A Stand-Development Approach to Oak Afforestation in the Lower Mississippi Alluvial Valley

Land-Use Patterns Before European settlement, forests covered much of the LMAV (National Research Council [NRC] 1992, Hefner and Brown 1985) although the exact extent of forests during this time is unknown because of the indeterminate role of American Indians and their clearing practices on the forest resource (Buckner 1989, Hamel and Buckner 1998, Fickle 2001). After settlement, documented forest clearing for agriculture began (Barry 1997). Deforestation reached a maximum of 300,000 ac yr 1 during the 1950s through the 1970s in response to high soybean (Glycine max [L.] Merr.) prices (Spencer 1981) and a prolonged drought that gave a false sense of security in farming floodplain soils that have a high clay content and are normally wet much of the year (Stanturf et al. 2001). By the early 1990s only about 5 million ac of forests remained in the LMAV (The Nature Conservancy 1992). Furthermore, all of the land in the LMAV has been subject to altered surface and subsurface water flow patterns caused by stream channelization and rerouting and the construction of levees, ditches, roads, and dams. Notable among these is the mainline levee system along the Mississippi River that has reduced much overland and backwater flooding (Barry 1997). The extensive land clearing for row-crop agriculture and urban development combined with changes in surface and subsurface water flow patterns has led to the declaration that the LMAV is one of the most endangered ecosystems in the United States (Noss et al. 1995). Afforestation in the LMAV—Single-Cohort, Single-Species Stands During the 1980s, concern was expressed over the loss of bottomland forest ecosystems in the LMAV (Haynes et al. 1993). Efforts began to restore these bottomland ecosystems through a variety of treeplanting projects (Allen and Kennedy 1989, Allen 1990, Haynes et al. 1993). With the advent of federal and state government programs to aid in costs of planting trees, especially the Conservation Reserve Program and the Wetland Reserve Program (Kennedy 1990, Haines 1995, Stanturf et al. 1998), about 500,000 ac of former agricultural land have been planted with bottomland hardwood species by 2001 (Gardiner and Oliver 2005). Although afforesting former agricultural fields is not considered complete bottomland forest ecosystem restoration, it is an important first step (Stanturf et al. 2000, 2001). Afforestation in the LMAV has involved planting hard mast species, primarily 1-year-old bareroot oak (Quercus spp.) and sweet pecan (Carya illinoinensis [Wang] K. Koch) seedlings, on a 12 12-ft spacing, in either single-species stands or mixtures of oak species or oak species and sweet pecan (Stanturf et al. 2000, Schoenholtz et al. 2001). These species were favored in planting on former agricultural fields for their function in wildlife habitat and because seed dispersal into such areas is limited. Native, light-seeded species, such as green ash (Fraxinus pennsylvanica Marsh.), elms (Ulmus spp.), American sycamore (Platanus occidentalis L.), sweetgum (Liquidambar styraciflua L.), and red maple (Acer rubrum L.), were assumed to naturally colonize old fields as their seeds are dispersed by floodwater (Stanturf et al. 1998, 2000). Stanturf et al. (2000) summarized problems with this traditional approach to afforestation in the LMAV: (1) light-seeded species reliably established only within about 300 ft of forest edges (Allen 1990, 1997; McCoy et al. 2002); (2) homogeneous oak plantations do not provide early complex stand structure that is valued for wildlife habitat (King and Keeland 1999, Twedt et al. 1999); (3) stocking densities typically achieved under federal cost-share programs usually limit timber management options, thereby restricting potential management objectives; and (4) carbon sequestration may be lower in oak monocultures than in mixed-species stands. Oak plantations, either single species or a mixture of oak species, typically do not develop satisfactory stem quality (Figure 1). Wide spacing on many afforested sites allows oak stems to develop large branches on the lower bole before the onset of crown closure (Oliver and O’Hara 2005). These branches slowly die after crown closure but persist on the bole for many years. Concurrently, slow diameter growth occurs from intraspecific competition between the oak trees. The slow shedding of large dead limbs and slow diameter growth results in large surface knots, which degrade the lower bole, significantly lowering stem quality and value (Kenna 1981, 1994). Furthermore, intense intraspecific oak competition places the stand under stress promoting epicormic branches along the bole. These branches further reduce stem value (Meadows and Burkhardt 2001). Precommercial thinning can alleviate this stress (Oliver and O’Hara 2005), but it is often not practiced because of the costs burdened by the landowner. More recently, managers have established species mixtures with oaks and native