To achieve these pockets of forest within a largely desert world you need to use a grids and a bit of variance. The main influences being rainfall and drainage grids. Since all desert creatures can spawn in any of the desert biomes, drainage is limited to prevent the other less desirable desert biomes and improve sand desert tile numbers. On the flip side rainfall in amounts enough to combo with proper drainage values to create forest is harder to achieve statistically. All this greatly decreases the amount of desert tiles.
Still there is a large amount of variation in the amount and way that forests are gen'd. Both grids essentially have to align by chance as well as the other various small influences on forest conditions. This being the case minimum initial tile restrictions are placed to allow for enough forest tiles in various places to spawn elves. Most of the time this will produce one or two large forest regions which don't provide enough dispersion of elven civs to really be effective. Restricting by initial forest regions slows down the filtering of worlds during gen greatly and seems to not be working correctly.
To gen this param set effectively in decent time a PC with a very good single core speed is needed to plow through rejections. Because of the minimum tile restrictions you will get thousands of rejections. Sometimes it'll gen on the first button press, other times I've seen it rejects hundreds of thousands of worlds. I use
AutoHotkey with a
repeating C key press to deal with the world rejection popups. With this function one can let it run with the game window in focus while watch a movie or something in the background on a separate monitor. Script could be improved as well but it is functional.
F7::
Loop {
Send {c}
sleep 2500
}
You could, of course, simply increase the amount of drainage, rainfall, and grid dispersion. This is just the balance and method I've found to produce worlds aesthetically and functionally pleasing.