It affects seedling recruitment for sure. Some species love to start beneath leaf mulch and there is probably something out there that would colonize your leaf field hard and fast if it were given the chance. I collect red alder seedlings in timber clearcutting areas, and often find that they've had to twist/push their way through 6-12" of slash (clearcutting debris) to reach light and finally grow upwards. Quite a few conifers can push upward through many inches of needle duff as well.