For other scenes, you want reasonably good trees, but lots of them, and you don't want 5 megabytes and a few hundred objects allocated to each tree. You don't want a hero for the featured close-up, but rather a bunch of cheap extras to fill the background.

This tutorial is on how to make extra trees to fill a background.

In the first illustrated panel (Extra Tree step one), we'll build the trunk and branches with one symmetrical lattice. So select or create a lattice and go into the Terrain Editor. The default grid is 128, but at that setting, your brush is too fat even at its smallest setting, so I always set the grid at 1024 to get the very smallest brush diameter. 

Then I start drawing the branches, dropping down to the trunk. With each new branch I draw, I purposely don't try to follow the exact same line as I go to the trunk, because I want it wider than the single branches. Then, when I have enough branches, I thicken the trunk and lower branch areas.

Set the grid back to 512 or even 256, set the level to lowest, set the brush size to biggest, and the flow to minimal. That done, click on the branches to lower them (but not the trunk) so the branches thin out compared to the trunk in the perspective view lower left of the TE. Simply you don't want the branches as thick (in height, as you see it in the perspective view) as the trunk.

Exit the TE and give your tree a nice tree/bark/wood sort of look.

To make the leaves, duplicate the lattice and go back into the TE, as seen in the second panel (Extra Tree step two). Set the brush to it's smallest size and click to create small circles of foliage around the ends of the branches. This step is the key to having foliage that really grows out of branches, instead of having foliage just hanging out in space unconnected to any branch.

Once you have nice leafy masses, simply erase the tree. Then give your new lattice a nice leafy texture. 

The resulting tree is good in a simple way, but it's completely flat on one axis, and we'd like to have at least some sense of three dimensional structure about it. So, as you see in the third panel (Extra Tree step three), you duplicate both lattices, move them to one side, and then edit each one as you like. I generally erase the trunk from tree number two, so it's just upper branches. Sometimes I change the leaf lattice too.

Then I group them and slide them back over to join the main tree, rotating them 30 degrees or so, resizing them, maybe even flipping them, to create a sense of their being different from the main tree and leaf mass. 

The end result is four lattices, not much data, and trees that can fill a background nicely if you duplicate a bunch of them and scatter them about.

The obvious benefit of this process is that you can control the look of your tree instead of just taking a library tree, and having lots of them without a massive file size.