Advance Regeneration

Seedlings of many species germinate beneath a partial or full overstory and exist as suppressed advance regeneration, responding to release when the overstory is removed. For most advance-growth dependent species, seedling-sprouts are the primary reproduction growth form; they arise from the recurrent dieback of shoots of reproduction that originate as seedlings (Johnson 1989).

Advance regeneration is the primary source of regeneration for oak species in the southern Appalachians, particularly red oaks. Other upland oaks typically occur on somewhat poorer quality sites, where mature stands contain a relatively large number of smaller diameter oak stems that sprout vigorously from stumps after a harvest cut. In mature stands on high-quality sites, however, red oak usually occurs as scattered, large individuals that have a low probability of sprouting after cutting. On these sites, therefore, advance red oak reproduction is the primary source of regeneration. Other species that also rely on advance regeneration include: white oak, chestnut oak, black oak, scarlet oak, white ash, cucumbertree, white pine, hickories, basswood, red maple, sugar maple, beech, buckeye, and hemlock. Even more intolerant species, such as yellow-poplar and black cherry, depend to some extent on advance regeneration.

Where advance reproduction is the primary source of reproduction, the number, size, and distribution of stems of advance reproduction collectively determine regeneration potential. The ability of advance-growth seedlings to respond to release depends on their size: larger stems have a greater likelihood of capturing growing space after canopy opening than smaller ones. However, the minimum size necessary varies considerably among species. Black cherry is the best adapted to respond when small (2 to 6 inches); at the other extreme are the oak species, which must be at least 4.5 feet tall to respond well to an overstory release. Therefore, manipulating mature stands to enhance the survival and growth of advance reproduction prior to overstory removal is crucial to the success of oak regeneration (Loftis 1990). This concept is integral to oak <shelterwood methods> (Kelty 1988)

The relative ability to survive in the understory and respond to release also varies among species. Advance seedlings of most species are present at some time, even in stands with relatively dense overstories. Intolerants such as yellow-poplar and cherry germinate, live for several years, then die. Intermediates such as the oaks, hickories, and ash may survive for a decade or more but grow very little beneath fully stocked stands (Beck 1970, Loftis 1983). Only the most tolerant species are likely to grow to any size in the understory without substantial overstory disturbance. (Kelty 1988)

Because advance regeneration plays such an important role in hardwood forests, some suggest thinking of regeneration guilds based upon their need to develop as advance growth and their ability as advance-growth seedlings to respond to release from overstory shading. This continuum would range from progression/pioneer species to very tolerant species

1. Pioneer species that can become established in open conditions, not requiring development as advance growth at all (e.g., yellow-poplar).
2. Species that can respond to release as fairly small seedlings, or on some sites become established in open conditions (e.g. black cherry, white ash).
3. Species that rely almost entirely on developing as advance regeneration and must achieve a large seedling size to respond to release (e.g., oak species).
4. Very tolerant species that must reach sapling size or larger before release, if they are to reach the overstory of the new stand (e.g., sugar maple, hemlock).

This scheme is only slightly different from Keltys division of species into three regeneration quilds. Placing species along this progression can be helpful in organizing thoughts about species responses to various silvicultural treatments; however, there are enough complexities in reproductive characteristics that it is important to know the details for each species and not depend too heavily upon such generalizations.