fir
椴 refers to a fir tree, a type of conifer. The meaning is specific to this tree species.
椴 combines the tree radical 木 with the phonetic component 段, which likely indicates the sound. The character was created to name the fir tree.
A tree 木 with the component 段: picture a fir tree whose wood is cut into steps or tiers, like the shape of 段.
For タン, imagine a fir tree with a tan-colored trunk: tan -> タン, and the tree is a fir.