Boron fertilization in a boreal Norway spruce forest: long-lasting effects on growth and nutrition

Background and aims Boron (B) deficiency is widespread in boreal forests, but it can be prevented by fertilization. As B deficiency reduces root growth, it may affect the uptake of other nutrients. We assessed the persistence of the effects of a one-time B application on growth and nutrition of Norway spruce (Picea abies). Methods A single-tree B-fertilization experiment was established in a highly productive stand in eastern Finland in 2000 and followed until 2018. The applied B dose was 2 kg ha−1. Results After 19 growing seasons, height growth was higher in B-fertilized trees and the effect was not waning. Diameter growth was not affected. The mean needle-B concentration without B fertilizer was 1.7 mg kg−1 and with B-fertilizer, 4.8 mg kg−1. Boron-fertilized trees had higher foliar aluminium (Al), carbon (C), copper (Cu) and sulfur (S) concentrations, and lower soluble silicon (Si). Conclusions The positive B effect on growth persisted after 19 years. Boron concentrations remained higher in fertilized trees, although lower than at the early stages of the experiment. Boron fertilization maintained Cu and S levels above deficiency limits. The small but consistent change in C indicates a change in needle compounds. Increased Al may result from reduced Si accumulation, as Al co-deposition with Si alleviates Al toxicity. Further studies are required to optimize B-fertilization practices and to elucidate the mechanisms behind the effects on height growth and the levels of other elements. A combination of soluble and sparingly soluble fertilizers could further increase the duration of the effect.