Root system origin and structure influence planting shock of black spruce seedlings in boreal microsites

Planting shock can last for several years during which seedling growth may significantly slow down. This duration depends upon the time that is required for the root system to acclimatize to its new environment. We hypothesized that characteristics of the root system are important factors for black spruce (Picea mariana [Mill.] BSP) seedling establishment. To test this hypothesis, we compared growth and physiology of three stock types: containerized seedlings, with a dense initial root system restricted to the volume of a root plug; deeply planted containerized seedlings, with the stem buried during production to develop dense adventitious and initial root systems; and bareroot seedlings, with a less developed but unrestricted initial root system. The three stock types were planted on three microsites that are typical of reforestation sites found in eastern Canadian boreal forest: (1) bare mineral soil; (2) in presence of ericaceous shrubs; or (3) in presence of Sphagnum spp. and ericaceous species. Labelled nitrogen fertilizer was added to half of the seedlings to compare nitrogen uptake. We measured seedling growth (height, diameter, above- and belowground biomass), nutrition (including ¹⁵N uptake) and ¹³C discrimination (as an indicator of water stress) at the end of the second growing season. Except at the root level, there were few interactions between stock types and microsites. For fertilized microsites, we speculate that ericaceous species captured most added nutrients since seedlings that were planted directly on mineral soil had greater height and diameter increments than seedlings with competing vegetation. In unfertilized microsites, Sphagnum spp. and ericaceous species had significant negative effects on seedling growth. For stock type, the presence of a root plug allowed containerized and deeply planted seedlings to have a lower δ¹³C, suggesting less water stress compared to bareroot seedlings. Strong initial root system development of containerized seedlings resulted in greater total biomass. We hypothesized that the presence of adventitious roots in deeply planted seedlings positively influenced nutrient uptake, particularly N. As nutrient availability is one of the most limiting factors in the boreal forest, we suggest using deeply planted containerized seedlings to reduce planting stress.