According to Wikipedia:

The bark dies annually and species can be roughly grouped based on its appearance. In smooth-barked trees most of the bark is shed, leaving a smooth surface that is often colourfully mottled. With rough-barked trees the dead bark persists on the tree and dries out. Many trees, however, have smooth bark at the top but rough bark on the trunk or its bottom. The types of rough bark is often used to broadly label a group of eucalypts.