White spruce (Picea glauca) structural root system development and symmetry influenced by disc trenching site preparation

Mounding site preparation is normally required prior to planting seedlings for sites with understory competition and cold and wet soils in order to increase seedling establishment and survival in the boreal forest. Concern has been raised that planting seedlings on mounded microsites may influence the spatial distribution of lateral roots and lead to potential windthrow. The objective of this study was to examine root symmetry for container white spruce (Picea glauca) seedlings planted on disc trenched berms and no disc trenched sites compared to naturally regenerated seedlings in mixedwood forest of Saskatchewan. No significant differences were found between treatments for root cross sectional area index (CSAI) at radii of 25, 50 and 100 cm from the stem center. Values of CSAI for planted seedlings on disc trenching, no disc trenching and natural regeneration sites all exhibited a degree of root asymmetry (range of 0.63-0.92) rather than being symmetrical (0.25) for all three radii. Disc trenching site preparation only impacted the direction of root development on <50% of planted container white spruce seedlings, for which the center of diameter (COD) positions were located on the berm side leading to a potential root asymmetry. However, the magnitude of this potential asymmetry was within the range observed on no disc trenching and natural regeneration sites. Rooting depth at 25-100 cm radii from the stem, was significantly shallower in planted container seedlings (<6 cm) compared to naturally regenerated white spruce seedlings (>38 cm). Container seedlings exhibiting both shallow rooting depth and root asymmetry on disc trenched microsites suggests an increased risk for potential windthrow as the trees mature.