Wouldnt you prefer to see them all in a main game? Hopefully you'l be able to see the trees in a demo. 
If a picture says a thousand words, a 3D demo says at least a thousand more... it's not so hard to setup / beats an image... especially for original stuff.
Those trees have around 500 vertices each. I dont think its possible to go lower than that, certainly if you want to make a nice looking tree.
It's hard to tell exactly what you've got going. But usually what helps in the bushy areas is to have a few different planes intersecting. For twigs anyway. For leaves something with volume usually looks better with a free roaming camera.
The amount of polygons is really not super important... what's most important what you do with them. The trees we have are really not efficient either in terms of conveying a tree or geometry... so it's good to see you're doing your own and they seem to be on the right track.
If you want sparse trees it's definitely important to leverage transparent texels to convey detail.
PS: Som actually has a (not well understood) 3D billboarding system. You can really make things look nice that way, but it can seem kind of unnatural if the tree is not too amorphous. Another thing you could do with it (once we figure it out someday) is create the illusion of lots of trees in the distance or on top of a cliff, pretty much anywhere the player can't actually go and can only see from pretty much one angle.
