Interactive effects of species composition, site quality, and drought on growth dynamics of Norway spruce and Scots pine stands in Norway

Data from the Norwegian national forest inventory spanning from 1994 to 2022 were analyzed to explore the growth dynamics of pure and mixed stands of Norway spruce and Scots pine. The derived large dataset enabled the development of models designed to assess how stand characteristics and drought interactively affect volume increment at the stand and individual tree level. The analysis revealed that pine-dominated stands outperform spruce-dominated stands at lower site qualities, while the opposite was true at higher site qualities. Mixed stands exhibited overyielding, with productivity exceeding the expected combined pure stand productivity of the individual species components. Based on model predictions, relative overyielding increased with stand age and declined with increasing site quality. Transgressive overyielding, where mixed stands outperform pure stands of either species, was predicted for medium site qualities. Drought-induced productivity losses increased with spruce proportion, especially at lower site qualities, and with stand density. The presence of pine in mixed stands mitigated the negative effects of drought on spruce. The findings of this study suggest that pure spruce stands should be avoided on lower-quality sites while mixed stands with appropriate thinning interventions should be promoted to maintain productivity under changing climatic conditions. • Norwegian national forest inventory observations were leveraged to create large dataset. • Predicted relative overyielding increased with stand age and declined with site quality. • Transgressive overyielding found in mixed stands on medium quality sites. • Mitigating drought effect of pine appeared to be more pronounced at lower site quality.