oak
檞 centers on a type of oak tree, specifically the kind that produces resin or is used for its wood.
檞 combines 木 (tree) with 解, which likely contributes the sound and may relate to the idea of splitting or loosening, possibly referring to the tree's resin or bark.
The tree radical 木 beside 解 suggests a tree that produces something that can be 'loosened' or extracted, like resin from an oak.
For カイ, imagine a kayak made from the wood of this oak tree: kayak -> カイ, and the tree provides the material.