Root plug effects on early growth and nutrition of container black spruce seedlings

The capacity of root plugs of containerized stock to serve as a ready nutrient source for growth and nutrition of newly planted seedlings is not well understood. Intact and barerooted container-reared seedlings were planted on soil bioassays of contrasting fertility to examine the contribution of the root plug to early field performance, and test if nutrient loading and (or) site enrichment may enhance this effect. The test species was black spruce (Picea mariana (Mill.) BSP) fertilized conventionally or nutrient loaded (at 10 or 64 mg N per plant for the season) in the nursery. The bioassays, retrieved from a boreal forest in northern Ontario, were topdressed with 20–20–20 fertilizer at 0 and 200 kg N ha^-1 at planting to simulate nutrient “poor” and “rich” sites. Without changing total biomass, nutrient loading raised root plug electrical conductivity by 67%, and built up plant N, P and K reserves by 152, 32 and 33%, respectively. One season after outplanting, nutrient loading increased biomass, N, P and K accumulation of seedlings as high as 35, 43, 27 and 41%, demonstrating the effectiveness of this practice in improving early growth and nutrition. Element content differences between barerooted and intact seedlings suggest that root plugs contributed as much as 22, 17 and 34% to respective plant N, P and K uptake, signifying that root plugs act as important reserves for meeting nutrient requirements of container stock. Barerooting reduced height growth, shoot:root ratio and nutrient uptake of seedlings, particularly on the fertile (rich) site, but nutrient loading countered this inhibition somewhat. Study results confirm the importance of root plugs as immediate nutrient sources for fast growing trees, and suggest that supplemental fertilization prior to outplanting may benefit initial plantation establishment.