well you have the option of using paint FX in maya, it's comes with maya unlimited...
and as a starting point there are preset "brushes" as they call them, which has default trees, clouds, plants, etc etc...
anyways, that's an option, another option is to model about 4-5 different kinda of trees, and texture them, up and then import them into your scene, but paint fx handles large forests relatively cheap (cheap as in processing power).
check the help files to see how to use these, etc...
if you need some more info, drop me a line, i'm busy tonite (valentines day

) but tomorrow or later this week i can show you some stuff if you need.
peace!