So I decided to dig up soil in them and take a look at what was going on.
Most noticeably was that the plant roots all congregated where clumps of wood chips were buried.
Also very noticeable, there was lots of worms throughout all the soil, soil that had wood chips and soil that didn't. In previous years I had never seen so many worms. I believe having the wood chip mulch encouraged more worms than the leaf and pine needle mulch that I used in previous years. I had wondered if worms would feed on wood chips, it seems that they do.
Here are two clumps from the same bed. On the left no wood chips had been mixed in. On the right lots of wood chips had been buried. Notice the almost total lack of roots on the left, yet these clump were only about 4" away from each other in the ground.
Another soil & wood chip aggregate from another wood chip bed. Notice the darker parts of the soil in this picture. These are worm castings. The soil had lots of worm casting deposits wherever wood chip clumps were, even over a foot deep.
Here I dug up soil from a regular hugel bed (horizontally placed logs). In the spring I did dig in some pine needles & leaves, but they had totally disappeared. The cucumber growing in this bed did ok, but not great. Cucumbers growing in the wood chip bed did great. Cucumbers growing in just a clay bed (no wood chips, no hugel logs) did very poorly (didn't produce any harvest).
There were lots of worms and worm holes in the this soil, but it was still thick clay and no where close to being as good as the soil in the wood chip bed.
It seemed the roots did best in soil that was about one-half wood chips!
Quite a surprise.
As another experiment, I had planted a couple seedlings above some wood-chip-only clumps in the soil that were about 3" deep and 5" diameter. The plant roots did not like growing into only wood chips and these plants didn't do as well.
Seeing how well roots did in "wood chip soil", has given me confidence to really go all out when digging in chips. I had wondered what would be too much. But it seems even 1/2 chips, 1/2 dirt is great for the plants. If I had seen this before I prepared my 4 new hugel beds, I would have dug in even more wood chips. For the 2 existing wood chip beds, I'm digging in a lot more chips now, before planting fava beans.