Tree Species-soil Relationships on Marginal Soybean Lands in the Mississippi Delta’

In the Mississippi Alluvial Plain, marginal soybean lands are those lands that are frequently flooded and have relatively low average soybean yields. These marginal farmlands might be regenerated to bottomland hardwood species if species-site relationships and silvicultural systems were better developed. Cost effective esteblishment.and management of these stands will require an understanding of species-soil compatibility as well as an appreciation of the impacts of long-term soybean cropping on this relationship. Published site selection systems are evaluated within the context of afforestation on marginal soybean land in the Delta. Projected site indices and species-site suitability is given for former soybean lands on soil series that are marginal for soybean production. Potential improvements in species-site selection methods for soybean land afforestation are discussed. INTRODUCTION Soybean price increases during the 1970’s resulted in a large amount of cleared land in the lower Mississippi Valley (Delta) (Sternitzke 1976). Some of this land is now apparently of marginal value for agriculture. In 1996, a consortium of industry, conservation and economic development agencies sought to evaluate the potential of these lands to enhance the economic and environmental integrity of the region (Amacher and others 1998). While the rolative costs and benefits of soybean agriculture are well known for these lands, productive potential following sforestation remains poor ly unders tood. Landowners , forest idustry and government agenc ies wou ld benef i t f rom nowing the productivity of afforested former soybean lands 3 aid in land use decisions, regionwide raw material nojections and development,of reforestation incentive brograms. Gu ide l ines mak ing s i te -spec i f i c spec ies ucommendations and productivity predictions would be nest useful toward these ends. ‘o date, three published systems are available for matching bpocies with site for reforestation efforts: I. Broadfoot (1976) provides specific species recommendat ions and es t imates a range o f productivity for a limited number of soil series in the mid-south reg ion. ;’ County soil surveys are widely available in this region. The newer editions include estimates of forest productivity and recommend tree species for planting by soil series. However, these estimates are based on llmited data and may be unreliable. S Baker and Broadfoot (1979) predict site index for several bottomland hardwood species by evaluating soil and site fIttributes. In the past, use of this system required Intensive sampling of the planting site. However, recent county soil surveys now include necessary information and soil mapping of the region is virtually complete. In Iho case of former soybean lands, soil survey data combined w i th assumpt ions regard ing so i l ’ and s i te nllributes can be made permitting generic modeling of polential productivity for soil series. The objectives of this study were to evaluate tree speciessoil productivity guides to make species selection and predict potential productivity for several soil series representative of marginal soybean lands. Improvements are suggested for the further development of species selection and productivity estimation tools for old fields. MiTHODS Selection of Soils for Consideration The study area was limited to counties located entirely or partially within the Delta in the states of Arkansas, Louisiana and Mississippi (fig. I). Ten Delta soil series were selected for study according to the following criteria: 1. Soybean cultivation is commonly practiced. 2. Soils are poorly or somewhat poorly drained. 3. Flooding frequency ranges from occasional to frequent. 4. Soils are classified as hydric. All soils used in this study met each of these criteria except the Dundee series, which is not hydric but often occurs in close association with several of the other soil series (Soil Conservation Service 1987). Tree species were selected for evaluation on the basis of potential to occupy bottomland sites and eventual merchantability. Site Productivity Estimates Estimated site productivity was calculated using the methodology developed by Baker and Broadfoot (1979). This system involves assigning point values for several soil and site criteria. For each criterion, point values are assigned in accordance with its relative importance to the growth of a particular species. When values have been assigned for each criterion, point values are summed to provide an estimate of site index for the given species and site conditions. In order to make generic estimates for economically-marginal soybean fields, several assumptions were made (table 1). In this way, species productivity estimates could be made for each soil series entirely on the basis of information available in published soil surveys. This evaluation eliminates the costs associated with on-site soil i)‘h~~,r presented at the Tenth Biennial Southem’SilviculturaI Research Conference, Shreveport, LA, February l&18,1999. lunl Professor, Department of Forestry, Southern Illinois University, Carbondale. IL 62901-4411; Associate Professor and Graduate rch Assistant, Department of Forestry, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0324; and Project Leader, fores1 Service, Southern Research Station, Stoneville, MS 38776, respectively.