There have also been long discussions on the MBTR Trail Building and Advocacy forum about this. The consensus was, it depends
I think the arguments can be summarized as:
- Over time, decomposing leaves do add bulk to soil, gradually building up over decades to significant levels. If you clear the trails, that bulk will not be added to the trail, eventually resulting in sunken trail bed as the surrounding topsoil builds in depth.
- Removing leaves helps the trail dry faster, resulting in less erosion and soil displacement in traffic.
- Decomposed leafy matter is light and dusty, and wind and traffic just blows it off a packed trail bed anyway.
- A cleared, defined path helps keep singletrack tight
There also seemed to be a pretty strong soil and geographic dimension to this... in sandy Florida soil, the organic matter was key in keeping the trail dense and solid, but in clay soils leaf covers meant that any rain kept the trail wet for days, and the resulting traffic impact far outweighed any loss in soil.
I don't think we've done any long term tests here in SE Michigan. I suspect that for places with very little traffic density we'd get one answer, and for places with high usage and traffic we'd get another.