Is Soil Covered with Climacium dendroides More Conducive to the Germination of Pinus koraiensis Seeds?

Our previous investigation has indicated that the natural regeneration of Pinus koraiensis occurs solely in soil covered by Climacium dendroides. This study aimed to determine whether C. dendroides-covered soil enhances seed germination or reduces seed decay. The experiment was divided into two parts: a simulated natural regeneration field experiment, and a greenhouse-based potted trial. For the field experiment, soils were categorized into three treatments based on C. dendroides coverage: high coverage (HCD), low coverage (LCD), and no coverage (NCD). Four forest microsites were considered: a gap in the mixed coniferous forest (GCF), a closed stand in the mixed coniferous forest (SCF), a gap in the mixed broadleaf–coniferous forest (GBC), and a closed stand in the mixed broadleaf–coniferous forest (SBC). The greenhouse experiment consisted of four treatments: HCD and LCD with similar C. dendroides coverage as the field experiment, litter-covered soil (LC), and bare soil (CK). P. koraiensis seeds were sown in each treatment’s soil in both the field and greenhouse experiments and collected after one year to analyze their germination rates, decay rates, and antioxidant capacity based on each treatment. Correlations of the germination rate, decay rate, and antioxidant capacity of P. koraiensis seeds with the soil water content (SWC) and secondary metabolites of C. dendroides in soil were examined. The results revealed that, compared to soil without C. dendroides, HCD exhibited higher germination rates (increased by 15.2% and 32.5% for dormant field seeds and dormancy-broken greenhouse seeds, respectively), ABTS⁺ free radical scavenging activity (RSA_ABTS) (22.84% and 5.98% increases, respectively), catalase activity (CAT) (5.49 U·min⁻¹·g⁻¹ and 1.71 U·min⁻¹·g⁻¹ increases, respectively), and superoxide dismutase (SOD) activity (0.33 U·g⁻¹ and 0.68 U·g⁻¹ increases, respectively). In the field experiment, seeds in HCD exhibited higher DPPH free radical scavenging activity (RSA_DPPH) (26.24% increase) and peroxidase (POD) activity (4.0 U·min⁻¹·g⁻¹ increase) compared to seeds in NCD. Greenhouse seeds in HCD showed a lower rot rate (27.6% decrease) than seeds in CK. In both the field and greenhouse experiments, SWC, soil p-hydroxybenzoic acid content (PHBA), soil p-coumaric acid content (PCMA), and soil vanillic acid content (VA) were significantly positively correlated with the germination rate and antioxidant capacity of P. koraiensis. Soil total phenolic content (TPH) and total flavonoid content (TFL) had minimal impacts on P. koraiensis seed germination. The primary findings suggest that C. dendroides may alleviate drought stress and enhance seed antioxidant and germination capabilities by increasing SWC, PHBA, PCMA, and VA.