Volume growth responses of Scots pine and Norway spruce to nitrogen fertilization: quantitative synthesis of fertilization experiments in Finland

Nitrogen (N) fertilization can enhance carbon (C) sequestration in biomass in boreal forests, which has potential to work as a tool addressing climate change and promoting sustainable forest management. The effects of N fertilization on tree growth have been studied widely in boreal forests in Finland, but a quantitative synthesis is still lacking. Therefore, we performed a quantitative synthesis of the effects of N fertilization on Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.) stands across Finland, utilizing data from 9 published studies encompassing 108 Scots pine and 57 Norway spruce observations. Our research involved building multivariate linear regression models that reflect the annual volume growth response induced by N fertilization, incorporating factors such as N dosage, site fertility, and climatic conditions. The models demonstrated that the N dose is the most significant predictor of volume growth response, which is positively correlated with average precipitation but negatively correlated with time since fertilization. Notably, site fertility had significant influence on growth increment for Scots pine. These findings underscore the importance of site-specific precision fertilization schemes to sustainably enhance growth and carbon sequestration, addressing key management implications for boreal forest resilience. Furthermore, this work contributes to the broader framework of forest system modeling by integrating multiple environmental variables and offers insights into adaptive management strategies.