Leaves Instead of Wood?

Like the title says, could one subsitute leaves for the wood normally used in a hugel?

I wouldn't think so.
Although they are both slow to break down, I would think that if you piled a bunch of dirt/soil on top of a pile of leaves, you would end up with an anaerobic pile. No space for oxygen, no place for water. Oxygen and water need to occupy approximately 25% of a soil's structure for the soil to be alive.

As John says, you have to make sure it doesn't get anaerobic, which can happen when lots of wet leaves stick together.

What I do is to use my bagging lawn mower as a leaf collection device, and in the process it sucks up twigs, small sticks, acorns, sand and dirt, and other things that act as spacers that keep leaves from sticking together when wet. It also shreds the leaves to a smaller size, and when I empty the bag on the hugelkultur I am building, it's excellent soil building material.

I suspected that lack of oxygen might be an issue.
Thank you both for your replies.

Part of the point of a hugel is water/nutrient retention. The mechanism of water retention is the structure of the rotting wood(micro pores and channels), as well as woody fungal networks, acting as a sponge. Since leaves are more like other green matter, it would be closer to a compost pile instead of a hugel. It may be fertile, but it won't be hugelly.