What is the grammatical role of "rich" in the following sentence and what does mean it? (also why it uses the comma?)
In the Pacific Northwest, the high level of precipitation ensures rich, green plant life.
"Rich" is an adjective. It modifies "plant life."
There is a comma between "rich" and "green" because both adjectives modify "plant life" and we separate consecutive adjectives with commas.
The meaning of "rich" there is
Existing in plentiful quantities; abundant.
It is an adjective, part of the noun phrase "rich, green plant life".
The comma is optional in a string of adjectives like this. When I read it aloud, because of the comma I pause briefly, and begin a new prosodic contour for "green plant life", which has the effect of putting more emphasis on "rich" than if I read it through as "rich green plant life".