I have to plant some trees on mounds. Until now I made them huegelcultureish, first dig out 30 cm pop in wooden material until it's a bit  over the ground, grass clippings on the top and then soil and manure and more grass clippings and comfrey leaves.
We build a dry stone wall around, but now we are out of bigger stones.
If I build the same but without stones, does it work? Do I have to shape the thing different? Do I need a bigger diameter then?

I have been building soil mounds on my terraces for my trees.  It works well and keeps the trees from drowning in the winter in my clay soil.

I make them from about 4 to 7 feet wide and a foot to 2 feet or so high depending on the hillside conditions.

I have done others with dry stack rock walls and they also work well.

As long as the mound slopes out properly, that can work fine.

The 'angle of repose' is a useful concept for dealing with piles of soil.  Depending on the soil, a pile with angles of around 30-40 degrees will be stable while it is just a pile of soil. Anything steeper can be inherently unstable.  The critical angle also varies with the moisture content, etc.  If there is vegetation holding the soil, it is more stable.

http://en.wikipedia.org/wiki/Angle_of_repose

i have terraces that are dry stack rock and others that are not. the thing with the ones that dont have any rock or walls. is you just need to get plants growing, the roots hold it together.

That's a good idea, just plant them then it wouldn't look ugly. I like the stones as they warm the soil too, but I always can do them later when I find more stones.

where I live, a lot of people planted a low clumping grass around their oak trees where their lawn grass does not grow. 6" to 9" tall plants. Some of these are now built up over a foot. Going on drive by measurements, but have seen it in multiple yards. Not sure low clumping grass is what you would want to plant around those trees, but something similar might work.