Forest fertilization does not cause any long-term effects on tree growth or vegetation composition

Abstract: Forest fertilization is considered one of the most effective management options to improve forest productivity. In Fennoscandia, nitrogen (N) fertilization has been practiced for over 50 years and is normally added in the form of granules of NH4NO3 at least 15 years prior to the final harvest. In addition to the intended effect of increasing tree productivity, N addition will induce changes in other parts of the forest ecosystem. Such effects include changes in the community composition of plants, insects, and soil fungi. Although many of these effects appear short-lived there are concerns that forest fertilization will induce long-term changes in soil nutrients and associated plant productivity. This study aims to investigate the long-term residual effects of commercial forest fertilization on tree growth and vegetation composition. The experiment was conducted in two regions of Sweden: the northeastern area of Uppsala and the Skinnskatteberg municipalities. It involves 36 forest stands, with half previously fertilized and the other half were unfertilized during the previous forest rotation. It is worth mentioning that these stands were prior to this study clearcut (5-14 years ago). The two forets types were used to evaluate effects of past fertilization. Most data were collected in August 2022, including tree heights, collection of trees cross-sections, vegetation records, and soil samples collection. However, the analysis of the soil data and stem radial growth measurements were conducted at the beginning of 2023. Although tree density, soil N and C/N ratio could affect tree stem radial growth and C/N ratio could influence tree height and density, past fertilization did not affect the radial and vertical growth of trees. Further, there were no differences between fertilized and unfertilized stands in terms of soil N, C/N ratio or on ground vegetation composition. These results suggest that the long-term effects of N fertilization are small or absent, with no major long-term effects on soil N content or vegetation distribution and productivity. The results highlight the resilience and sustainability of forests and provides useful insights for the rational use of fertilization and conservation of forest ecosystems. The forest manager should focus on planning and nutrient management to better balance forest development and promote more sustainable forestry practices.