Interspecies Plantings to Improve Future Oak Log Quality

The booklet entitled, Forest Trees of South Carolina (SC Forest Commission 2018) contains 18 oak species commonly found in South Carolina. Six of these species are considered bottomland oaks and may exist in the stream floodplains. Silvicultural researchers have investigated the beneficial relationship between different bottomland tree species (oak and non-oak) growing in proximity to one another in natural and artificially regenerated stands. Clatterbuck and others (1985) and Clatterbuck and Hodges (1988) observed within even-aged, mixed-species hardwood stands (on Mississippi Gulf Coastal Plain Region) that faster-growing species are dominant early in stand development but tend to become subordinate to oak during the stem exclusion stage of stand development. In these studies, sweetgum established dominance with high stem density early, but after a couple of decades, cherrybark oak surpassed the gums in height to occupy dominant crown positions. Research has suggested that when sweetgum stems averaged 1.8 – 5.5 meters (5.9 feet – 18 feet) from a cherrybark oak, oaks would be restricted with lower height and diameter. If the cherrybark oak, however, was not completely overtopped and could benefit from some overhead sunlight, the oak would catch and stratify above the sweetgum by age 25 (Clatterbuck and Hodges 1988, Lockhart and others 2005). In natural settings or on sites recently disturbed (such as timber harvesting), colonization by light wind or water disseminated species should supply stems to train the oaks’ vertical form. This colonization may only exist up to 300 feet from forested edges, however (Stanturf and others 2000). Should stocking density be higher with stems less than 5 feet from the preferred oak reproduction, some form of woody release (directed foliar sprays or basal bark herbicide applications) is warranted to ensure oak reaches superior crown positions in the developing stand.

Greater cherrybark oak development was observed in an artificial regeneration study looking at said species planted with sweetgum. Lockhart and others (2006) stated that at age 17, the oaks were similar in height and diameter. At 21 years after planting, both height and diameter (see table 1) at breast height (dbh) were statistically greater than sweetgum. Height growth for cherrybark oak ranged from 4.26’ – 9.8’ greater than sweetgum across planting densities. Cherrybark oak had displaced 97% of the formerly dominant sweetgum into either overtopped or intermediate crown classes regardless of spacing (8’x 8’ or 5’ x 5’) treatment. Cherrybark oak that reached either dominant or codominant crown classes was calculated at 74% (8’x 8’ spacing) and 53% (5’x 5’ spacing).

Table adapted from Lockhart and others (2006). Measurements from journal article were converted from metric to English units.

Scientific reasoning for this occurrence may be attributed to a number of factors. These include:

1. crown form – sweetgum exhibits an excurrent crown form represented by branching remaining close to the main bole (conical shaped) even in “open” growing conditions. Oak species tend to be more semi-excurrent when competing with adjacent trees but convert to a decurrent (wide, spreading branch crown or rounded shape) (Lockhart and others 2006, Lockhart and others 2008)
2. crown abrasion – smaller, more brittle twigs (such as exhibited by sweetgum) can break when battered against more stout oak limbs during heavy wind events (Lockhart and others 2006, Lockhart and others 2008). This is important in terms of crown expansion.
3. phenology – the onset of bud break in oak begins from the top of the crown and advances downward (basipetally) whereas sweetgum does the opposite (acropetally). Sweetgum also begins several days later (Young 1980).
4. branching pattern – species with opposite branching patterns (such as ash, maple) may respond more slowly following terminal twigs (from abrasion) given the emergence of new twigs occurs at 45° – 90° from the boles. These new twigs require time to “curve” up towards overhead light.

These trainer tree attributes result in adjacent oaks displaying more elongated, “cleaner” boles meaning the increased probability of higher log grade or quality.

Lockhart and others (2008) developed a ranking system for bottomland species that could serve as potential trainer trees. The previously mentioned factors were given point values based on whether the species had desirable or less desirable characteristics. The following abbreviated table was developed from a more extensive list of species found in the publication. The species presented here are what the author suspects landowners in South Carolina may be most interested in planting around bottomland oaks or have a high likelihood of naturally colonizing (maples).

Table adapted from Lockhart and others (2008)

Planting density can also influence the future potential log quality for oak. Tighter spacings are preferred as wider spacings delay crown closure and intraspecific competition among the planted oak trees. Self-pruning is thus later which promotes defect occurrence and reduces merchantable bole length (Clatterbuck and Hodges 1988). In non-forestland receiving afforestation (planting to create forestland from non-forestland), a tighter spacing, such as 8’ x 8’, should promote quality stem development.

