oak
栢 centers on the oak tree, a type of tree with strong wood.
栢 combines 木 (tree) with 百, which likely contributes the sound. The exact historical development is uncertain, but it is used for oak.
A tree 木 with a hundred 百 acorns: that's an oak.
For ハク, imagine a hundred acorns being hacked open: hack -> ハク, and the oak tree drops them.