Tim, correct me if you think I'm wrong please, I looked up the information for a loquat tree on
PFAF, and it says a loquat prefers well-draining soils and needs warmth to produce fruit. Because of this, I think planting the loquat on the slope of berm facing the sun would be the best place; the sun-facing side will absorb the heat- storing it for later- and the gradient of the slope will help will draining water. I do not think placing it near the valley would be a good idea because moist cold air would flow down the hill, unless stones are on that side to store and release heat.
Christine, the general idea, I think, for a hugel bed is to store water for the duration of the wood being there, and after it has decomposed, you have soil. How well the hugel holds its shape probably depends on the roots of the plants living there and how the soil life is there. I looked up the
Japanese Maple, and I think it would do well on the slope right before the valley because there is more moisture there. I guess you could keep adding stuff to the hugel bed to keep enriching the soil; it isn't really a straight
answer, but there is nothing wrong with not adding stuff to it because the plants produce their own mulch and should be able to manage just fine without you. Though, I'm sure they would appreciate anything you give them.