If the area has a high potential to be invaded with seed from adjacent forestland or was previously harvested/disturbed forestland, colonization by natural means (such as by sweetgum) may provide the higher stocking density needed to improve future oak bole quality.

Less research appears to have been performed to investigate trainer species to improve upland oak. Schubert and others (2020) suggest that black cherry may be a good option for trainer trees on moderately productive upland sites. The species has smaller-sized leaves, a more distributed leaf arrangement, and an alternate branching pattern which contributes to a low canopy light extinction rate. In other words, more sunlight can penetrate through this species’ crown compared to other species, such as ash. The Schubert and others (2020) study also found that black cherry had a larger average dbh (after seven years of growth), regardless of spacing, over yellow-poplar but statistically similar to sweetgum. Thus, all three of the species have rapid early rates of growth. Black cherry also provides soft mast forage for wildlife and may have the potential for producing high-demand lumber in the Appalachian region.

Another potential species that can grow on upland sites is common persimmon. This species had a high rating (Lockhart and others 2008) for serving as bottomland trainer trees. Logic would stand to assume that given the species-wide site tolerance and acceptable growth form that the species can also be utilized in mixed planting on upland sites. The wildlife preference/usage of the species is also a strong factor for selection as a component of stock to be planted.

One other consideration in addition to planting spacing is the sequence among species planted on a given row. Most of the previously mentioned studies followed alternating rows with a row of trainer tree only, followed by alternating oak with trainers on the subsequent row. An illustration of this species planting arrangement is depicted in figure 2.

This row arrangement may also enable easier guidelines for conducting a thinning of the stand. The individual rows comprised of only trainers can be removed initially after oak crown stratification and clear bole appearance on oak has occurred. This will enable oak crowns to expand into the created additional growing space which should support increased volume production.

In conclusion, land managers that are attempting afforestation of non-forestland to forestland may choose to consider a mixed plantation containing both oak and one or more other species for economic, wildlife, biodiversity, improved pest resilience, periodic revenue from thinning, or any other reason. Properly placing tree species that are adapted to particular soil conditions found in either bottomlands or uplands is critical for long term success and survival of planted stock. Oaks established on recently disturbed forestland or in immediate proximity (less than 300 feet) may rely on natural colonization from early successional tree species (such as sweetgum or maples) as opposed to planting trainer trees. In the latter case, some form of stand-thinning or woody release around planted oaks may be necessary. Based on scientifically observed physical development of species like sweetgum, it has been suggested that oak bole quality and more dominant crown positions by oak can be achieved through mixed-species planting.

References:

Clatterbuck, W.K., Hodges, J.D., and Burkhardt, E.C. 1985. Cherrybark oak development in natural mixed oak-sweetgum stands-preliminary results. In: Proceedings of the 3^rd Biennial Southern Silviculture Research Conference. USDA Forest Service General Technical Report SO-54, pp. 438-444.

Clatterbuck, W.K. and Hodges, J.D. 1988. Development of cherrybark oak and sweetgum in mixed, even-aged bottomland stands in central Mississippi, USA. Canadian Journal of Forest Resources. 18:12-18.

Lockhart, B.R., Meadows, J.S. and Hodges, J.D. 2005. Stand development patterns in southern bottomland hardwoods: management considerations and research needs. In: Ecology and Management of Bottomland Hardwood Systems: The State of Our Understanding. Gaylord Mem. Lab. Special Publication No. 10. Pp. 439-448.

Lockhart, B.R., Ezell, A.W, Hodges, J.D., and Clatterbuck, W.K. 2006. Using natural stand development patterns in artificial mixtures: A case study with cherrybark oak and sweetgum in east-central Mississippi, USA. Forest Ecology and Management 222:202-210.

South Carolina Forestry Commission. 2018. Forest Trees of South Carolina.

Schubert, M.R., Clatterbuck, W.K., and Zobel, J.M. 2020. Cherrybark oak 7-year growth response in intermixed species, competitive neighborhoods. Ed. Bragg, D.C., Koerth, N.E., and Holley, A.G. In: Proceedings of the 20^rd Biennial Southern Silviculture Research Conference. USDA Forest Service General Technical Report SRS-253, pp. 193-199.

Stanturf, J.A., E.S. Gardiner, P.B. Hamel, M.S. Devall, T.D. Leininger, and M.E. Warren, Jr. 2000. Restoring bottomland hardwood ecosystems in the Lower Mississippi Alluvial Valley. Journal of Forestry 98:10-16.

Young, N.L. 1980. Phenology of plantation grown cherrybark oak, yellow-poplar, and sweetgum. Masters Thesis. Louisiana State University.

Author(s)

Stephen Peairs, Cooperative Extension, Forestry and Wildlife Specialist

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