Effects of canopy structure and cultural treatments on the survival and growth of Pinus palustris Mill. seedlings underplanted in Pinus taeda L. stands
Longleaf pine restoration is a common management objective in the southeastern United States and requires artificial regeneration of longleaf pines on sites currently dominated by loblolly pine. In many cases, retention of canopy trees during stand conversion may be desirable to promote ecological function and meet conservation objectives. We tested the effects of seven harvesting treatments that varied residual canopy density and distribution, in conjunction with additional cultural treatments (herbicides and fertilizer), on the mortality and growth of longleaf pine seedlings underplanted in loblolly pine stands. We observed no change in the root collar diameter of longleaf pine seedlings planted in plots with no canopy removal (residual basal area of 16 m²/ha) over three growing seasons. Clearcutting resulted in the greatest seedling growth and the greatest percentage of seedlings that had emerged from the grass stage, although mean seedling size within canopy gaps did not differ from that within clearcut plots. Within canopy gaps, seedling root collar diameter did not significantly increase beyond 10 m from the forest edge. Canopy trees provided an apparent facilitation effect on longleaf pine seedling survival, with the highest mortality in clearcut plots and on the northern half of canopy gaps. Releasing planted longleaf pine seedlings with herbicides resulted in a moderate increase in the percentage of seedlings in height growth but had no effect on root collar diameter. Our results demonstrate trade-offs between longleaf pine seedling survival and growth associated with canopy retention but also suggest that managers have some degree of flexibility in prescribing harvesting treatments to meet restoration objectives on sites currently dominated by loblolly pines.