Ancient forest and biological diversity
In Tyresta the forest has retained its ancient character. Here, there are trees of varying age and species — primarily spruce and pine, with elements of broad-leaved trees such as aspen and alder.
First and foremost, the forest is characterised by a steady large supply of dead wood. Many of the forest’s threatened species of fungi, mosses, lichens and insects are linked to dead wood in various stages of decay. Death is clearly linked to life in the Tyresta forest. In a logged forest, dead wood does not accumulate since trees are harvested before they become old and die.
Here and there old trees have fallen, leaving a light opening in the tree cover where new life can thrive. On rocky ground fire may have raged and left nature to start over, with broad-leaved trees dominating for up to 100 years before evergreen forest takes over again. This can be seen most clearly in the area of the 1999 forest fire, which covers 20% of the national park’s area.
There is great biological diversity in Tyresta National Park, due to the fact that the forest has been allowed to be forest for very long time — since 10,000 years ago, when the ice sheet retreated and the trees made their entrance.