Thanks for your comments, jfrazierjr.

The two textures I used for the ground are just two of Xtreme's standard textures. I didn't take the time to find anything better, as I focused on developing drawing techniques for the buildings and trees.

Adding shading to the trees is certainly possible, but would be a bit problematic. I could do it the same way as I did the buildings, but that would mean placing the trees by hand.
Another way is to create two brushes, one for the trees' shapes and colours, and one for the shading. The settings of the randomization of spacing, scaling, rotation and offset have to be exaclty the same in both brushes, which is problematic in Xtreme, as you cannot enter a seed number (just press the Randomize button), and most other settings are done with horribly inaccurate sliders. But it can be done. This would result in a much more complex cleaning-up phase, as I would have to move or delete twice the number of objects, and most of these will be difficult to select due to the fact that a lot of trees overlap.

I really hope Xtreme's developers will add support for shadows and live effects in brushes, so it could all be combined in one brush, but I doubt that is going to happen anytime soon.