I recently planted several saplings in my yard. It may be too soon to tell, but i have observed that the plants that i put directly on my hugel berm are not budding, while some (not all) of the others (placed directly in ground) are budding.

I also noticed that one of the saplings was falling into an air void in the mound. This one was replanted in more solid ground near the hugel.

This led me to wonder about potential problems caused by air pruning. I know that instructions that sometimes come with transplants often include excessive watering to remove air pockets to avoid such issues... Has anyone experienced problems with trees being planted directly on a hugel<whatever>?

Really good question. I think the air pockets around the roots would be the problem. Getting all the air pockets out of a hugel would be difficult though. If you find any concrete answers please let us know.

Perhaps I'm asking the question wrong... Let's try it from the other perspective.

Has anyone has SUCCESS with planting trees directly on a hugel<whatever> that was known to contain significant air pockets?

I've been told to keep trees off my hugel bed, under no circumstances to plant them on top of or in the hugelkultur. From what you are saying it may be correct. I put two long Hugel beds on contour, and don't plan on planting trees, but if I do plant them someday I would do it below the bed, similar to planting trees below a swale instead of on top of a swale. I have very sandy soil, so I want water to stick around as long as possible. Good luck!

In thinking about this it would seem to me that planting any trees on a newer Hugel would be bad for a few reasons. Potential air pockets is one, but it is also likely that in time, the hugel will settle as the wood decays. This could be unsettling (couldn't resist) for any trees. Perhaps it's best to plant any trees alongside or in front/back of the hugel?
An exception might be a well established hugel that has settled out.

Another issue you might consider with planting directly on the hugel (which I've also been told never to do with trees) is the rapid cooling they can experience in the winter months. Being exposed to the prevailing winds and having a relatively small contact area with the more temperature stable earth below can create rapid cooling (as well as heating) when the seasons change. This phenomenon is partly responsible for the self tilling nature of these beds, but could prove troublesome for some of the more tender/marginal for your conditions tree specis. Just another thought on what may be going on, along with the air pocket prroblem.

I agree that planting trees on a hugel bed may not be ideal. I do think that planting trees on the lower slope at or near finish grade has merits.
Lots of nutrients, water and some buffering to be had at the bottom of a slope.