How can a tree trunk get thicker each year?

Have you ever hugged a big tree in the park? Press your hands on its bark. It feels rough and bumpy under your fingers.

A young tree has a trunk as thin as your arm. A very old tree has a trunk so wide that your arms cannot reach around it. So how can a tree trunk get thicker each year?

The answer hides just under the bark. There, a thin, soft, wet layer wraps all the way around the trunk. We call it the growing layer, which means the part of the trunk that makes new wood.

In spring, the leaves open and the tree starts to grow. The growing layer gets busy too. It makes a thin new sheet of wood around the old wood inside.

Why does the tree need this new wood? The new wood is full of tiny tubes. They carry water from the roots up to the thirsty leaves.

The old wood does not go away. It stays deep inside and holds the tree up. So the new sheet wraps around it, and the trunk grows a little wider.

Now the bark feels tight, like a jacket that is too small. The bark stretches and cracks into those bumpy lines you felt.

When cold winter comes, the tree rests. The growing layer stops making wood and waits quietly. What do you think the growing layer will do when spring comes back?

When spring comes back, the growing layer wakes up. It makes another sheet of wood, right on top of last year's sheet. Year after year, the sheets pile up, and the trunk gets thicker and thicker.

Each year's sheet makes one new circle of wood. We call it a tree ring, which means one year's layer of wood in the trunk. More tree rings make a thicker trunk.

Growing layer and tree ring. Each year, the growing layer under the bark wraps a new tree ring around the trunk.

Next time you hug a big tree, remember the tree rings hiding quietly inside it.