This is because silvicultural methods applied at the end of the 19th century and in the early 20th century created thin, low-productive forests. Major restoration work was started in 1950 which led to large areas of high-growth forest that are not yet mature. A relatively large portion of Swedish forest older than 100 years is set aside for nature conservation purposes.
The forest's rotation period is taken as 60-120 years, depending on where in the country the forest is located.
Source: Swedish Forest Agency and Sveaskog.