FO8 Forest age structure

>> Background information

Development

The age structure of Finnish forests has changed considerably during the past one hundred years. From the point of view of biodiversity the greatest change has been the steep decline of the old forests of Northern Finland. In the 1920s 65% of the forests of northern Finland were over 100 years old and nearly half had reached 140 years. By the 2010s the share of over 100 years old forests had fallen to 23% and the share over 140 years old forests to 14%. The rate of decline of the old forests of northern Finland has remaining practically unchanged for the past 50 years.

The age structure of the forests of southern Finland was already heavily influenced by human activity in the 1920s. Due to the collecting of timber and the wide-spread practice of slash-and-burn cultivation the share of over 100 years old forest was as low as 6% and the share of over 140 years old forest a mere 0.7%. During the following 80 years the share of over 100 years old forests doubled as the abundant young and middle aged forest cohorts (20 to 80 years old) aged. However, since the turn of the millennium the share of over 100 years old forests has begun to decline again due to forest management that aim at a more even age class distrubution.

At the same time, the share of young forests and sapling stands has increased. In one hundred years the share of under 40 years old forests increased from 30% to 43% in southern Finland and from 5% to 27% in northern Finland.

Impact on biodiversity

Each forest development stage supports an associated species assemblage. A particularly high species diversity can be found in heterogeneous old forests which, today, nearly always also host more dead wood than the younger forest stages. Dead wood plays a central role in increasing species diversity as 4 000 to 5 000 species live on dead wood – every tenth species in Finland depends of dead and decaying wood.

35 % of all threatened species are primarily old-growth forest species. The decline of dead wood is the most important cause of threat to 21% and the decline of old forests and large trees to 14% of all threatened forests species. On the whole, nearly 300 species have been classified as threatened due to these intertwined causes. Most of the threatened forest species are fungi, lichens of insects.

An old forest with a closed canopy upholds a microclimate that differs from the adjoining areas. This may have a crucial impact on the survival of species. An old forest typically also includes patches where trees have died or fallen to the ground as a result of storms or snow, for example. Such variation increases species diversity because the age structure within one forest patch becomes more varied and the different aged trees support different species assemblages.

Young managed forest stands are, on the contrary, normally even-aged and often dominated by one species. This makes them species poor. On the other hand, natural disturbances such as forest fires and windfalls may create young forest that host a high species diversity. Such forest, however, are extremely rare in present day Finland. Natural young forest may also occur along the land uplift coast at the Bothnian Gulf and rather diverse young forest may also develop on fields that have been abandoned and turn into forest by themselves. The majority of Finland’s young forests are mechanically regenerated managed forest, however.