Taller and slenderer trees in Swedish forests according to data from the National Forest Inventory.

• Trends of basal area growth and mean height were studied in the period 1983–2020. • The study was made for 20 to 60 year old pines and spruces in Sweden. • On average, mean height at a given age of both species has increased by 2 m. • The basal area growth level was stable in the period. • Current trees are becoming taller and slenderer in Swedish forests. Changes over time in annual basal area growth and mean height for Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.) over the period, 1983–2020 were studied using sample tree data from temporary plots recorded in the Swedish National Forest Inventory. The annual basal area growth was derived from the last measured full ring on increment cores. Using 20 to 60-year-old dominant trees, the mean height and annual basal area growth were examined as functions of tree, stand and site conditions, and trends were assessed mainly using residual analyses over time. A significant increase in mean height at a given age was found for both species, but the annual basal area growth level remained stable over the 38-year period. Currently, at a given age of 50 annual rings at breast height, the mean heights of pines and spruces increased on average by 10.1% (i.e. ∼2 m), compared to 50 year-old pines and spruces in the 1980s, and the increase was similar in the different regions. The results suggest that trees have become taller and slenderer in Swedish forests. Increasing tree height over time at a given age in Northern Europe has been documented in several reports and many causes have been suggested, such as changed forest management, increasing temperatures and nitrogen deposition. We suggest that elevated CO 2 in the air and improved water-use efficiency for the trees might also be strong drivers. [ABSTRACT FROM AUTHOR]