Longleaf pine restoration is a major objective of land managers throughout the southeastern United States, including across Department of Defense installations. Within the Atlantic Coastal Plain, hydric conditions are common on flatwoods and savanna sites. These site types can support highly diverse plant communities, but establishing longleaf pine trees on such sites is difficult due to the wet soil conditions and the abundant competition from other vegetation. Site preparation is commonly used to improve establishment of planted trees but may have negative impacts on the ground layer plant communities. Short-term results from site preparation studies are useful but do not provide information on trajectories of plantation establishment. This study remeasured a previously established field experiment (SI-1303) to understand effects of alternative site preparation treatments on plantation development through 15 years after planting.

This project used a manipulative field experiment to contrast eight site preparation treatments that included a factorial combination of two treatments that controlled competing vegetation (chopping and herbicide) with three treatments that manipulated soil conditions (bedding, mounding, and untreated/flat). In addition to the six factorial combinations, the study included an untreated control and a more intense treatment that combined bedding, chopping, and herbicide. Data collected in SI-1303 during the first three years after study initiation (2004-2006) were combined with data collected in 2017/2018 to report on longer-term responses of the planted trees and the ground layer plant community.

The site preparation treatments had long-term effects on longleaf pine establishment and plantation development. The untreated control and the chopping treatment (without the addition of bedding and mounding) resulted in the lowest survival of planted trees and the poorest tree growth. In contrast, the higher intensity treatments of bedding or mounding combined with herbicide resulted in the greatest tree survival and the largest longleaf pine trees. In general, successful trees emerged from the grass stage within the first three-years of the study, with the chopping treatment having later average grass stage emergence. The site preparation treatments resulted in sustained growth increases across the 15-year period. While resulting in the greatest benefit to planted longleaf pine trees, the site preparation treatments of greater intensity (bedding, mounding, herbicide) generally also had greater impacts on the ground layer plant community. Herbicide in particular had long-term effects. The herbicide treatment was designed to control woody vegetation and was effective in short-term control, but woody plant abundance had recovered and dominated these treatments after 15 years. The herbicide treatment also reduced the abundance of bunchgrasses, and wiregrass in particular, compared to chopping. The mechanical disturbances of bedding and mounding also reduced wiregrass, but this effect was locally variable in the bedded treatments.

Understanding long-term patterns in plantation development is critical for management for restoration objectives. Maximizing the rate of plantation establishment will be achieved with more intensive site preparation, including bedding and herbicide. These treatments will also result in greater dominance of woody vegetation and shifts in plant community composition, including reductions in wiregrass. Site preparation decisions can be made based on specific restoration objectives.