The bark dies every year. In smooth-barked trees most of the bark falls off the tree, leaving a smooth surface that is often colourfully marked. With rough-barked trees the dead bark stays on the tree and dries out. Many kinds of eucalyptus trees have smooth bark at the top but rough bark lower down. These different kinds of bark are used to arrange the eucalypts into groups – this is useful because there are hundreds of kinds of eucalypts, and breaking down the group in this way makes the group easier to understand.