It really depends on how you build your hugels. I'm very careful to fill in all the gaps in mine so they don't settle as much. But I still don't plant trees on top of them. Though you can plant trees along the base of the north side (or south side in southern hemisphere). That gives them a really nice sheltered environment. When I did this along mine I was getting 3 to 5 feet of growth in the first year and they're still growing strong several years later.
I do plant shrubs on hugels. They seem to do fine overall and don't seem to have issues with the beds settling. Though if you don't fill in the gaps the hugel will settle more and some shrubs may not like that.
I also like to plant fast growing nitrogen fixing plants like lupines to help hold everything together as the shrubs get established. Lupines also get nice taproots.
I wrote a blog post a while back about how I create hedgerows.
https://www.wildhomesteading.com/start-a-hedgerow/
It doesn't have to be a hugel bed but I used the methods covered in that post on a couple hundred feet of hugel beds. And the pictures in that post are mostly of my hugel hedgerows.
Hope that helps!
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