Phosphorus addition alter the pine resin flow rate by regulating tree growth and non-structural carbohydrates in a subtropical slash pine plantation

Pine resin is a widely used forestry by-product, and its yield can be determined using the resin flow rate (RFR). The application of phosphorus (P) fertiliser is prevalent in plantation forests; however, its effects on the RFR are currently unclear. Here, a P fertiliser addition experiment was conducted in a slash pine (Pinus elliottii Engelm) plantation in subtropical China with four treatments - P1 (25 kg P ha−1 yr−1), P2 (50 kg P ha−1 yr−1), P3 (100 kg P ha−1 yr−1), and a control. Tree growth represented by diameter at breast height (DBH), height (H) and timber volume (V) increments, the RFR over 9 months, nine needle parameters, and non-structural carbohydrates (NSC) in the organs of the slash pine were measured. The RFR was highest from June to August. The P addition, especially after the P2 and P3 treatments, enhanced the RFR overall, except during March, April, and November. Compared with other treatments, the P3 treatment improved the increments in DBH and V. Meanwhile, the P addition regulated the synthesis of soluble sugars (SS), starch (S) and other NSC, which varied among organs. P addition increased needle morphogenesis and chlorophyll content while reduced the dry matter content and the ratio of dry to fresh weight. RFR was positively correlated with tree growth and needle length, area, chlorophyll and NSC but negatively correlated with the dry matter content and the dry:fresh weight ratio of the needle. The structural equation model showed that P addition not only had a direct positive effect on the RFR but also indirectly altered it through tree growth and organ NSC. These results indicate that P fertiliser application can effectively promote resin yield by altering tree morphology and physiology in subtropical slash pine plantations. The trade-off between the P fertiliser application rate and economic and ecological benefits should be considered in the future research.