Growth and root development of black and white spruce planted after deep planting

The capacity of trees to produce new roots is essential for rapid early growth of planted seedlings. Black and white spruce trees (Picea mariana and Picea glauca) are widely planted in the eastern boreal forest of Canada and are known to develop substantial adventitious root systems. In this study, we compared root development and growth of 17-year-old trees that had been planted at two different depths (ground level vs 10-12 cm) to see if partial stem burial would hasten adventitious root development, and in turn, growth. Root number (total and adventitious), root total area, rooting depth, year of root formation, tree height and basal diameter were measured in black and white spruce trees. Both species developed adventitious root systems, and adventitious roots size and area were greater for deeply planted trees than for trees planted at ground level. The number of adventitious roots and the speed of adventitious root development were greater for deeply planted black spruce but not for deeply planted white spruce, compared with trees planted at ground level. For the latter, site conditions could explain the absence of a planting depth response. Deep planting increased tree height and basal diameter of white spruce, but only height for black spruce trees. However, tree growth was related to total root cross-sectional area (not just adventitious roots), underscoring the importance of both types of roots for tree growth.