Growth Responses of Acer velutinum Boiss. to Soil Compaction in a Hyrcanian Forest Nursery, Northern Iran: Implications for Sustainable Forest Restoration and Nursery Management

Soil compaction alters soil structure and hydrology by increasing bulk density and penetration resistance, disrupting aggregates, reducing porosity and infiltration capacity, and enhancing shear strength. These changes can impede seedling establishment, particularly by restricting root development. The objective of this study was to evaluate the effects of a controlled soil penetration resistance (SPR) gradient on velvet maple (Acer velutinum Boiss.) seedling growth, morphology, and architecture under nursery conditions in northern Iran’s Hyrcanian forests. We hypothesized that increasing compaction would reduce seedling size and total biomass while altering the pattern of biomass partitioning between above- and belowground organs. Seven compaction levels were induced using a 4.736 kg hammer dropped from 45.7 cm, applying 0 (control), 1, 2, 4, 6, 8, or 10 blows per layer across 35 pots (5 replicates per level). After the 227-day growing period, seedlings were carefully excavated; root systems were gently washed, and all morphological indicators (main root length, lateral root length, and stem length) were measured manually using a ruler. Soil compaction resulted in a marked increase in soil penetration resistance (SPR), rising from 0.32 ± 0.03 MPa in the control treatment to 2.09 ± 0.08 MPa at the highest compaction level (p ≤ 0.01). Elevated SPR elicited significant polynomial responses in seedling morphological traits, including stem and root dimensions, as well as in biomass accumulation (total, shoot, and root). Notably, the main root biomass exhibited a significant decline with increasing compaction intensity, whereas lateral root length increased correspondingly, despite a reduction in lateral root biomass. These findings indicate that, in loam to clay-loam soils under optimal moisture conditions, total seedling biomass increased with moderate compaction, peaking near 1.5 MPa, whereas significant root growth reduction occurred above approximately 1.2 MPa. Above this level, compensatory lateral root elongation and a shift in biomass partitioning toward shoots were observed, but severe compaction (>1.5 MPa) ultimately suppressed whole-plant productivity. The study identifies a novel root architectural trade-off characterized by a 198% increase in specific root length and a tripling of the lateral-to-main root length ratio under severe compaction, providing species-specific thresholds and early-warning indicators for nursery soil management. These findings provide valuable insights into adaptive seedling responses to soil compaction and contribute to the development of sustainable forest nursery practices. Improving seedling quality under compacted soil conditions can enhance plantation success, soil conservation, and the long-term sustainability of forest restoration programs.