Growth, nutrient dynamics, and outplanting performance of Picea mariana (Mill.) B.S.P. seedlings in response to fertilization during the hardening period

Traditional nursery culture of containerized black spruce (Picea mariana [Mill.] B.S.P.) seedlings involves a late-season hardening period when irrigation and fertilization are withheld to stress seedlings and promote frost-hardiness. Nutrient uptake is limited without supplemental fertilization, hence growth during hardening results in severe dilution of internal nutrients. The main objective of this study was to promote nutrient uptake and avert N dilution by fertilizing seedlings during the hardening period. Growth continues during this time (15-weeks), but gradually declines at the end of the rotation. Hence, nutrient supplementation progressively diminished with time to match seedling growth rate. Fertilization initially increased N uptake and elevated N concentration above initial pre-hardening status, but uptake declined after 6 weeks when fertilization ceased, causing growth dilution and N concentration decline. Based on these findings, fertilization dosages were increased and delivery schedules were prolonged in a follow-up experiment. Prolonged high-dose regimes promoted N uptake and prevented dilution, as reflected by a sustained increase of N concentration with time. When transplanted for 12-weeks on Boreal forest soil substrates to test outplanting performance, seedlings that received prolonged high-dose fertilization during hardening resulted in 115% more biomass and 350% more N than conventionally reared seedlings. The results demonstrate that fertilization during the hardening period improves early outplanting performance of seedlings, and may benefit future reforestation in northern forests.