Why does birch bark feel smooth and pine bark rough?

Have you ever touched a tree on a walk? A birch tree has white bark that feels smooth under your fingers. A pine tree has brown bark that feels bumpy and rough.

Both trees wear bark, like a coat around the trunk. So why does birch bark feel smooth, and pine bark feel rough? Let's find out together.

A tree's trunk grows a bit fatter every year. The tree needs a thicker trunk to hold up its growing branches. Fresh new bark grows on the inside, right next to the wood.

The old bark sits on the outside. As the trunk grows, it pushes on that old bark. It is like your body pushing on a shirt that is getting too small.

A birch tree's bark grows in layers, which are thin sheets lying one on top of another. Each layer is as thin as paper. You can picture the pages of a book, pressed together.

When the trunk pushes, a birch's thin layers stretch and grow wider along with it. The oldest outside layers peel off in curly paper strips. So the birch trunk stays smooth, with no deep cracks.

A pine tree does something else. Its old bark is thick and stiff, and it cannot stretch. What do you think happens to that thick bark when the trunk pushes on it?

It cracks open! Deep cracks split the old bark into plates, which are thick, flat chunks with gaps between them. The plates stay stuck to the tree and hold on tight.

Each year the trunk grows fatter, so the cracks grow deeper and the plates grow thicker. Your fingers bump over the plates and dip into the cracks. That is why pine bark feels rough.

A growing trunk pushes on its old bark. Thin birch layers stretch and stay smooth, but thick pine bark cracks into rough plates.

In the forest, a white birch stands smooth and still, with curly paper strips at its feet. Beside it, a brown pine holds its rough plates tight.