Thereby drying out the sponge and sucking up more water from the soil.

The flow of water determines the peak height a tree can grow, as well as its shape at different ages. The higher a tree gets, the harder gravity pulls on the water. There comes a point where the pull of earth’s gravity becomes larger than the sucking power of the sponge. As a tree reaches its peak height, it will soon stop growing in height. And with the main trunk loosing in power, side branches take over, widening the tree.

Flow of sugar

Apart from evaporating water, leaves also use water to perform photosynthesis. Which brings us to the flow of sugar. Absorbing the energy from the sun, leaves take water and carbon dioxide and bind them to form sugar molecules. The entire tree uses these sugars as a store of energy. The flow of sugar takes place in the living tissue of branches – the thin layer just below the bark. Leaves continue to fill up this store, and when a connected cell is in need of energy, it takes it out. Roots, trunk, branches and buds are all connected.

When a cell needs energy, it takes the sugar and some oxygen from the air and lets them react. That’s right, trees breathe! As the molecules fall apart, they release the energy it took to first bind them together. Our own breathing has the same effect. However, only plants can create their own sugars – we need food to keep us up and running, as we need much more energy to walk around.

Flow of hormones

Both the flow of sugars and the flow of water are also used to distribute hormones. Take the apical meristems – the tips of branches that grow new cells – which produce a hormone called Auxin that regulates growth. This hormones tells branches lower down the branch to lay low.

A tree creating much of this hormone will give precedence to its main branches, yielding tall, pointy trees. Botanists call this effect Apical Dominance. Low Auxin means equal growth in every branch, yielding round, bushy shapes.

Twigs

Now we know how a tree regulates its growth on a grand scale. Next we’ll take a look at twigs. Although twigs do take their orders from the hormones flowing through the plumbing system, they are very individualistic plants. The life of a twig starts in a bud.

At the end of each branch, and at the base of each leaf, grows a bud. It contains the embryonic beginnings of a twig, encapsulated in a protective cocoon that helps it survive the upcoming winter. Early next year, the protective bud scales open like a flower and the new twig shoots out.

Senses

Every year, a huge number of twigs are grown from these buds. Twigs are the eyes and ears of the tree. Well, trees can’t hear – but other than that, they have just as many senses as animals. Twigs determine where to grow depending on these senses.

A tree has the sense of balance which it uses to grow contrary to gravity. Biologists call this gravitropism. It makes a tree grow up, and its roots grow down.

Light is of the utmost importance to trees as it is their food. So it’s not surprising to see that trees have developed far superior senses for light. A twig senses many hues of light. When a twig sees too little light, it starts vigorous growth in the hope to find it – just like when a seed first germinates below ground. This also occurs in twigs that are shaded – these twigs will grow longer, but much weaker, making them likely to break in autumn storms. This natural pruning evolves the tree into an airy and efficient structure.

Contrary to popular belief, trees do not grow toward sunlight. It is way too strong for them and they much prefer the blue hue of the overall sky. That’s what makes them grow into a balanced shape.

A twig knows whether it’s day or night, how long the day is, and whether the days are getting shorter or longer. The latter determines when to open their buds, when to flower, and when to drop their leaves in fall. To illustrate the individualism of a twig as a separate plant, look at a tree through the seasons. In spring, buds with the longest exposure to light open up first. Sunny buds are the ones to flower. It’s not uncommon to see a tree that is flowering on just one side. Most obvious is the transition to fall, where twig after twig turns yellow. Instead of an overall effect, it’s every twig for itself.

Turning to wood

During winter, twigs become woody and do not increase in length after that. The only living cells in this branch grow in a thin layer just below the bark. This layer grows out, each summer thickening the branch and adding rings. It is important to know that internodes only grow in length the year they came from a bud. After that, the dead wood within fixes its length.

Learn more

Without a doubt, the most popular mystery in art and nature is the golden section. This “divine” proportion seems to pop up in everything from sea shells to roman architecture. Trees are no different, they too show this magical proportion. It lets trees grow branches in just the right angle. Learn about the golden angle in trees.