light-seeded species (Jon Wessman, pers. comm., US Fish and Wildlife Service, July 11, 2006). These mixtures were established to increase tree species diversity, which is assumed to result in improved wildlife habitat. Although the trend toward establishing species mixtures is warranted, most mixtures are assigned with little concern for species compatibility. Stand Development Approach—Single-Cohort, Mixed-Species Stands Overview Problems inherent to single-species oak stands can be overcome by planting species mixtures that encourage interspecific competition instead of intraspecific oak competition early in stand development. Mixed-species forest plantations are typically plantings of two or more tree species wherein each species has a specific role in the mixture. Unlike single-species plantations, mixed-species plantations require knowledge of each species silvical characteristics and the interaction of these characteristics between species and site conditions (Larson 1992, Oliver 1992). Failure to consider individual Figure 1. Twenty-year-old water oak plantation in Sharkey County, Mississippi, showing little canopy stratification and poor self-pruning of individual trees. (Photo by Brian Roy Lockhart.) SOUTH. J. APPL. FOR. 32(3) 2008 121 species requirements and the effects of the mixture on requirements can lead to plantation failure. Advantages and Disadvantages The benefits of mixed-species plantations are many compared with single-species plantations (Kelty 2006, Nichols et al. 2006). Some mixed-species plantation mixtures can provide greater yields than single-species plantations (Binkley and Greene 1983, Binkley 1984, Schlesinger and Williams 1984, DeBell et al. 1985, Mielikainen 1985, Kelty 1986, Tham 1988, DeBell et al. 1989, Paschke et al. 1989, Groninger et al. 1997, Nichols et al. 2001). Greater structural diversity of habitat conditions in mixed-species plantations provides for improved wildlife habitat compared with singlespecies plantations (Twedt and Portwood 1997, Twedt and Wilson 2002). Mixed-species plantations may also increase carbon sequestration (Montagnini and Porras 1998, Kaye et al. 2000) and enhance recruitment of natural regeneration compared with singlespecies plantations (Parrotta 1999, Carnevale and Montagnini 2002, Twedt 2006). Finally, mixed-species plantations may reduce the risk of plantation failure compared with single-species plantations by providing greater resistance to damaging agents such as insects, pathogens, and wind (Watt 1992, Montagnini et al. 1995, Nichols et al. 1999). A disadvantage to mixed-species plantations is their high establishment and maintenance costs (Montagnini et al. 1995, Gardiner et al. 2002) when control of competing herbaceous plants is required for successful establishment (Bowersox and McCormick 1987; Ponder 1987; von Althen 1991; Ezell 1995, 1999; Groninger et al. 1997; Ezell and Catchot 1998; Ezell et al. 1999). On former agricultural fields in the LMAV, woody vines are particularly difficult to control (Stanturf et al. 2004). In addition, care must be taken to match species with site requirements (Putnam et al. 1960, Hodges 1997) and ensure compatibility of development patterns of selected species (Bhatnagar et al. 1993). Mixed-species plantations may also reduce harvesting efficiency compared with single-species plantations because harvesting equipment must navigate around selected species. Key Considerations for Constructing Single-Cohort, Mixed-Species Stands Ashton et al. (2001), who worked on restoration of tropical ecosystems, forwarded several principles that should be considered when designing mixed-species forest plantations. Many of these principles, which account for ecological relationships between individual species and site conditions, can be applied to mixed-species stands in the LMAV. An inherent assumption made by Ashton et al. (2001) is that mixing shade-intolerant species with shade-tolerant species or pioneer species with later-successional species is preferable to planting species of similar shade tolerances or successional status. Stands planted with species of similar shade tolerances or successional status develop similar to single-species stands (Guldin and Lorimer 1985) with few of the benefits associated with mixtures of contrasting species. When species selected for planting in mixtures are successionally compatible (i.e., shade-tolerant species with shade-intolerant species or pioneer species with later-successional species), early successional species will enhance site conditions for subsequent development of later-successional species, similar to pathways found in autogenic succession (Hodges 1997). These mixtures also facilitate the development of stratified canopies that increase the number of niches for wildlife while concurrently providing interspecific competitive conditions that enhance the development of quality boles on desired later-successional species. Thus, a primary consideration is that the successional pathways of species or species guilds must be known