Leaf mold has not been given the amount of study and discussion that I think it deserves, so I enthusiastically welcome your questions. I don't have an
answer here, but I can offer my observations.
Let me first identify my experimental variables.
I have not done much with
leaf mold before 2010.
I'm in northern Florida, USDA Zone 8b/9a, depending on which map you look at. I get a few frosts each year, maybe a couple weeks of daytime highs in the 40s.
Average annual rainfall here is 54", more in the summer, quite dry in the winter. This area has been in some level of
drought ranging from D1 to D3 from August '10 until earlier this year. I have added copious amounts of
water to a particular heap which was close to the hose, but I don't bother adding water anymore. With daytime temps in the 90s for several months, there is no way I can keep a heap moist. I let the rain do the job.
My soil is over 98% sand.
Most of the
trees around here are water oak, a couple of small pecans, a few laurel, 2 southern yellow pines, 3 loquat ( I leave these leaves in place), a couple of magnolia over the
fence, a couple of pignut hickory, 2 tiny
apple trees, and a lone lime that has yet to make it through the winter but keeps growing back despite my neglect. The preponderance of the leaves I work with are water oak.
I think results are highly dependent on the environment in which the leaves decay.
I've got several heaps of leaves around here of various ages.
When the leaves have been recently heaped, there are not many worms, bugs, or even fungi. It takes a while for this stuff to get going.
After a few months, when the weather is warm, I have examined a heap to find it thriving with critters, including worms. Mind you, 2 years of drought had decreased the worm population considerably. I am still hard pressed to find worms. I can find them in the middle aged heaps here and there when the heaps are moist.
In the older heaps, the once thriving critters are indeed fewer, including the worms.
As for the ground on the forested side of the back field, I have made no observations.
In areas of the garden where I have added leaves and leaf mold, I do find some worms. Not an abundance, but one here and there. I don't do much digging out there. I smother with leaves, let it do its thing, then start building
compost where the beds will be. Any digging is done when I toss the compost with a bit of soil and when I set in plants. I'm not actively searching for worms. I'll strive to get out tomorrow to dig around, see what I find.
You have observed no worms in a mature woodland duff. What we need is more observations, more data points to come up with a pattern. Along with observations, it would be handy to pay attention to other details in order to come up with an explanation.
It is my understanding that worms
feed on bacteria, which is why they thrive in composting environments and environments rich in bacterial decay. Leaf mold is produced primarily through fungal decomposition. There would be some bacterial activity, especially at the start. Leaves do have some nutrients when freshly fallen. Gathering them up can bring in some green material/weeds/grass, but not so much, I think, to maintain prolonged bacterial activity. I think a leaf heap would be attractive to worms for a limited period, after which they die off or move on.
Leaves tend to be acidic. While it is true that whatever passes through a worm tends to come out the back closer to pH neutral, once the bacterial phase winds down, the pH of the decaying leaves will stay more acidic, which may be unappealing to worms.
Leaf mold can be produced in low oxygen conditions. Leaf mold in a forest floor would not be turned and aerated. The worms need oxygen. Such an environment would not help them out. Worms dig plenty of tunnels, which greatly improves aeration, but decaying leaves do not maintain structural support which would result in destruction these tunnels. Then again, the ground is spongy and the mycellium gives the soil plenty of structure.
Near the edge of a woodland, there would be more diverse plant waste on the ground, but deep into the woods past the edge, the floor litter would be leaf dominant. This says less diet diversity. I'd think at least some worms would be around, but not as many a near an edge.
How much leaf mold is too much? I've got a pile of compost out there with weeds growing all over. I've got a pile of leaf mold that is ready for use that only has a couple of vines. In the
thread Building soil: what to do with leaf mulch I talked about how much leaf mold to add, offering 10% leafmold as still improving soil. The percentage of leaf mold in the soil of a forest floor can be significantly higher.
Hmmm...
Science lives on data, but is driven by hunches. I have a hunch you are right in that very few worms live in a mature duff.
All we gotta figure out is the Why.
CALLING ALL PERMIES!
Give your woodland floors a good look. Where do you find worms, what are the environmental conditions, and specifically, do you find worms in forest floors with abundant leaf mold?
As for using it as a soil conditioner, I strongly support this notion.
In my garden area I have 4'x50 areas to which the only thing I have done is cover them with 6 inches of leaves. These areas have grown up with grass and weeds, as have areas that have been left untouched. The amended areas show more growth, faster growth (I mow the whole place), and a noticable difference in color. The weeds have more stems, leaves, and blossoms. The grass is a darker green, with a clear line of demarcation. You can pick out the treated areas at a glance from 100 feet away.
If you are hesitant, consider experimenting with a small area, perhaps 4'x4' to see what happens.
I would love to hear your results.