Comparing to 林, 森 have more trees than 林, and as mentioned before, number three in Chinese language means a lot. 森, which is three 木, means lots of trees.
So 森 means Forest.
In the Forest, all plants are very closely compacted, thus 森 is extended to mean Dense.
As Forest also appears depressing or frightening, thus 森 is also extended to mean Gloomy.
In our spoken Chinese language, we normally use a phrase 森林 to mean forest. It is because while we speak to each other, we would like to add extra characters with similar meanings so that even if we did not hear part of the sentence clearly, with redundant information, we could still understand each other correctly.
Now let us check 森's evolution history: (image get from http://www.vividict.com)
|Learn one Chinese Character a day - 森|
Now let us enjoy a Chinese calligraphy with 森 inside: