Justification for Shelterwood Cutting

The shelterwood method appears to be especially well suited for regenerating species that are intermediate in shade tolerance and have slower initial growth. In the southern Appalachians, this fits the description of the oaks. The shelterwood method for oak regeneration is primarily used to establish new seedlings of the desired species and/or to encourage the development of large seedlings from existing smaller ones, so that they will be able to successfully compete with other vegetation when the overstory is removed (Hodges and Janzen 1987, Loftis 1990). In the central hardwood region, shelterwoods work for other species of intermediate tolerance such as ash, basswood, hickory and others whose advance regeneration must be relatively large in order to compete successfully. (Johnson, 1993a)

Where oak regeneration problems occur, the shelterwood method may be the most effective solution, especially in the troublesome mesic ecosystems (Isebrands and Dickson, 1994). In mesic ecosystems, successful application of the method requires coordinating overstory density control with competition control and, in some cases, acorn production. oak regeneration is easier to establish on the poorer upland sites (Carvell and Tryon 1961). (Johnson, 1993a)

Research in the southern Appalachians has demonstrated the potential of the shelterwood method to regenerate northern red oak on highly productive sites (site index 70 to 90 feet at 50 years) (Loftis 1983b, 1990a, 1990b). This research focused on finding a level of disturbance that enhanced growth of red oak seedlings, but also impeded the establishment and growth of yellow-poplar, the primary shade-intolerant competitor.The resulting prescription called for a single shelterwood cut that reduces stand basal area by only 30 to 40% of full stocking. In this initial cut, stand density is thinned from below by removing the subcanopy of tolerant non-oaks using herbicides and leaving the main canopy intact. The shelterwood is then removed about 10 years after the initial cut treatment. Studies in the southern Applachians have shown that if understory competition is not controlled after this initial cutting, sources of competition from red maple stump sprouts and dogwood will displace oak regeneration. These studies have also verified that over a broad range of residual basal areas (25-66 sq ft/ac), species composition of the new age class is a function of the regeneration sources present at the time of the initial cut. (USDA Forest Service 1995)

It is critical to understand that the shelterwood method practiced on non-accumulating oak ecosystems, such as mesic sites in the southern Appalachians, generally does not result in the establishment of new oak seedlings (Janzen and Hodges 1987, Loftis 1990).  Non-accumulating oak ecosystems are those where no significant new oak seedling establishment is occurring over time.  Loftis (1990b) points out the results of several authors (McGee 1975; Beck 1977; Auchmoody et al. 1993) who found that opening up oak stands did not necessarily result in increased acorn production. Oak regeneration takes many years to form, and often develops only in years with (1) a heavy seed crop; (2) low predation by seed-eating insects and animals; (3) seedbeds of only thin leaf litter; (4) favorable weather conditions; and (5) freedom from interference by understory vegetation or faster-growing competing plants. These conclusions seem to be verified by Schuler and Miller (1995) in central West Virginia, where they reported that 10 years after a shelterwood seed cutting, desired regeneration of northern red oak was still inadequate. Thus, Loftis proposed that the objective of the first cuts in an oak shelterwood was as a release cut to enhance already existing oak regeneration, rather than preparatory and seed cuts to create regeneration (Tryon and Carvell 1962). (Johnson, 1993a)

Contrasting experiences in the southern Appalachians and other areas in the central hardwood region emphasize the need for shelterwood prescriptions that are both species- and ecosystem-specific. The minimum duration of the shelterwood period, and the length of time between the seed cut and shelterwood removal, varies among sites. Xeromorphic oaks such as black oak, white oak, scarlet oak, and chestnut oak growing in xeric ecosystems may require two or more decades in the understory before they accumulate the root mass necessary for competitive shoot growth after overstory removal (Sander 1979a, Sander and Clark 1971). However, because seedlings of these species typically accumulate in the understories of xeric ecosystems in which they typically occur, use of the shelterwood method is often unnecessary there (Sander and Clark 1971). In contrast, studies on regenerating northern red oak in the Driftless Area of Wisconsin indicate that a shorter establishment period of 1 or 2 years may be sufficient. Results from one case history in southwestern Wisconsin even indicate that northern red oak can be successfully regenerated with little or no oak advance reproduction provided that the final harvest occurs during the dormant season following a good acorn crop (Johnson et al. 1989). In mesic and hydric ecosystems, a longer accumulation period may be required when applying the shelterwood method. To be effective, this may require control of both overstory and competing understory density (Johnson, 1993a).

The shelterwood system also can be used in combination with underplanting (Johnson et al. 1986, Johnson 1989). This method provides the opportunity to introduce genetically improved stock into stands. However, there is little theoretical evidence that application of the shelterwood system is dysgenic (Howe 1989).