Top tree of the triple stack - the indexing radical. Sits centred above the two lower trees, slightly compressed to fit the narrow top slot. One tree is 木 (a tree), two stacked side by side make 林 (a grove), three together make 森 (thick forest).
Lower-left tree - the right diagonal is replaced by a dot so it can sit beside its twin. Together with the lower-right 木, it forms the second tier of trees that turns 林 grove into 森 thick forest.
Lower-right tree - completes the triple. Three trees together visualize a dense, gloomy primeval forest. The semantic extension to 阴森 eerie/gloomy comes directly from the darkness inside such a thicket where sunlight cannot reach.
