A realistic tree is about 10,000 polygons
have you tried this? : use a very very hi res tree...Xfrog or arbaro(free), ..then , do a render rotating camera around it...like 16 angles minmal, but th emore the merrier....I have never tried this way, as last time I did some low pol plant or tree, was -really- low pol, and only made two orthogonal planes....the idea is you first model a realistic trunk and main thick branhces (through its extruding power, that's seconds in Wings3d) as no fake thingie. Then, with the several renders you have (put light before render simply exactly above the tree,(at a distance) to avoid possible probs) , and make planes (quads) each on the corresponding camera angle.This is pretty easy in Blender, for example. So yo go making your planes, just rotating th ecamera (in blender, you can set pivot of rotatinon to cursor, rotate with z axe, and have a viewport making the rotation numerically, of the camera) . So you make like your 16 planes or more, forming like a "star" if seen from above.(almost filling a cilindric volume) In the middle, you already had your tree trunk. You surely'd better had added plannar UVs to your SINGLE faced plane. Which you cloned for the rotations. Asign as texture, each of the renders to each plane. Following of course, same order (sorry, i meant the rotation of camera etc, must be same when you do the low pol and hi res version thing, indeed, can just use same camera moments to do the stuff) that you did for renders of the hi res version. One important matter, is...the renders of the hi res, if using blender, you need to render as RBGA, for example, PNGs, as this way, you have transparency in all which is not leaves or branches. (don't put more extra meshes) .This way, once you play the renders as textures, you can later on make in Irrlicht those planes as alpha mapped, and the idea is do them double facing renders (double facing in Blender, as would show normals probs) , but do in irrlicht.I dunno if possible in irrlicht, I've only done in other engines. (the art side, of course.)
This way, and unless you get quite close...I....suespect....is gonna look quite realistic....Dunno if some one here has tried this already.
BTW, for hi res trees, and having xml templates of REAL trees, and free, open source, multiplatform, go here, is a quite nice software, it needed some trick, but i used it quite succesfully ... :
http://arbaro.sourceforge.net/
that would be a great shame!
So I will have to think of something else for the trees (or less realistic) (Klasker tree are working but I've not yet tested them)