Defense keeps pace with growth in intensively managed Southern US pine plantations: Co-scaling of resin ducts and radial growth

Bark beetle attacks are a major threat to the U.S. timber industry, and the primary defense mechanism trees possess against these insects is oleoresin, which is produced and delivered in resin ducts. Silvicultural practices that increase resin duct formation could strengthen tree defenses. We used neural network–based resin duct and tree ring detection to examine how pine plantation management practices influence resin duct traits in the southeastern United States. We analyzed 272 loblolly and slash pines from four experimental plantations in Georgia and Florida that included four levels of planting densities and two regimes for fertilization and competition control. Diameter at breast height (DBH) was 29.0% (loblolly) and 26.1% (slash) lower at the highest planting density than the lowest and 11.4% (loblolly) and 17.6% (slash) greater under the intensive management treatment than the operational treatment. This relationship also occurred in unstandardized resin duct metrics such as total duct area which was 23.6% (loblolly) and 26.4% (slash) lower at the highest planting densities and 17.5% (loblolly) and 26.0% (slash) higher under the intensive management treatment. Relative duct area changed little with tree age while duct density decreased slightly after the first years before stabilizing. These results suggest that plantation management in southeastern U.S. pine plantations to increase yield will also increase total axial resin duct production, while leaving proportional resin duct allocation largely unchanged.