Interactive Effects of Soil Compaction Intensity and Soil Texture on Morphological Traits, Biomass Accumulation, and Allocation Patterns of Persian Honeylocust (Gleditsia caspica Desf.) Seedlings

Ground-based logging operations cause soil compaction that constrains natural regeneration in the Hyrcanian forests of northern Iran, yet how soil textural properties modulate these effects on endemic tree species remains poorly understood. This study assessed the interactive effects of soil texture and compaction intensity on seedling growth and biomass allocation of Persian honeylocust (Gleditsia caspica Desf.) under controlled conditions. A factorial greenhouse experiment with three soil textures (loam, sandy loam, and silty clay loam) and six compaction levels (0–5 Proctor impacts) was conducted. After a 110-day growing period, morphological traits and biomass partitioning were analyzed using two-way ANOVA. Significant soil texture × compaction interactions (p < 0.05) were found for lateral root length, primary and lateral root dry biomass, stem biomass, total biomass, leaf mass ratio, root mass ratio, and the lateral-to-primary root length ratio. In sandy loam, mild compaction (Level 1) increased total biomass by 39% (2.15 g) and lateral root length by 34% (448.5 cm) relative to the controls, indicating a beneficial compaction window. Loam soils exhibited an initial reduction in growth at low compaction levels, followed by recovery at moderate levels, suggesting physiological acclimation. Silty clay loam, with its high compressibility, suppressed root proliferation and aboveground growth across most compaction treatments. Axial traits, including stem length, primary root length, and root collar diameter, remained stable irrespective of treatment, highlighting a conservative developmental strategy. These results reveal that G. caspica employs a hierarchical adaptive strategy involving dynamic modulation of lateral root development and whole-plant carbon partitioning while preserving core axial architecture. The strong texture dependence of these responses highlights the need for texture-specific forest management. For restoration in degraded Hyrcanian landscapes, soil compaction thresholds should be calibrated to the dominant textural class to avoid exceeding the species’ ecological plasticity.