Growth responses of Norway spruce to weather variation across Finland over the past 60 years

To understand the effects of climate change on boreal forest productivity, it is crucial to examine tree growth variation and identify the climatic factors impacting it. Norway spruce (Picea abies (L.) Karst.) is a common and economically important tree species in Northern and Central Europe. Despite being sensitive to drought, it has been the preferred species for regeneration. Using an extensive tree-ring dataset collected as part of the Finnish National Forest Inventory, we identified the primary weather factors affecting annual radial growth variation in 4979 individual trees over the 1961–2023. We also analysed the spatial patterns of these relationships across Finland. This large-scale analysis revealed that high June temperatures promote tree growth at northern latitudes while the growth response to precipitation is weak. A clear south-north trend was observed in the weather-growth relationships. In southern and central Finland, warm, dry summers were detrimental to spruce growth. However, high June temperatures the previous year were negatively related to radial growth across the country. Furthermore, radial growth negatively responded to high winter temperatures. These results imply that temperature-induced water stress already limits spruce growth more severely and over a larger region than expected. It is likely that the anticipated benefits of global warming due to higher temperatures are diminished or negated by lower soil moisture availability or increased atmospheric evaporative demand.