Competition from hardwood species has long been considered a constraint on longleaf pine (Pinus palustris) regeneration in the fire suppression era. However, observational evidence suggests that the presence of midstory hardwoods may improve longleaf seedling establishment on xeric sites. Clarifying and defining this relationship is of great importance for the restoration of the longleaf ecosystem, as current management practices, including frequent prescribed burning and overstory retention for red-cockaded woodpecker (Picoides borealis; RCW) habitat, may be inadvertently contributing to regeneration failures. To explore the importance of midstory hardwoods and overstory density, we established a manipulative experiment on a longleaf dominated site in the North Carolina Sandhills currently being managed for RCW habitat. Plots were located across a gradient of overstory density (9-25 m2/ha) with a consistent hardwood midstory, and planted with containerized longleaf seedlings. Hardwoods were chemically controlled by removing prior to planting (R0), removing after one year (R1), or left untreated (UN). Plots were grouped over a gradient of overstory density based on RCW management guidelines, as follows: < 12 m2/ha (low), 12-18 m2/ha (prime), >18 m2/ha (high). Seedlings were measured for survival (percent per plot), growth (mass), and brown spot needle blight (percent infected seedlings) after two growing seasons.
Results/Conclusions
Overall, seedling survival averaged 80% and differed significantly among treatments (p=0.0329), with UN plots (84%) supporting the highest survival and R1 the lowest (76%). Overstory density had the strongest effect on seedling survival (p=0.0001) with low density (86%) supporting significantly higher survival than prime (79%) or high (73%) density. Seedling biomass averaged 10.5 g and differed significantly (p=0.0002), with the greatest average dry weight under low overstory density (10.9 g) but did not differ among hardwood treatments (p=0.1550). Brown spot needle blight infection averaged 44% and differed significantly (p=0.0008), with the greatest infection under low overstory density (49%) but did not differ among hardwood treatments (p=0.2187). Competition for moisture with overstory pines was most strongly associated with seedling performance, as seedling moisture stress was negatively correlated with overstory density (-0.29, p=0.0062) which in turn was positively associated with seedling growth (0.30, p=0.0046) and survival (0.31, p=0.0037). Our early results indicate that maintaining overstory density greater than 12 m2/ha and controlling midstory hardwoods after seedling establishment is negatively impacting longleaf regeneration.