alright, I'm using the world gen, allowing elevation between 1 and 400, quite high differences allowed, further no actual requirements, everything free, reasonable settings
My last 20 attempts ended with a pretty much shrublands only, therfore no dwarf civ to start from in DF
erosion too high, too much metals in the ground killing the trees? what am i missing. I'm too lazy to post the exact settings, and i did ask for the log of "problems", but nothing so far
previous attempts with almost the same settings had a good result.
I'd say yes, erosion too high, if you suspect that might be the case. High erosion can make your mountains wear away into nothingness even as you watch. Metals in the ground can't kill the trees, as the amount of soil placed is related to the biome the game generates for each area, but somehow I doubt you were serious about that one.
Try lowering the erosion. Then, if that doesn't work, ramp up the mountains just a bit (be careful about this one, it can lead to rejections of death). I would suggest setting the elevation mesh size to 2x2 (or higher if you want blockier, less random mountains), then setting the following values to 1,1,1,1,3 or something similar - it's the last number in the sequence you want to raise. This will make the generator more likely to place higher terrain.
If that doesn't work, it's also possible you have the XY variance for elevation too low. Raising it a few hundred (or less, if you're generating a smaller world) might help.
If all else fails, there's always
magmawiki.