Little question, no little answer. That comes into the artistry of each and every forest, even "yours". Certain forests by design have some bases of treetrunks touching (thus the roots were crowded to each other). Depends on how many trees, how large the trees, the kind of trees, the shape of the trees, etc., etc. Strive for "natures" positioning which tends to be random (no rows, no equal spacing, not the same height, shaped according to conditions where growing, etc., esthetics tends to say odd numbered amounts, just simply do what nature does) but then the trees in nature grow according to their environment. If close together and crowded there will be some stunted growth, branches reaching out beyond/around others, without crowding trees will tend to be fairly similar on both sides, (following the unimpeeded form of that species of tree) etc., etc. To say nothing of what size the base/pot should it be? "You" are the artist, how many trees will "you" use, and what..., and...? Hopefully this is vaguely specific enough to be of help. May help to google gobs and gobs of bonsai forest images (keeping "gobs" out of the search). Late last fall, I assisted ( I drove) a friend in harvesting some wild oaks nearby but on the way back I noticed (while also keeping my eyes on the road) many mature trees in their natural settings. It was dusk and the trees were backlit by the setting sun, which lit them up in dark contrast and very vividly. At the time I wished I had a camera to record exactly what I was seeing and the obvious ways certain branches grew according to their position, or confinement, or lack of confinment on each tree. Oh, and do post pics of the forest "you" decide on because there's some bnutters that will critique the crap out of it for you.
I pruned out the worst of the burned leaves and getting some regrowth further inside the trees. 
More cuttings started.
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