Tree shape

[ExpectedIdentifier]

I think the best way to make low poly trees that are sub 1000 polys is to make a cylinder for the trunk, say 8 sides, and just use intersecting planes with an alpha texture for the branches. In Photoshop its just a black background, and the branch is selected and made into an alpha channel, and saved out as a DXT5, then I just use the same file in the alpha slot in the material editor. As soon as you start to make the branches in 3D, the poly count will skyrocket if you end up modelling a lot of them. Here's a tree I made, its a bit crap but its poly count is only 300, you could probably do a lot better with careful placement of the planes. It would be a fair bit less without the broken twigs near the bottom too.

Adding the leaves is a similar process right?

Yes. If you want leaves I would say use a diffuse texture that has branches and leaves together. I would recommend something similar to what Jonny said, but model some of the branches in 3D, just keep them as low poly as possible and only do this for larger branches, then use planes for smaller branches coming off of them and for leaves. Also don't forget to duplicate all the planes and invert the normals. If you don't do this you will be able to see through the back of the plane and it won't look right (I guess this depends on the angle of your planes though).