Speedtree seems to have some very high polycounts then.
Another option is to use trees designed by an artist so they are very low in polycounts. You could alto try out ForesterPro(google that and see some awesome stuff) I am using that software as well.
The trees made with ForesterPro are at most 6000 Faces and 3-4000 verts at high resolution. You can get them down to 10 procent of that. I still remember me making my first forest ever with high resolution trees and seeing my machine running out of memory and the game engine crash Ah that was some fun times testing out the max limits of the stuff
If you then use some more awesome texture on those trees from like game texture then you have some really good looking low poly trees that wont eat up all your machines memory.
Also use some fog, make the view distance smaller etc.
Also I never make that big worlds/levels unless they are split up in zones and levels(use to do so and testing them out in other game engines). By doing that I can better control the memory use, easily finish a level one at the time(lots of triggers, events and level design in each area have to be done). Anyway some zoning or streaming has to be done or else no normal machine can run that.
Are you making a MMO or is it a landscape for show?
Wish you a happy new year!