Best way to keep objects from overlapping

So I assume that at the moment you're placing them completely randomly. One thing you could do is something like this:

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That would create them in a semi-randomised way. They would be spaced 50px apart on average, but each individual tree would have 40px of randomised variation within that. Here's what it looked like when I ran it:

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Because it's not entirely randomised, the spacing is likely to look a little artificial (ie. not 'patchy' enough), so you may want to create a few more genuinely randomised trees on top of that. And possibly delete a few random trees too, to create a few gaps.

Another thing you could do would be to find overlapping trees, but instead of destroying them, just re-randomise them. Perhaps run a fastloop 5 times or so where you re-randomise a tree's position and re-check for overlap. If it still overlaps another tree after 5 tries, then destroy it (but hopefully most trees would have randomly found a suitable non-overlapping spot before the 5th try).

Fusion won't progress to the next event until a fastloop is finished. So if in the example I screenshotted above there was an event #4 that played a sound, the sound wouldn't play until all 10,000 loops were finished from events #1-#3 (100 of the first loop multiplied by 100 of the second loop)*. So to answer your question, just do your checking for overlaps in a separate event that comes after your tree-creation fastloops, and it won't run until all the trees have been created.

I'm guessing that randomising X and Y should work ok. Give it a try and see what happens.

* Blimey, 10,000 trees - I think I went a little overboard in that example