This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The historical origin of 樷 is uncertain. It visually combines 林 (woods) and 取 (take), but the exact development is unclear.
With no clear meaning, picture a dense grove (林) where something is taken (取) — perhaps a hidden thicket where things are gathered or collected, but treat this as a placeholder image only.
For ソウ, imagine a soul (ソウ) lost in the woods (林) trying to take (取) the right path: soul -> ソウ.