maple tree, colorful autumn foliage
栴 centers on the maple tree and its most celebrated feature: the brilliant, colorful autumn foliage it produces.
栴 combines 木 (tree) with 丹, which likely contributes the sound and may relate to the red color of maple leaves in autumn.
A tree 木 with a red 丹 component: picture a maple tree whose leaves turn a vivid red in autumn, making the whole tree look like it's glowing with color.
For セイ, imagine a serene maple tree whose leaves say 'say' as they rustle in the wind: say -> セイ, and the tree shows off its colorful foliage.