beech tree
橅 centers on the beech tree, a specific type of tree. The meaning is narrow and concrete, referring directly to the tree itself.
橅 combines 木 (tree) with 無, which likely contributes the sound. The character was created to represent the beech tree specifically.
The tree radical 木 stands beside 無, which looks like a complex, spreading shape. Picture a beech tree with a broad, intricate canopy: 木 + 無 = beech tree.
For ボ, imagine a bow made from beech wood: bow -> ボ, and the tree provides the strong, flexible wood.