This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development is uncertain. The modern form combines 木 (tree) with 叢 (cluster, thicket), suggesting a dense growth of trees.
A tree 木 beside a dense cluster 叢 suggests a thicket or grove, but the exact meaning is unclear.
For ソウ, imagine a dense grove of trees where the wind makes a soft 'sou' sound as it blows through the leaves: sou -> ソウ.