I have been working with an experienced permaculture practitioner in Denver. He is building hugel sheet mulches, two feet deep, and planting trees in them. First a layer of manure about half an inch think goes down. Then a layer of cardboard, not overlapped. Then newspaper covering the seams and well soaked down. Then another layer of cardboard, and another layer of news paper. (The bindweed here is fierce! If cardboard is overlapped, it grows right through.) Over this goes about two inches of wood chips, and then several inches of manure. Then comes a layer of logs, cottonwood and Siberian elm. The gaps in this layer are covered with a thick layer of smaller sticks and twigs, stamped down. Then comes another layer of wood chips, more manure, then more wood chips again, and then a final layer of manure. Then stuff is planted, including trees, right into the mulch. He thinks he will be able to go thirty years without adding any more fertility to the bed beyond nitrogen fixing plants and dynamic accumulators. Very little water is needed in these kind of beds, even in Denver.
However, I am not sure how long his oldest tree has been growing in these. I will have to ask him.