Long-Term Ecological Impacts of Norway Spruce Plantations on Biodiversity and Microhabitat Conditions

Reforestation efforts have sought to counteract deforestation and to provide a nature-based solution against climate change. However, they often involve monoculture plantations of non-native species, which may have unintended ecological consequences. Yet, the long-term ecological impacts of planting trees have been poorly estimated. Leveraging historical reforestation conducted in northern Italy during the 1930s by the fascist regime, we assessed the long-term impacts of Norway spruce (Picea abies) monoculture plantations on biodiversity of plants and soil fauna along with soil properties. We found that plant diversity in tree plantations was 50.3% lower than in native forests and 74.5% lower than in grasslands. Ecological indicator values for temperature and light were reduced in tree plantations. Additionally, functional evenness was reduced by 30% in spruce plantations, suggesting lower ecological stability. In tree plantations, organic carbon content was 25% higher due to litter deposition and slower decomposition rates. Soil fauna diversity was marginally less affected, suggesting a faster recovery over the last one-hundred years of arthropods as compared to plants. These findings highlight the need for monitoring reforestation interventions, suggesting strategies that incorporate diverse tree species rather than planting tree monoculture to support functionally and resilient ecosystems.