The Persistence of Container Nursery Treatments on the Field Performance and Root System Morphology of Longleaf Pine Seedlings

In recent decades, container stock has become the preferred plant material to regenerate longleaf pine (Pinus palustris Mill.) forests in the southeastern United States. We evaluated the effects of container nursery treatments on early and long-term field performance in central Louisiana. Seedlings were grown in four cavity volumes (60–336 mL) with or without copper oxychloride root pruning (Cu or no-Cu) and fertilized at three nitrogen (N) rates. Across treatments, 91% of the seedlings emerged from the grass stage by the second field season, and 88% of the seedlings survived eight years after outplanting (Year 8). Seedlings grown in the largest cavities had greater total heights and stem diameters than those cultured in the 60- and 95-mL cavities through Year 8. Seedlings receiving the least amount of N in the nursery were consistently smaller in stature through Year 8 than seedlings receiving more N. Field growth was unaffected by copper root pruning through Year 8. Foliar mineral nutrient concentrations and seedling nutrient contents of Year 2 seedlings did not respond to nursery treatments. Independent of nursery treatments, seedlings excavated in Year 2 had at least 60% of their first-order lateral roots (FOLRs) originating from the top 4.0 cm of the taproots. The Cu-root-pruned seedlings had twofold the percentage of FOLRs egressed from the top 8.0 cm of the root plug when compared with the no-Cu seedlings. Moreover, the Cu root pruning treatment decreased the percentage of root plug biomass allocated to FOLRs, total within root plug FOLR lengths, and FOLR deformity index. The effects of increasing cavity volume or N rate on the root plug FOLR variables were opposite those of the Cu root pruning treatment. Our results suggest that a tradeoff may exist between seedling stature and a more natural FOLR morphology in outplanted container longleaf pine seedlings.