This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 桼 is uncertain. It visually combines 木 (tree) and 水 (water), but the original meaning and component relationship are unclear.
The shape shows a tree 木 over water 水, but the meaning is unclear. Think of it as a mysterious kanji whose meaning is lost.
For シツ, imagine a sheet of paper floating on the water under the tree: sheet -> シツ, but the meaning remains a mystery.