Large trees have increased greatly in Finland during 1921–2013, but recent observations on old trees tell a different story

Abstract Large and old trees have a vital role in preserving biodiversity in forest ecosystems. We used National Forest Inventory data from 1921 to 2013 for studying changes in densities (stems per ha) of large trees (diameter ≥40 cm) in Finland. In addition, densities of old trees (age ≥150 years) are reported from 1971 to 2013. We present results separately for the three subzones of the boreal biogeographical zone. Large trees have increased as much as 325%. The change has occurred mainly since the 1970s. On country level, old trees have become slightly less common (−4%) since the 1970s, although a decrease was actually observed only in the northern boreal subzone. The large majority of old trees in Finland are quite small in diameter, however. Trees that are both large and old show a notable increase from 1971 to 2013. During the 2010s, densities of large trees were higher in the southern boreal subzone than in the northern boreal subzone, but in the 1920s the opposite was true. Densities of old trees have been much higher in the northern boreal subzone. The observed densities of large trees are still considerably smaller than those observed in unmanaged old-growth forests in Scandinavia. High densities of large and/or old trees were observed in areas with restrictions on wood production emphasizing their role in maintaining biodiversity. The results reflect the destructive effects of former land use and the transition from dimensional cuttings to clear cuts and thinning from below after the 1940s. Proportionally larger changes were observed for southern Finland, where a higher human population density and the resulting more intensive land use had more severe detrimental effects on forests. As the densities of large trees and old trees have developed in a completely different manner in Finland, our results suggest that monitoring only the size distribution of trees will not sufficiently describe the role of old trees as constituents of biodiversity. Thereagainst, densities of large trees and large old trees developed in a similar manner.