tree
棪 refers to a type of tree. The given meaning is simply 'tree', so the kanji's core idea is a specific kind of tree, though its exact identity is not detailed in the source.
棪 combines 木 (tree) with 炎, which likely contributes a phonetic element. The exact historical development is uncertain, but it is a phono-semantic compound referring to a type of tree.
A tree (木) that looks like it's on fire (炎) — imagine a tree with flame-colored leaves or blossoms, making it a distinctive type of tree.
For エン, picture a tree with fiery red leaves that looks like it's in an 'en' (エン) during autumn, like the 'en' in 'enflamed'.