Simplify the following text for a Grade 9 native English-speaking person.
Original: The trunk is covered by the bark , which is an important diagnostic feature in tree identification , and which often differs markedly from the bottom of the trunk to the top , depending on the species .
Simplified: The trunk is covered by the bark , which protects the tree against damage and often differs markedly from the bottom of the trunk to the top , depending on the species .