Fertilization Differentially Enhances Stem Growth and Oleoresin Production in a Dual-Purpose Slash Pine (Pinus elliottii) Plantation

Slash pine plantations managed jointly for timber and oleoresin require fertilization regimes that support both stem growth and resin production. We conducted a two-year field experiment in a 14-year-old, newly tapped Pinus elliottii plantation in subtropical Jiangxi, China, comprising an unfertilized control (CK) and nine fertilization treatments representing three nitrogen (N) rates, three phosphorus (P) rates, and three compound-fertilizer formulations (three 20 × 20 m plots per treatment). Fertilizers were applied in spring 2022 and 2023. Relative to CK, the two highest P rates increased the 2022–2023 DBH increment by 0.71–0.74 cm (Dunnett’s test, p < 0.05), and DBH and volume increments increased linearly with P rate, although the overall treatment effect on increments was marginal (p = 0.080–0.112). In 2023, resin yield was 20.6%–25.7% higher than in CK under all three N rates, the highest P rate, and the highest compound formulation (p < 0.05), and treatment effects remained significant after adjusting for stem basal area. Growth and total resin yield did not differ significantly between the N-only and P-only series, whereas resin yield per unit basal area was higher under N than under P fertilization (p = 0.022). Soil variables explained 50.6% of the variation in needle traits (adjusted R2 = 0.20; p = 0.004). Stem growth was correlated with soil total and available P, soil Cu, and needle P, whereas resin yield was correlated with soil available B and needle N. These associations identify candidate nutrient constraints for dual-purpose slash pine management that should be tested in factorial, multi-site trials.