type of oak
柞 refers to a specific type of oak tree. Its meaning is narrow and botanical, without broader abstract extensions.
柞 combines 木 (tree) with 乍, which likely contributes a phonetic element. The character is used for a type of oak, but the exact historical development of the phonetic choice is uncertain.
The tree radical 木 beside 乍 suggests a specific tree. Picture an oak with a distinctive shape that looks like the strokes of 乍 growing from its trunk.
For サク, imagine an oak tree with a sack hanging from a branch: sack -> サク, and the sack is tied to the oak.
Chinese oak silkmoth (Antheraea pernyi); Chinese oak silkworm; Chinese oak tussar moth